Trans 1MarE, Vol 107, Parr?, pp 27-45

Ships and offshore cabling where to now?

REVIEW OF CABLE STANDARDS

Naval cables

Understandably, perhaps, the Royal Navy took the lead inthe late 1970s and 1980s when they introduced a new familyof Naval Engineering Standards (NES). What is perhaps notso readily understood is the fact that the most radical changeinvolves that of the sheathing compound. NES 518 coveredits sheathing compound requirements, later to become De-fence Standard 61-12 Pt 31, with actual cable constructionscovered by NES525, 526 and 527. Included in this family ofstandards are the specialised tests required to check compli-ance, such as NES711 and 713.

These standards introduced the term 'Low Fire Hazard'(LFH). NES525 provided thin wall insulation systems for upto 2.5 mm2 conductors, based upon engineering polymers orirradiation technology with zero halogen, low smoke (LFH)sheaths. It included a toxicity requirement covering all thematerials used in the construction of these cables.

These cables save on both weight and space when used toconnect the complex electronics installed high up in thesuperstructure of modern warships or throughout the hullsof modern submarines.

NES525 was itself converted to a Defence Standard: DefStan 61-12 Pt 25 in 1990.

NES526 replaces DGS212 covering a more standard, eas-ily recognised ship's cable, but still having such advancedfeatures as dual wall, oil resistant, flame retardant insulations

P WaterworthDelta Crompton Cables Ltd, UK

Fortune has not smiled kindly upon the UK Shipbuilding Industry, either the commercial orthe military aspects, and the Offshore Industry, once seen as the salvation, is now itself at afairly low ebb. Driven by buoyant markets and sales, developments in both materials andthe actual construction of electric cables advanced during the 1980s. Many new standardswere prepared and issued, existing standards were updated - NES525 and 526, BS6883, theIEC92-35X and 27X series being typical examples. These are reviewed and their meritsdiscussed. During this review the reaction to fire that each type of cable exhibits must alsobe considered. What, contentiously, needs to be asked is: Has the industry gone too far whenaddressing this particular aspect? Has it sacrificed too many of the virtues, that it perhapstook for granted, ie ease of installation, toughness and performance in fluids, in the searchfor the smallest, lightest, 'best' fire performance cable, or is the industry just selecting thewrong type of cable for the particular application? One area not tackled very scientificallyduring the advances of the 1980s is that of current rating relating to cables. A comparisonbetween onshore and offshore practices is made. Finally, an attempt will be made to answerthe following questions: Whereto now? What new materials and manufacturing processesare becoming available? Does a subdued Marine Industry want them or need them and,finally, can it afford them?

(see Fig 1) in place of standard butyl or ethylene propylenerubber (EPR); these cores are contained within the same typeof LFH sheath and covered by the same overall toxicityrequirement.

The Navy's current circuit integrity, fire survival, cable isstill based upon the well tried and tested silicone rubberextrusion, glass fibre braided core or, in the case of largersizes, multiple layers of a silicone rubber impregnated glassfibre tape. The sheath is the same as in the former twostandards, complying with Def Stan 61-12 Pt 31.

Perhaps surprisingly, the current test for fire survivalspecified by the Ministry of Defence (Navy) (MOD(N)) issimilar to the popular IEC331 but with a flame temperatureof 650°C.

Reviewing the sheathing material in more detail theMOD(N) drew up quite stringent requirements when com-piling NES518, the combination of requirements and bal-ance of properties exceeding those of any other material inservice throughout the world.

The requirements laid down cover virtually all situationsa shipboard cable is ever likely to encounter. The physicalproperties specified match those of the successful heat, oiland flame retardant (HOFR) compounds but have, in addi-tion, excellent resistance to fuel oil, hydraulic and lubricat-ing oils, as well as water, both distilled and salt.

If this in itself was not enough the compound, as the termLFH suggests, exhibits a very useful balance of properties inthe event of a fire; flame retardancy, low evolution of smokewhich is halogen free, as well as fumes considered lessharmful to humans.

Author's BiographyPeter Waterworth is the Technical Manager for the Special Cables Division of Delta Crompton Cables Ltd, based at Derby. He has over25 years experience of the Cable Industry, having been employed by several manufacturers of ships and offshore cabling. He is a convenoror member of IEC, CENELEC, BSI and TEE committees and working groups relating to cables.

Paper read on 01.11.1994 27

P Ware rwoeth

28

Offshore cables

The Oil Industry in its growth period produced a variety ofin-house standards and specifications supplementedby, bu tregrettably not replaced with, United Kingdom OffshoreOperators Association (UKOOA) Guidance Documents cov-ering both instrumentation and control cables, as well aspower and control cables.

Table II BS 6883 thickness of insulation: comparison of 600/1000Vcables to 1969 and 1991 editions

Fig 1 Large three-core naval cable with dual wall insulation system

NB: The values in the 1991 edition now align withthose in IEC92-350.

BS 7655 Type RS 3 (ie HOER) compound.Typically, this material was standard CSP or CPE.

BS 6883: 1991 offers a choice of the following four sheathing materials,which possess different degrees of improvement over RS 3:

Type A

As RS 3

Type B

MRS 3

Type C

As RS 3

Type D

As RS 3

+ enhanced oil resistance

+ enhanced oil resistance+ halogen acid gas emission 5 5%

+ tear resistance 55.0 N/mm

+ halogen acid gas emission 5 0.5%

+ reduced smoke emission+ elongation at break 5 150%

+ enhanced oil resistance+ halogen acid gas emission 5 0.5%+ reduced smoke emission+ elongation at break n 50%

Cable types introduced during the 1980s have tended tofollow the trend set in the particular practice applicable tothe actual installation, fixed platforms broadly followingland based practice, with 'mobiles' and 'floaters' firmly inthe Marine Engineering camp.

Therefore, as has happened onshore, simple PVC/ SWA/PVC BS6346 armoured cables have given way to XLPe/OHLS/SWA/OHLS BS6724 types. The shortcomings of theearly varieties of thermoplastic zero halogen, low smoke(OHLS) compounds have now largely been addressed andproperties currently match those of standard PVCs -except-ing that of oil resistance. It should be noted that this thermo-plastic OHLS should not be confused with elastomeric OHLSor LFH.

Being insulated with cross-linked polyethylene (XLPe)the maximum allowable conductor temperature can be ashigh as 90°C, against that of 70°C for PVC, with the conse-

Table I Summary of important new features of BS 6883: 1991 Table III Insulation and sheathing in accordance with BS 6883:1991 edition

IL 150/250V pairs and triples with individual and/or collectivemetalled tape screening. Insulation

2. 600/1000V 2,3 and 4 core power cables and control cablescontaining up to 48 cores with or without metallic braid.High voltage cables up to 10/15kV.

The EPR insulation required by the 1991 edition of BS 6883 enablesthe cable to be operated at a continuous conductor temperature of 90°C.

4.

5.

EVE insulation now rated at 90°C.

Choice of four sheathing materials, including two low smoke zero

The 5°C increase over the previous issue of this specification isreflected in higher current ratings.

halogen types.

Detailed cable identification on outer surface.Cables made to BS 6883: 1969 were required to be sheathed with a7. Tests for flame propagation on bunched cables (BS 4066: Part 3).

Nominal area of

conductor

Thickness of

insulation

BS6883: 1969 BS 6883: 1991

inm2 min 1)1771

1.0 0.8 0.8

1.5 BS 0.8

2.5 0.8 0.8

4 1.0 1.0

6 1.0 1.0

10 1.2 1.0

16 1.2 1.0

25 1.4 1.2

35 1.4 1.2

50 1.6 1.4

70 1.6 1.4

95 1.8 1.6

120 1.8 1.6

150 2.0 IS

185 22 2.0

240 2.4 2.2

300 2.6 2.4

400 2.8 2.6

500 3.0 2.8

630 3.0 2.8

Sheath

materials

Fig 2 Operating temperature envelope for therrnoplastics

Materials

Fig 3 Operating temperature envelope for elastomers

quential beneficial effect upon the current rating assigned toa particular cable.

This family of cables is lighter and smaller than either PVCor marine types but has much higher minimum bend radii due,in the main, to the single wire armour (SWA), terminationtechniques which are comparable to standard ship types.

Commercial ships' cables

In the commercial Marine Engineering camp, BS6883: 1969has been radically updated, with the publication and issue ofthe 1991 edition.

This much enlarged document now covers 150/250Vpairs and triples, high voltage power cables for use inearthed neutral systems, up to and including 15 kV, as well

Trans 1MarE, Vol 107, Part 1, pp 27-45

BR

PAIn Temp, Reel

0 Max Operating Temp.

0 Min. Temp. Fixed

0 Mex. Operating Temp.

as retaining the 600/1000V power cables of the supersededstandard. Opportunity was also taken to uprate the insula-tion material, EPR, from 85°C continuous conductor tem-perature to 90°C and align radial thicknesses with the Inter-national Electrotechnical Commission (IEC) standards. Fourelastomeric sheathing compounds are now available, in-cluding two OH LS types. Flame propagation, as determinedby IEC 332-3/BS4066: Pt 3, is also addressed. Tables 1,11 andIII give a more detailed review.

Internationally, IEC TC 18 SC 18A is making gradual butslow progress in converting the old IEC92-3 standard into anew multipart 1EC 92-35X and IEC 92-37X series of stand-ards.

These standards now derive the radial thickness of insu-lation used from IEC502. These are less than those tradition-ally found in the marine environment and certain countries

29

PR CSP/CPe SILICONE XLPe PCP EVA EMA

P Waterloo*

Table IV Status of tests on cables for reaction to fire

30

IEC publication Further work in IEC CENELEC TC20 position British Standard position

Tests for flame propagation

IEC 332- Tests on electriccables under fire conditions

Part 1(1993): - HD 405.1 to be BS 4066: Part 1Test on a single vertical rewritten as an EN implements HD 405.1insulated wire or cable

Part 2(1989): HD 405.2 endorses BS 4066: Part 2Test on a single small IEC 332-2 implements HD405.2vertical insulatedcopper wire or cable

Part 3 (1992): 20(SEC)20 HD 405.3 endorses BS 4066: Part 3Tests on bunched (amendments plus conversion IEC 332-3 implements HD 405.3wires or cables to International Standards)

Tests for fire resistance

IEC 331 (1970): Proposed revision Draft for requirements New edition ofFire resisting 20(SEC)23 relating to construction 856387 publishedcharacteristics Products Directiveof electric cables (CLC/TC20(SEC)972)

approved for UAP

Tests for evolution of acidic and corrosive gases

IEC 754 - Test on gasesevolved duringcombustion ofmaterials from cables

Part 1(1994): Revision published Agreed to issue as an BS 6425: Part 1 is

Determination of the EN based on the IECamount of halogen acid gas document

Part 2 (1991): Minor amendments HD 602 endorses BS 6425: Part 2

Determination of degree proposed in IEC 754-2 implements HD602of acidity of gases evolved'

during the combustion ofmaterials taken from electriccables by measuring pH andconductivity

Tests for smoke opacity

IEC 1034:

Measurement of smoke densityof electric cables burningunder defined conditions

Part 1(1990): Revision proposed - HD 606.1 endorses BS 7622: Part 1

Test apparatus 20C(SEC)21 IEC 1034-1 implements HD 606.1

Part 2 (1991): Revision proposed Published as HD606.2 BS 7622: Part 2

Test procedure and 20C(SEC)22 with modification to implements HD 606.2

requirements include cable sizes downto 2 mm overall diameter

AM D1-1993

5. ''Tests for toxicity

None approved IEC SC20C requested by

ACOS to examine subject in

collaboration with IC TC89

IEC TC89 WG7 has

working drafts

Awaits 1EC. Some existing

(national) specificationscalled up for PPD documents

WC;12 (Railways) looking at

UITP method in interim

Will reflect IEC/CENELEC work

-NB: Publication 1liC695-7: Guidance on the minimisation of toxic hazards due to fires involving electrotechnical products - Section 1: General, is recommendedreading for those having an interest in this topic.

2.

3.

4.

have commented unfavourably upon this. As would be ex-pected perhaps, being truly international, the scope of thesestandards goes beyond that of BS6883 even in its latest form.

Materials allowed for insulation include PVC, EPR, XLPe,silicone rubber and, at present in draft form, an irradiatedpolyolefin insulation system favoured by France.

For sheaths two PVCs, polychloroprene (PCP),chlorosulphonated polyethylene/chlorinated polyethylene(CSP/CPe) and, again in draft form, two OHLS materials, athermoplastic favoured by France and an elastomeric pro-moted by the UK. Appendix A lists the standards issued todate and the new work being considered. Figures 2 and 3show the operating temperature envelopes for thermoplas-tics and elastomers, respectively.

Various thin wall cables have been offered and havefound use in commercial shipping and in the offshore indus-try, however high their costs and, unless special techniquesare adopted, problems with terminations at present out-weigh any size or weight savings, and has limited wide-spread adoption of these types.

REACTION TO FIRE

It would, perhaps, be a good idea to take this opportunity toreview the arena of 'Reaction to fire' and try to arrange thecables previously discussed into an order of merit.

In the UK, Europe and to a lesser extent the rest of the world,cable makers and users have, through the vehicle of BSI CIL/20,CENELECTC 20 (European Committee forElectrotechnicalStandardization Technical Committee 20) and IEC TC 20,split a cable's reaction to fire into five distinct categories:

Flame propagation. The property that determines the rateat which fires develop and spread.

Fire resistance. The maintenance of electrical circuit in-tegrity under fire conditions.

Smoke opacity. The property that determines the timepersonnel have to evacuate a ship or installation and, mostimportantly, mount an effective fire-fighting operation.

Acidic and corrosive gas evolution. The property that de-termines the propensity to corrosion of structure andequipment during or after a fire.

Toxicity. In a joint paper' presented some two years ago,it was stated that toxicity is: 'a very emotive subject,whereby any discussion of toxic emissions usuallygenerates many conflicting views'. Nothing hashappened in the intervening period to change thisview.

Table IV gives a list of some common standards relatingto reaction to fire properties.

The principle presently adopted by Technical Commit-tees' (TC) dealing with cables is one whereby the maincommittee (TC20) lays down the type of test, the equipmentto be used and general methodology to be followed. Theactual product working party then lays down the pass/failcriteria, based upon their more expert knowledge of theapplication and duty.

Flame propagation

The 1980s saw a move from flame retardant to reducedpropagation, the Limiting Oxygen Index (LOT) being aug-mented with the Flammability Temperature Index (FTI).Both are useful laboratory scale indicators but on their own,as a means of assessing a cable's fire performance, their valueis limited.

The simple, single sample, flame propagation test re-mains IEC332-1BS4066-1, although in the 1993 edition theburner is more closely defined and its output is now anominal 1 kW flame.

The LOT test is included in the updated and revisedIEC332-3: 1992 BS4066 Pt 3: 1994 but only as a relativelysimple routine check on the components of a cable, to re-move the need to repeat the large full-scale type test.

Reviewing the changes to this document it is perhapswise to point out and highlight certain sections:

within the National Foreword:

'It should be noted that IEC332-3: 1992 is a TechnicalReport of Type 2. It is not to be regarded as an internationalstandard. '(Author's italics.)

within the Scope:

'Three categories are defined and distinguished by testduration, and the volume of non-metallic material ofthe sample under test; they are not necessarily related todifferent safety levels in actual cable installations.' (Au-thor's italics.)

The Introduction should also be read in its entirety as itoffers guidance on features to be considered, which have abearing upon propagation of flame along a bunch of cables.

The main changes to this document in its latest formconcern the manner in which the cable samples are mountedon the ladder.

Category A, the 7 1/m of non-metallic material nowhaving two designations for the method of mounting, nowbecomes:

332-3A F/R 300 mm wide ladder332-3A F 600 mm wide ladder332-A3B F 300 mm wide ladder332-3C F 300 mm wide ladder

with the F/R signifying cables are mounted on both the frontand rear of the ladder, and F signifying front mountedonly. As can be seen, Categories B and C remain substan-tially unchanged. Table V gives a diagrammatic explana-tion.

The document rather usefully now includes Table I: 'Sum-mary of test conditions' and Table II: 'Summary of guidancedata for the selection of cables for type approval tests'. Bothare reproduced as Appendices B and C.

What has not changed markedly is the mistaken beliefthat Category A is in some way the most severe test. Forcertain cables this would be the case: typically large cablesmounted and spaced, front and rear, thus providing theclassic 'chimney effect' for the 70 000 BTU flame. For othercables, however, mounted more akin to ships and offshore

Trans 1MarE, Vol 107, Part 1, pp 27-45

31

-

3.,

4,

5.

-

P Waterworlh

-Table V Revised IEC 332-3 installation configurations,

practice, that is to say in one large bundle on the front of theladder, it is not.

This leaves the specifier or installer with the age olddilemma does the test one intended to simulate and repro-duce accurately, provide actual installation conditions andtherefore gives an exact measure of how a cable will performin service, or is it merely a test to confirm the cable has beenmanufactured to the correct specification? Certainly theextracts mentioned previously would point away from the'former.

Practically speaking, what is clear is that extensive testingby both manufacturers and approval authorities2has shownthat where cable is installed in large groups or bundles withlittle if any spacing, as is usually the case in the majority ofmarine installations, then propagation is greatly reduced.

Also, elastomeric insulated and sheathed cables tend toperform better than XLPe insulated, thermoplastic zerohalogen sheath types and that the inclusion of a metallic.layer,, be it SWA or braid, also helps.

Fire resistance

asfar as ships are concerned not much has happened duringthe past 10 years, the test for electrical circuit integrityremaining IEC331, which is a single temperature (750°CXfixed time test not involving impact or water spray.

Driven by the European Construction Products Directive(CPD) work is in hand internationally to correct what is, in'essence, a fairly simple, almost crude test. Opportunity is

N32

being taken to allow the flame application time and tempera-ture to vary by the adoption of a standard burner fed withcarefully controlled gas and air flow rates. The burner is alsorepositioned to avoid blockage by falling debris.

Cables meeting Naval Standards based upon silicone/glass insulations would meet a standard 750°C, IEC331 testbut, partly because of a lack of metallic braid armour wouldnot always pass the enhanced 1000°C test.

In the offshore arena the spectre of a hydrocarbon firecaused the simple IEC331 test method to be upgraded toincrease the flame temperature from 750 to 1000°C (actually950°C ± 50°C). Also, as the distance between new oil fieldsand land increased, so too did the required levels of safety.

BP with their Magnus Field were one of the first compa-nies to address fire, impact and water, which resulted in thedevelopment and issue of BP Engineering Standards 235and 236 which remain current to this day.

These standards should not be confused with BS6387 inthat one sample of cable is subject to a 1000°C flame for 3h,whilst subject to mechanical impact directly onto the cable,followed by a water spray of 151/min for 5 min. Needless tosay cables meeting these extremely onerous requirementsare of a costly special design, incorporating metallic tapebarriers or thermal barrier tapes under the braid armour.

BS6387 was developed and issued for onshore use in firealarm, detection and emergency lighting circuits. Updatedand re-issued in 1994 this standard allows a matrix of flametemperatures, ie 650°C, 750°C and 950°C as a stand alone firetest, with symbols A, B,C and S, the latter symbol signifyinga short 20 min test. A test is included of a 650°C fire with

_

Category < 35mm2 > 35mm2,

1

1

AJF

_t _00 Cl UGULI 0taillmr, 1

I

:A/F/R=reammetei

111.0.111.111.111,11111116MISOft),lifeellifilo 0

realt_

1 I

Iree66565etall

UU00f6M6656644651

C . LCCOCOrriri1

{ I0 0 ,

,

Fig 4 Small two-core circuit integrity cable with mica/glass taped conductors

water spray, symbol W, and finally fire, with impact at thethree temperatures mentioned previously with symbols X,Y, Z. The most severe requirement is categories C, W, Z.

It has to be stated, however, that these three tests areconducted on three separate samples of cable. Also, a furtherlimitation of the standard is that it is only applicable to cablesup to 450/750V and of small diameter, typically up to 21 mmoverall diameter. Where cables are to be subject to impactthey are mounted on a fire resisting board, and it is this boardthat is subject to the impact upon its upper edge, no directcontact being made with the cable.

Where cables are to be subject to water spray they areclipped to the front edge of two parallel metal strips andwater is applied via a commercial fire sprinkler.

This standard has now found its way into both ships andthe offshore field but, as stated, its value to ships andoffshore cabling is limited due to its size limitation andcareful thought is required when calling it up.

To obtain circuit integrity in the offshore market, earlydesigns relied upon the silicone/glass insulation system. Itsshortcomings were very quickly highlighted and alternativesystems sought. From this early work emerged the almostuniversally accepted mica/glass taped fire barrier applieddirectly onto the conductor over which is extruded an EPRor XLPe insulation (see Fig 4).

At various times people have made claims that a cheapercable has been produced using PVC as the insulating layer overthe mica/glass tape. In the author's opinion this is highlyimprobable due to the fact that halogen ions are conductive inthe extreme, and, enclosed within the confines of a cablesheath, dramatically lower the insulation resistance by form-ing a conductive path through the mica/glass tape.

Perfectly acceptable for use in 600/1000V and 1900/3300V cables, and even 4150V systems, problems start toappear when this construction is applied to 3600/6000Vcables and above. The inclusion of the mica/glass tapeprevents the application of the normal semiconducting con-ductor screen and therefore unwelcome discharges can maketheir presence felt over a prolonged period of use.

Fortuitously the effect of these discharges has little effect inthe short term and so this design of cable can be considered for'short life' applications, typically emergency fire pumps andthe like. For cables which are energised continually therewould appear to be problems in addition to those mentionedpreviously, but these can be overcome. In the main, these highvoltage cables tend to be quite large by their very nature, andthe consequential high thermal inertia can be used to goodeffect; thermal barrier layers can be incorporated into the outerlayers of these cables and a 3h rating achieved withoutcorn pro-mising the electrical integrity of the inner insulation.

Fire testing of these cables at high voltage can also beproblematic due to safety and equipment considerations.

Smoke opacity

In the early days of this inexact science levels of smoke weremeasured by testing small samples of individual cable com-ponents, such tests being the Arapahoe Smoke Chamber testand the NBS Smoke Chamber test.

Essentially both of these tests subject a small plaque ofmaterial (100 mm x 100 mm) to a source of heat, eitherflaming or radiant. In the case of the Arapahoe test the smokeevolved is passed through a simple filter and the amount ofdebris collected, thereon weighed and expressed as a per-centage of the initial plaque. In the case of the more popularand widely accepted N BS Smoke Chamber test the level ofsmoke evolved is measured optically.

Although still accepted and used by both the MOD(N)and the USA to predict and classify the smoke evolutionperformance of complete cables, within the UK and Europetheir use, although valuable, has been relegated to that of alaboratory development tool.

The European Cable Industry is now quite firmly com-mitted to the use of the '3m cube' to test samples of thecomplete cable as would be supplied to the user. IEC 1034 Pt1 and Pt 2 are now reflected in both a CENELEC Harmoni-sation Document (HD) and a British Standard, BS7622 Pt 1and Pt 2.

In simplistic terms when a cable burns it is the actual basepolymer and processing aids (oils and waxes) that contrib-ute to the evolution of smoke. The materials used as fillers,typically chalks or whiting, do not contribute appreciably tothe levels of smoke generated. To make a low smoke'compound therefore, one has merely to remove polymerand process aids, to be replaced with a higher loading ofinert filler.

Unfortunately, as history has repeatedly proven, life isnever so simple. Whilst following this route would producea 'low smoke' cable other properties would suffer. Thecompound produced by this action would have much re-duced physical properties, tensile strength, elongation atbreak and, importantly in the marine environment, tearstrength.

Also, performance in fluids would suffer, with fluiduptake increased due to the hygroscopic nature of simpleuntreated fillers.

Because of this a more complex approach is taken, involv-ing the selection of polymers that produce light, whitesmoke rather than the dense black smoke exhibited by suchmaterials as natural rubber or the chlorinated polymers.Polymers that will accept higher filler loadings with mini-mal effect upon ultimate physical properties are also veryuseful. Active fillers such as aluminium trihydrate are cho-sen, as these also contribute to reducing flame propagationand can aid reduction of acidic and corrosive gas evolution

Trans IMarE, Vol 107, Part Ic pp 27-45

33

P Waterworth

by way of dilution. Filler particle size, as well as activesurface area, all play a part, as do various coupling agents.

Finally, the size of the smoke particles produced also hasan important part to play in preventing obscuration, a fewlarge agglomerated particles being preferred to a myriad ofsmall particles, the analogy being heavy rain and fog.

What is clear is that the traditional cable sheathing mate-rials, such as polychloroprene (PCP/OFR),chlorosulphonated polyethylene or chlorinated polyethylene(CSP-CPe/HOFR), in unmodified form do produce denseblack smoke when involved in a fire scenario, although theyhave good physical properties. This smoke evolution can bereduced by the careful selection of compounding ingredi-ents, but ultimate physical properties do suffer. These mate-rials can also never be compounded in a practical form tomatch the low levels of smoke emission exhibited by theethylene vinyl acetates (EVAs), ethylene methacrylates(EM_As) and similar polymers.

Various 'low smoke' materials are now commonly avail-able in both the military and commercial market places.

Acidic and corrosive gas evolution

This property is closely linked to flame propagation, smokeopacity and toxicity.

Early test protocols (IEC 754 Pt 1 BS6425 Pt 1) pyroliseda small sample of material in a known and controlled airflow; the gases evolved were bubbled through a sodiumhydroxide solution and then back titrated to express theresult in terms of hydrochloric acid (HCI). A limit of 0.5%maximum acid gas is usually applied.

More modem thinking has produced IEC754 Pt 2 BS6425Pt 2, which follows a similar test protocol, but in this case thegases are bubbled through distilled water from which thepH and conductivity are determined. Recommended valuesare a pH not less than 4.3 and a maximum conductivity of10 microsiemens/ mm.

All of the halogenated polymer systems fail both parts ofthis test, ie PCP, CSP, CPe as well as PVC. Insulations basedupon EPR and XLPe pass, when properly compounded, asdo sheaths based upon EVA or EMA, as well as hydrogen-ated nitrile butadiene rubber (HNBR) or nitrile butadienerubber (NBR), again when properly compounded. The rea-son for stressing this latter statement is that although thebase polymer itself is halogen free and meets test protocols,certain synergistic flame retardant packages available to thecable maker contain halogens, typically bromine, and thesecan be difficult to detect by Pt 1 of the above standards. Aswith smoke opacity, materials are now commonly availableto meet this requirement.

Toxicity

There are presently more test methods in existence to meas-ure 'toxicity' than any other reaction to the fire parameter,yet none are recognised or approved internationally.

All suffer from two basic failings in that the method usedto pyrolise the sample does not reflect a true fire scenario andthat once collected, not in itself an easy task, the gases that are

34

detected are subject to a highly subjective weighing proce-d u re.

Various tests, such as NES713, can be used for laboratoryscreening procedures, but it is most unwise to rely on themto give a true and accurate indication of what happens in areal fire situation.

In practice, if common sense is applied, then cables andmaterials which liberate copious amounts of dense smokeand acidic gases should be avoided, as there is reasonablecorrelation between the levels of acidic gases generated andImmediately Dangerous to Life and Health (IDLE) values.

CURRENT RATINGS

If the IEE Ships Regulations,' the Blue Book, and LEE Off-shore Recommendations,' the Green Book, have a weaknessit is that the current ratings given in Section 12 have a basisthat is not accurately known. Derived from the ratingscontained within IEC92 standards their origins go back sofar as to make tracing them difficult, if not impossible.

They are known to contain 'safety factors' in that applica-tion of the tabulated rating will not cause the temperaturerise stated but a slightly lower value. These safety factorswere included to reflect the nature of the application, eg aship far from land and assistance. But, conversely, the in-creasing level of electrification onboard ships and offshoreinstallations results in bunches of cables far larger than anypresent, or considered, when the ratings were first pub-lished. For this reason the simple 0.85 bunch correctionfactor must be viewed with caution.

Finally, the present tables fail to differentiate between thetypes of circuit protection used: coarse-fuses or close-circuitbreakers.

Naval vessels have their own ratings based upon NESStandards and associated supplementary reports. Onshoreratings have benefited from major reviews in, first, theFifteenth Edition of the I EE Wiring Regulations and, latterly,the Sixteenth Edition, now 5S7671: 1992. These are supple-mented by a wide variety of papers and reports published byERA Technology Ltd of Leatherhead.

Table VI gives the size of cable required for variouscalculated currents, based upon IEE ships and offshoreratings compared with 1EE Wiring Regulations; as can beseen these vary with no apparent pattern.

Whichever basis is chosen to calculate current ratingswhat cannot be avoided are the basic laws of physics.

When a voltagepotential difference is applied to a cablethen a current will flow. Copper conductors are not perfectconductors and have a finite resistance, therefore a tempera-ture rise and a power loss occur when current flows. Thegreater the current flow for a given conductor the greater thetemperature rise (power loss and also voltage drop).

Modern materials will run quite happily at high tempera-tures and so they can be used with advantage to allow highercurrent ratings per given conductor size. However, there isa practical limit.

The heat generated can only escape from the cable eitherby conduction through the terminals or mainly by radiationfrom the outside surface of the cable. The optimum rating for

Table VI Comparison of conductor sizes for BS68 83 multi-core braided cables for land and offshore applications

Conditions

Ambient temperature 45°C; Conductor operating temperature 90°C.Cables installed on perforated metal tray.Multi-circuits based on cables touching.600/1000V, 3 phase operation.

Protection

'Close' defined as an enclosed fuse to BS88 or BS1361, or a mcb to BS3871.

'Coarse' defined as a semi-enclosed fuse to BS3036.

Land ratings based on 16th edition of TEE Regulations.For electrical installations (B5767:1992).Ships ratings based on 6th edition of the WE Regulations for Electrical and Electronic Equipment of Ships.

a cable is one where the amount of energy capable of beingradiated away equals the energy generated, ie power loss, inthe conductor.

This needs to be carefully considered when current rat-ings are assigned to thin wall cables or standard wall cablesare installed in large bunches. In the former, reduced surfacearea can negate increase in temperature due to new materi-als; in the latter, surrounding cables act as thermal insula-tion. Overheating, premature ageing and ultimate failurecould result. The actual power loss at higher conductortemperatures also needs to be considered against installedgenerated capacity.

CABLE MAKING PROCESSES

The late 1970s and 1980s saw a virtual revolution within thecable making industry. Up until this period only thermosets(rubber, etc) and thermoplastics (PVCs, etc) were available,both confined within clearly defined boundaries of perform-ance. The arrival of irradiation technology, silane technol-ogy, engineering polymers and thermoplastic elastomershas breached these once solid walls.

In the elastomeric field the old taped finish batch process-ing methods have given way to continuous productiontechniques. Liquid Curing Medium (LCM) and PressurisedLiquid Continuous Vulcanisation (PLCV) machines give toelastomers a smooth finish approaching that seen on PVCcables. The substitution of a eutectic salt as the curingmedium, in place of steam, removes the risk of core deforma-tion and braid push back due to applied process pressure.Cables produced by the LCM method, although meetingcurrent standards, do not quite equal the performance influids of the PLCV method due to the need to incorporatedesiccants into the compound to prevent porosity during

Trans 1MarE, Vol 107, Part 1, pp 27-45

processing. Superheated steam vulcanisation, the latest proc-ess development, now matches that of a PLCV in producingsmooth, tape mark free, braided cables, the smooth finishaiding installation and glanding.

Higher vulcanisation temperatures have allowed the sub-stitution of sulphur cure systems with those of peroxide, ahalogen free system.

Elastomers can also be cross-linked by means of an elec-tron beam - irradiation technology - and, although special-ised and relatively costly, it opens the door to compositethermoplastic /elastomeric cables due to the absence of heatinherent in conventional steam or salt vulcanisation proc-esses.

An electron beam also allows the cross-linking of materi-als that would be impossible to cross-link chemically byheat, and this has greatly assisted the introduction of the thinwall insulation systems seen.

Silane technology has made available cost effective XLPeinsulated cables, as well as EPR types, although it must bestressed these are not the true flexible elastomeric types ofEPR given in BS6883 and the like.

Improvements in raw materials, mixing and extrusion ofcompounds has allowed the reduction in radial thicknessesseen, without compromising electrical performance of ca-bles.

Engineering polymers are gaining in popularity and areused extensively in the thin wall insulation system market.Although having a very high raw material cost, the smallamount used per metre run of cable, allied to weight andvolume savings, makes for a more acceptable total installedcost.

Thermoplastic elastomers have yet to penetrate the shipand offshore markets but are starting to find applicationsonshore in, at this stage, low technology products. As thename suggests, these materials have certain characteristicsin common with conventional elastomers, yet they can be

35

No of circuits

on line Protection 15A 50A 100A 300A

Land Sea Land Sea Land Sea Land Sea

Close, 1.02 1.5' 102 102 252 352 1502 1852

1 Coarse 1.5' 1.52 102 BY 352 352 1852 185'6 Close 1.5= 1.52 162 102 352 352 2402 1852

6 Coarse 2.52 1.52 162 102 502 352 3002 1852

10 Close 2.5' 2.52 16' 162 502 502 2402 240210 Coarse 2.5= 2.52 252 162 702 502 3002 2402

P Waterworth

Table VII Elastomeric materials selection guide

36

PVC

PolyethyleneEVA

PolyamidePEEK

Pe/PVDF"

PVC

PolyethyleneEVA

PolyamidePEEK

Pe/PVDF4

Maximumoperating operating

temperature temperature

"C installation

Maximumoperating

temperature

"C

Minimum Minimum Insulation Sheath Tensile

operating strength

temperature

as fixed

installation

processed on conventional PVC plant, ie no vulcanisationprocess. Whilst the advent of these materials could providesome opportunities in the future, it is important not to forgetthat they remain essentially thermoplastic in nature and

Table VIII Thermoplastic materials selection guide

Minimum Minimum Installation Sheath Tensileoperating operating strength

temperature temperature

installation/ as fixed

handling installation

70-85 +5 -15 Y V 10-20

60 -40 -40 Y N 10-20

70 -0 -20 YIN Y 8-15

200 -100 -200 Y YIN 165

200 -40 -70 Y YIN 70

150 -40 -65 Y N 23

Limited Flammability Smoke Flammability Halogens

oxygen temperature emission

index index

25-40 200-250 n E P

21 50 E P E

20-40 250-300 E VG E

60 360 E VG E

43 325 E E E

27 400 G VG F

Abrasionresistance

Oil Flexibility

resistance

thus retain some of the inherent problems associated withthe use of thermoplastic materials in these applications.Their possible use will be investigated as their performanceenvelope expands.

EPR 90 -35 -40 Y N 8

CSP/CPe 85 -20 -30 YIN Y 10-20 VG VG

Silicone 105-150 -35 -40 Y N 5-15

XLPe 90 -35 -40 Y N 10-20 VG

PCP 70 -25 -35 N Y 10-15 VG VG

EVA 85-125 -40 -40 Y/N Y 5-15 VG

EMA 85 -40 -40 NI, Y 5-15

HNBR 100-125 -30 -35 N Y 8-12

Limited Flammability Smoke Flammability Halogens

oxygen temperature emission

index index

Key: Y = Yes suitable E = Excellent VG = Very goodN = No not suitable G = Good F = Fair P = Poor

Key: Y = Yes suitable E = Excellent VG = Very good

N = No not suitable G = Good F = Fair P = Poor

Thin wall engineering polymerIrradiation cross-linked thin wall

EPR 21 55 F-P

CSP/CPe 30-40 250-350

Silicone 20-25 100-150 VG

XLPe 21 50 VG

PCP 25-35 200-250

EVA 20-40 250-350

EMA 20-40 250-300 VG

HNBR 30-40 250-300 VG

Abrasion Oil Flexibilityresistance resistance

VGVG

VG VG

VG

VG VG

P

F

E E

IE

P

CONCLUSION THE FUTURE

The number of materials and processing techniques avail-able to the cable maker has never been greater. As a conse-quence the number and variety of cables available to thespecifier and installer are quite large.

Far from being a panacea this variety is creating its ownproblems by way of confusion, giving rise to the very realpossibility of the wrong cable being specified and installed.

Although physical properties have been compromisedslightly in achieving improved performance in the event ofa fire, the current overall balance of properties will ensurethat tried and tested 'traditional' elastomeric cables are usedin power and control circuits onboard ships and offshoreinstallations for many years to come. These cables are still'user friendly' and forgiving of abuse.

Encroaching into this domain, especially offshore, are thetypes of cable used onshore, typically XLPe insulated witheither PVC or OHLS sheaths. Although on paper a costeffective alternative to traditional elastomeric types, care isneeded in deciding where and how they are to be installedand used.

The operating temperature limits, during both normaland overload conditions, the minimum bending radii andperformance near liquids need to be considered when speci-fying this type of cable.

In instrumentation and low current circuits the movetowards the new generation of small, lightweight thin wallcables will continue. With this type of cable potential dam-age during installation, termination techniques and ultimateperformance in the event of a fire, due to their small size, areareas that require consideration.

If cost were not a consideration then the range of materi-als and cable designs developed by, and for, the Royal Navywould reign supreme. With the ever present drive to reducecosts to a minimum these cables will never be as widely usedin the commercial field as traditional EPR-CSP types. How-ever, they should not be discounted out of hand for, inproblematic or demanding applications, they could providea cost effective solution. Hopefully, increasing use will drivedown the cost of what is currently the speciality polymerupon which they are based.

With regard to performance in the event of a fire, al-though the Royal Navy are looking towards developing andimproving their current cables into the ultimate, universalcable, cost would preclude its widespread adoption in thecommercial marine industry.

Commercially, where circuit integrity is required, themost widely used types of cable will rely upon the mica/glass taped conductor with a variety of further finishes.Mineral Insulated Copper Covered (MICC) cables will stillcontinue to find limited use due, in the main, to terminationprocedures.

If, at this stage, the move towards higher flame tempera-tures of 1300°C and above continues, the use of MICC will beput in doubt, but mica/glass can still be considered.

Evolution of smoke and corrosive gases will continue tohave the highest priority within accommodation areas, pub-lic spaces and control rooms and, as materials are developedfurther, the current low levels of light obscuration willreduce even further. Where a reduction in physical proper-

ties and fluid resistance can be tolerated, these very lowsmoke emitting materials are available now. Tables VII andVIII give a generalised elastomeric and thermoplastic mate-rials selection guide.

The area of greatest debate concerns that of flame propa-gation. Until the limitations of TEC332 Pt 3BS4066 Pt 3 areformally recognised and international regulations and rec-ommendations reflect this, confusion and acrimonious ex-changes between manufacturer, installer, owner and certify-ing authority will continue.

Hopefully the work of [EC SC18A WG1 when started willlead to a speedy recognition and resolution of this long-standing problem.

Gaining more and more importance as a subject is that oftoxicity; this topic more than any other is one where the moreone knows the more one realises one does not know. Thesheer scope and depth of this subject leads to the many wellintended but conflicting test methods and interpretations.

The work of IEC TC89 will not resolve all the problemsbut should give an internationally recognised and acceptedbasis for a test and presentation of results. For anyone withan interest in this subject then IEC 695 Part 7 Section 1produced by TC89 makes invaluable reading, in reviewingand comparing the more commonly existing test methodol-ogy and presentation of results.

Finally, although the present fortunes of the UK MaritimeIndustry are clouded, its engineers, engineering practices,rules and regulations are recognised and respected through-out the world, although of course not always followed.

The future format of the IEE Ships Regulations and Off-shore Recommendations will remain as now, with no plansto convert them to British Standards, as was the case with theonshore regulations.

Continuously updated to match current practices thesetwo publications are a valuable source of information andguidance, their prominent position on any Marine Engi-neer's desk being highly recommended.

REFERENCES

T L Joumeaux and P Waterworth, 'Update on cable standards',ERA Conference (1992).F D Sydney-McCrudden, 'Fire performance of electric cables',Trans IMarE, Vol 101 (1989).The Institution of Electrical Engineers Regulations for the Elec-trical and Electronic Equipment of Ships with RecommendedPractice for their Implementation, Sixth Edition (1990) plusSupplement (1994).The Institution of Electrical Engineers Recommendations for theElectrical and Electronic Equipment of Mobile and Fixed Off-shore Installations, Second Edition (1992).

BIBLIOGRAPHY

NES518: Requirements for Limited Fire Hazard Sheathing forElectric Cables.NES525: Requirements for Electric Cables, Thin-wall Insulated,Limited Fire Hazard.NES526: Requirements for Cables Electric, Rubber Insulated,Limited Fire Hazard Sheathed for General Services.

Trans 1MarE, Vol 107, Part], pp 27-45

37

1..

2

3

2.

P Waterworth

NES527: Requirements for Cables Electric, Fire Survival, HighTemperature Zones and Limited Fire Hazard Sheathed.NES711: Determination of Smoke Index of Products of Combus-tion from small specimens of material.NES713: Determination of the Toxicity Index of the Products ofCombustion from small specimens of materials.Defence Standard 61-12 (Part 25). Cables, Electrical Limited FireHazard, up to 2.5 mm Cross-Sectional Area.Defence Standard 61-12 (Part 31). Sheaths Limited Fire Hazard.BS6346: 1989 Specification for PVC-insulated cables for electric-ity supply.BS6387: 1994 Specification for performance requirements forcables required to maintain circuit integrity under fire condi-tions.BS6724: 1990 Specification for armoured cables for electricitysupply having thermosetting insulation with low emission ofsmoke and corrosive gases when affected by fire.BS6883: 1991 Specification for elastomer insulated cables forfixed wiring in ships and on mobile and fixed offshore units.BS7655: 1991 Specification for insulating and sheathing mated-als for cables (replaces BS6746 and BS6899).BS7671: 1992 Requirements for electrical installations. IEE Wir-ing Regulations. Sixteenth Edition.IEC695 Part 7: Guidance on the minimisation of toxic hazardsdue to fires involving electrotechnical products - Section 1:General Guidance.BP Engineering Standard 235: Fire resistant power cables.BP Engineering Standard 236: Fire resistant instrument cables.

APPENDIX A

IEC PUBLICATIONS PREPARED BYSUB-COMMITTEE NO 18A

92: Electrical installations in ships.92-3 (1965) Part 3: Cables (construction, testing and in-

stallations).Amendment No 1(1969)Amendment No 2 (1971)Amendment No 3 (1973)Amendment No 4 (1974)Amendment No 5 (1979)Amendment No 6 (1984).

An ongoing, albeit slow, programme of work was, untilrecently, attempting to update this document and convert itinto a new multipart series of specifications as given below.

A challenge to this programme has been lodged and theissue is now under consideration by IEC Central Office andIEC TC 18.92-350 (1988) Part 350: Low-voltage shipboard power ca-

bles.General construction and test requirements.

38

A draft document intended to extend the voltage rangeup to and including 10/15 kV has been agreed but not yetpublished.92-351 (1983) Part 351: Insulating materials for shipboard

power cables.Amendment 1 (1992).

Recently the concept of low smoke zero halogen insula-tion systems was actively discussed and this standard willbe revised to reflect this. Also to be included will be anirradiated polyolefin insulation system.92-352 (1979) Part 352: Choice and installation of cables for

low-voltage power systems.Amendment No 1 (1987).

A new Working Group has been formed, but has not yetmet, to revise this specification. A major part of this workwill concern reaction to fire.92-353 (1994) Part 353: Single and multi-core cables with

extruded solid insulation for rated voltages0.6/1 kV.

This now follows the general move internationally toinclude 3 kV cables with 600/1000V. Amendments havebeen agreed but not yet published.92-354 (1994) Part 354: Single -and three-core power cables

with extruded solid insulation for ratedvoltages 6 kV, 10 kV and 15 kV.

92-359 (1987) Part 359: Sheathing materials for shipboardpower and telecommunication cables.

Low smoke zero halogen sheathing materials are now tobe included, France favouring a thermoplastic type, the UKa more conventional elastomeric material.

Amendment agreed but not yet published.92-373 (1977) Part 373: Shipboard telecommunication ca-

bles and radio-frequency cables. Shipboardflexible coaxial cables.

92-374 (1977) Part 374: Shipboard telecommunication ca-bles and radio-frequency cables. Telephonecables for non-essential communication serv-ices.

92-375 (1977) Part 375: Shipboard telecommunication ca-bles and radio-frequency cables. General in-strumentation, control and communicationcables.

92-376 (1983) Part 376: Shipboard multi-core cables for con-trol circuits.

IEC92-3XX The UK has submitted a proposal anddraft specification covering 150/250Vscreened cables for control and instrumenta-tion circuits - pairs and triples based uponBS6883: 1991. SC18A to circulate an Commit-tee draft.

'At least one conductor greater than 35 mm2tNo conductor cross section exceeding 35 mm'

APPENDIX C

Table II Summary of guidance data for the selection of cables for type approval tests

*Examples for category A, designation F:

Example 1: Single core cable, 1 x 70 mm' conductor cross section; outside diameter 17 rim; 0.2 litres per metre of non-metallic material.Maximum width available for test sample is 600 mm. To achieve 7 litres per metre would require 35 test pieces giving a total widthof: 35 x 17 mm 34 x 8.5 mm = 884 mm.This cable cannot comply with the limitations on choice. Type approval testing arrangements would therefore be made by agreementbetween the manufacturer and customer or test authority.

Example 2: Three-core cable: 3 x 50 min' conductor cross section; outside diameter 29 mm; 0.55 litres per metre of non-metallic material.Maximum width available for test sample is 600 mm. To achieve 7 litres per metre would require 12.7 test pieces. 13 test pieces givea total width of: 13 x 29 mm + 12 x 14.5 mm = 551 mm.This cable complies with the limitations on choice.

Trans 1MarE, Vol 107, Part 1, pp 27-45

APPENDIX B

Table II Summary of test conditions

39

Category and designation A FIR AF BF CF

Range of conductor cross sections (mm 2) >35" 35t >35* 35t >35" 35t >35"

Non-metallic volume per metre

of test sample (I)

7 7 7 7 3.5 3.5 1.5 1.5

Number of layers:

For the standard ladder:Maximum width of test sample: 300 mm 2 1 1 1 1 1

(frontand rear

of ladder)

For the wide ladder:Maximum width °fittest sample: 600 mm

Positioning of test pieces Spaced Touching Spaced Touching Spaced Touching Spaced

Flame application time (min) 40 40 40 40 20

Number of burners 1 1 1 2 1 1 1

A FIR AF BE CF

Maximum Maximum Maximum Maximumtwo layers one layer one layer one layer

(front and rear) 600 mm wide 300 mm wide 300 mm wide300 mm wide

including specified gapsincluding specified gaps" including specified gaps including specified gaps

At leasttwo test pieces

At least

two test pieces

Size of cable cross section Cables with conductors Cables having conductors with at least one cross section > 35 mm

having cross sections

35 mm 2and

telecommunication cables

Category and designation AF BF CF

Limitation on cable choice At least

to provide the required two test pieces

nominal volume ofnon-metallic material

1

P Waterworth

DiscussionJ S Williams (BP Shipping) Electric cabling for the marineindustry is probably one of those topics that all too oftencomes within the 'fit and forget' category and yet, as thispaper has amply illustrated, it is an area in which there hasbeen considerable change within the last decade or so. I wasparticularly interested to see the data presented in Tables WIand VIII of the paper. A few years ago a paper was presentedat this Institute on behalf of the late Frank McCrudden,which reported on a considerable amount of research under-taken by Lloyd's Register into how the fire performance ofcables varied according to the manner of installation. Thatpaper is now very much a standard reference within ourindustry. I suspect that certain aspects of this paper, espe-cially these two tables, could attain a similar status.

When talking to cable manufacturers' marketing peopleone is often presented with seemingly conflicting viewsconcerning the most appropriate cable for a particular instal-lation or application. The information contained in the paperprovides a useful basis for making an impartial decision, andI would like to thank Peter Waterworth for providing uswith such a tool. Selection of cables does, however, need toconsider cost, something which can vary remarkably aroundthe world.! wonder if the author could give an indication ofrelative costs for different cable types manufactured withinthe UK?

The paper rightly cautions us to remember that the stand-ard laboratory testing of cables may not be indicative of theiractual performance once installed. In the early to mid-1980smy own company was concerned with the specification andbuilding of the oil production vessel Seillean. This vesselcontains vast quantities of cable, many times more than in aconventional merchant vessel, frequently installed in con-fined locations. The choice and installation of cable was,therefore, an important cost and safety consideration. Atthat time considerable emphasis was being given to thegases produced during a fire. Analysis of video recordingsof the then IEC 332 Part 3 Category A propagation testconducted on various cables by Queen Mary College, Lon-don University, assisted us in making our decision. It wasfound that the flame propagation of EPR insulated, halogenfree sheathed cables was dramatically worse than the EPR/CSP eventually chosen, and that this cable could re-ignitesome time after the fire had apparently extinguished follow-ing removal of the burner flame. By comparison EPR/CSPdisplayed virtually no fire propagation by the cable, extin-guishing almost as soon as the source was removed.

Could the author comment on how cable manufacturersachieve a sensible balance between conflicting requirementsfor insulation and sheathing requirements, eg flame propa-gation and gas evolution.

A characteristic of merchant shipbuilding is that, particu-larly within the machinery spaces, virtually everything,including cables, will be painted. For Seillean a typical run ofcabling was assembled by Harland and Wolff and tested atQueen Mary College, the burner being repositioned to givethe same clearance as in the normal IEC test. Althoughlargely a subjective assessment, much to our surprise andrelief the painting did not affect the fire performance at all.

40

There has been considerable development in interna-tional standards for cables. However, many countries stillhave their own seemingly independent national standardswhich can become an accepted regional standard, eg J1S inthe Far East. Figure 1 of the discussion shows the aftermathof a 20-30s flash fire in a steering gear flat with JIS EPR /PVCcable having an outer steel braid, the standard ship wiringcable in thatpart of the world. The shot was taken after a highpressure detergent wash had been carried out and the cableclips removed. It may not be particularly clear from thephotograph, but the PVC sheathing has bubbled through thebraiding. Despite the short duration of the fire and theabsence of any damage to the deck head paint work all thecabling within the space was completely written off. Com-prehensive data for such cables is not readily available.Could the author please comment on any moves on theinternational scene to get nations, who are members of IEC,to adopt agreed international standards.

P Waterworth (Delta Crompton Cables Ltd) On the ques-tion of relative cost it is somewhat difficult to carry out adirect cost comparison due to the different cable construc-tions. For example, cables contained within NES 526 haveflexible class 5 conductors in place of the traditional strandedclass 2 conductors of BS 6883 cables.

It would perhaps be more helpful, therefore, to try to givean indication of actual material costs, but I must stress theseare indicative only (see Table I of discussion). Commercialdecisions could change the ranking, such as if a manufac-turer were determined they were going to win a particularproject, regardless of cost implications.

Since the presentation of this paper in November 1994 theindustry has been rocked by severe shortages and very largeincreases in the price of CSP. Current world manufacturingcapacity of the actual base polymer is believed to be 35 000t,whereas demand is running at 38 000t. The author is unableto predict what the outcome of this supply shortage will be.However, common-sense dictates it could be the start of theprocess to phase out halogenated polymers from ships andthe offshore industry on grounds of cost effectiveness.

Turning to your second question then: yes, halogen freecables have a totally different reaction when involved in afire. Halogenated compounds, as their name suggests, sup-press fires by shrouding the cable in non-flammable gaseswhich prevent oxygen reaching the cable, thus extinguish-ing the flames. These gases are still released after flaming hasceased and thus act to prevent re-ignition.

Of course, with halogen free materials this mechanism isabsent. With this type of compound the cablemaker must, forexample, make use of active fillers that decompose to releasewater, and thus remove heat from the fire and cause extinctionby this mechanism. The compound cannot contain too high aloading of these fillers or physical and electrical propertiessuffer, so the cablemaker must balance out the conflictingrequirements of performance in a fire situation with those ofthe traditional virtues of ships and offshore cables.

A second route followed by cablemakers is the inclusionof materials which form or promote a layer of char, which

Fig 1 Aftermath of a 20-30s flash fire in a steering gear flat with JIS EPR/PVC cable

forms a barrier between the flame and the underlying com-pound.

Fortuitously, material suppliers are working upon non-halogenated synergistic additives that improve perform-ance in water for instance, yet also enhance performance ina fire.

When one considers that we have been supplying CSPsheathed cables for nearly 30 years one cannot expect therelative newcomer, OHLS, to have reached the same state ofdevelopment and effectiveness.

Finally on this subject, for the record it should be notedthat in general the insulation used in both halogenated andhalogen free cables is the same, with only a modified flameretarded insulation being used in extreme cases.

With regard to the adoption of International Standards,unfortunately as we know to our cost, it is not a level playingfield out there and although the UK is quick to adoptstandards and open its markets, some of the competition isnot. Certainly, by and large, technical consensus has beenreached but until commercial or even nationalistic pressuresare removed, universal adoption will elude us.

M G B Wannell (Trinity House) Is there any indicationhow the more modern sheath materials stand up to UVdeterioration when in exposed surface run installations,over, say, 10 years?

If noticeable UV deterioration does take place can youindicate how soon one would experience water absorptioninto the cable?

Are there any indications of cost factors for these newmaterials against, say, EPR/CSP to BS 6883?

P Waterworth (Delta Crompton Cables Ltd) It is perhapsfortunate that most of the traditional materials used tosheath ships' and offshore cables are, in general, classed asUV resistant, examples being PVC, CSP/CPe.

Trans IMarE, Vol 107, Part 1, pp 27-45

Table I Relative volume cost of materials

Elastomeric

Thermoplastic

It is also well documented that where cables are exposedto solar radiation, compounds containing high levels ofcarbon black perform much better than lighter colouredcompounds.

As to the question of UV degradation and water uptakeone does need to be more careful in the selection of some ofthe modern zero halogen low smoke compounds. The au-thor is not aware of any specific work in this field buttheoretically, dependent upon the wavelength of the lightincident upon the cable, excitation of side groups can takeplace. If severe enough, this can lead to disassociation andthe formation of radicals and ion rich areas. Given water'sattraction to charged areas this could result in an increase inmoisture uptake.

As I stated in my paper, most simple zero halogen lowsmoke compounds contain a higher ratio of filler to polymer,so the effect would be more pronounced and noticeable with

41

XLPe insulation 45

OHLS 110

PVC sheath 50

PEEK (thin wall) 2300

EPR insulation 60

C5P/CPe sheath 100

PCP sheath 90

OHLS

typically EVA 120

OHLS

typically EMA 180

MOON) sheath 300

Silicone insulation 350

P Waterworth

simple compounds of this nature. That said, provided theold basic rules are followed, use black, high carbon loadedcompounds on cable runs exposed to high solar radiation;then their performance should not be dissimilar to the moretraditional materials.

As to cost factors these are covered in my response to MrWilliams.

H Rush (ASA Consulting Engineers Ltd) The paper pro-vides a very comprehensive review of ships and offshorecabling and current standards. It contains much useful infor-mation in a form which will, I feel, be readily referenced bymany engineers.

In the review of naval cables at the start of the paper andin the conclusions, our appetites were whetted about theirproperties. It is of interest to learn in which situations theauthor would speculate naval cables to be cost effective. Ifwe are to increase use of them, we need to know more abouthow they can help us.

P Waterworth (Delta Crompton Cables Ltd) By and large,electrically and mechanically, the new generation of navalcables do not have anything significantly different to offerthe Marine Engineer. The two areas that would be attractiveare the area of weight/space saving and that of resistance tofluids.

Cables manufactured to NES525 /Def Stan 61-12 Pt 25 aresignificantly smaller in diameter and hence mass than moretraditional types of marine cables. Typical examples are:a 3 core 2.5 mm2 unarmoured cable 8.4 mm against 11.2 mm(125 kg/km against 350 kg/km), and for a 7 pair 1 mrn2individually screened cable 16.9 mm against 22.8 mm (420kg/km against 580 kg/km).

With regard to resistance to fluids the outer sheathingmaterial complying with NES518 /Def Stan 61-12 Pt 31 hasproperties approaching those of the legendary 'Niplas 915'.In areas where oil or chemical contamination has provedproblematic and costly due to frequent replacement of ca-bles, these new materials should be considered.

Work at the author's company laboratories involvingspecific drilling MusD revealed acceptable retention of prop-erties for this material, when in the same series of compara-tive testing CS lost almost all of its physical properties andsamples of PCP actually disintegrated and fell apart!

J R Bennett (P&O/Princess Cruises) I would like to thankthe author for a most interesting paper.

My company have several large cruise vessels on orderfrom European shipyards. One shipyard has proposed acable rated at U /UN (UM) = 3.3/6 (7.2) kV for a 6.6 kVinstallation, instead of the more commonly used 6/10 (12)kV cable. The system employs high resistance neutralearthing, with earth fault alarm and tripping by manualintervention. Could the author please comment on thisproposed cable, both in terms of the UN and U figures,respectively.

Is painting cables likely to cause any premature ageing orother problems, particularly with modern elastomeric ca-bles? Also, does the paint affect the flame retardancy, orevolution of smoke, toxic and corrosive gases from OHLScables in the event of a fire?

42

Lastly we have heard OHLS cables described as both'zero halogen' and 'halogen free'. Is there any difference inthese two terms?

P Waterworth (Delta Crompton Cables Ltd) Guidance onthe selection of appropriate cable for a particular system canbe found in the Supplement of the Sixth Edition of the IEEShips Regulations (Blue Book).

The system proposed, automatic fault alarm but manualdisconnection, would appear to be a Category C system, andon this basis I would personally offer the more common6/10(12) kV cable. Had the system been a Category B or evenCategory A then the 3.3/6 (7.2) kV cable would suffice. Thisguidance and categorisation is derived from IEC502 and soshould be known on the Continent.

With regard to painting, the general advice given is not topaint cables but we all know it goes on. Because there aremany different types of paint, specific advice cannot begiven but again, in general, most popular paints do not causeproblems.

Certainly the thinners and solvents used in the manufac-ture of paints, if used neat and in prolonged contact, aremore than capable of causing damage to cables, however inpractice they are not neat and are free to evaporate off, socontact is not prolonged. Paints which could cause problemsare the thick bitumastic types, where solvents do not evapo-rate so quickly and freely, thus causing degradation.

A further general statement is that usually rubbers andcross-linked materials resist paints better than, say, simplethermoplastics such as PVC. This is because the solvents,although having no effect upon the actual PVC resin, canleech out and extract the plasticisers used to change basicPVC, a rigid material, into the flexible varieties used in themanufacture of electric cables.

As regards paints' effect on properties in the event of alfire, in theory it could assist propagation along a cable runalthough the author has no personal experience of this.Being dependent upon the actual formulation of the paint itcould most definitely contribute to the generation of smoke,toxic fumes and corrosive gases. Obviously chlorinatedpaints will release, for instance, smoke and acidic gas whendecomposed in a fire, the amount released depending uponthe volume of paint present.

With regard to the terms 'zero halogen' and 'halogenfree', they are practically the same. Of the two the latter is themore technically correct, mainly because 'free' does notmean absolutely none, rather none has been added inten-tionally or knowingly as a deliberate part of the formulation.An analytical chemist could, with his sophisticated equip-ment, detect halogens down to the parts per million or evenparts per hundred million level, so to claim 'zero halogen' tohim is not strictly correct, hence 'halogen free'.

Prof I D Stewart (BP Exploration) The author is to becongratulated on an excellent paper charting the develop-ment and specifications of cables. As a point of interest Ifwould like to correct a minor error in his paper. On the topicof Fire Resistance and the references to the BP EngineeringStandards 235 and 236 being current to this day, I would liketo inform him that they were replaced in July 1993 by a newdocument, namely: S112-12 Requirements for Flame Retard-

-

ant and Fire Resistant Cables. (It should be noted that it alsoreplaced the old BP Engineering Standard 242.)

On the topic of Fire Resistance the performance require-ments are given in clauses 6.1 and 6.2 which I reproducebelow:

'6.1 Fire and Impact Resistance

The cable shall be capable of operating when exposedto a source of heat as detailed in IEC 331 and simulta-neously sustaining a degree of mechanical impact forthe full time of the test detailed in IEC 331. The me-chanical impact shall be substantially similar to thatdetailed in Norme Francaise C32-070, Section 2.3 orNorme Beige NBN C30-004 Section 3.3.3.2, or BS 6387Section D.4.1.4.

6.2 Water Resistance During a Fire

The cable shall be capable of operating when exposedto a water spray during a fire. This shall be demon-strated by compliance with the test specified in 7.1.Fire and impact resistance tests and water resistancetests are to be carried out on the same piece of cable.'

Additional guidance is given for the design engineers tomake selection of suitable cables, as supplied into the marketplace:

'The fire performance requirements specified here(effectively 750°C for 3h) are meant as a compromise.One could specify a need for almost any temperaturewithstand and time (1000°C and a period of 20 minhave been employed in the past) but there wouldinevitably be manufacturers offering a differing set ofvalues. The adopted approach is to specify an Interna-tionally recognised standard (albeit with the addi-tional impact test specified) with the expectation thatthere would be a greater number of manufacturerseither having carried out tests to that standard orhaving the test arrangements for doing so. Clearlythere will still be manufacturers who will offer alterna-tives to those specified, eg 800°C for 1h. No specificguidance is available on equivalents and it is left to thespecific application engineer to determine whetheracceptance of alternative fire resistant proofs would bereasonable.The impact test references above were, at the time ofdrafting this document, the only ones considered ap-plicable and substantially contained within standardspublished within Europe. Alternative arrangementswhich offer similar shock treatment to the cable shouldbe considered if the particular arrangements of thespecified test arrangements are not appropriate. Theintention of this requirement is to be assured that a fireresistant cable can be robust enough to withstand atleast some degree of mechanical shock during the timeit is exposed to fire.'

There is, therefore, a continuing range of possibilities tomeet the offshore requirements of one operator, let alone anumber of operators. What is clear is a need to have aconcerted effort to standardise cables to a limited numberand type, in order to make a significant impact on cost

Trans IMarE, Vol 107, Part I, pp 27-45

reduction. Can the author see a way to get all parties togetherin an effective way to achieve this end?

Secondly, I would like to bring to the author's attentionthe activity under MC TC18 Work Group 18, which isdeveloping a set of new IEC Standards to cover electricalinstallation on mobile and fixed offshore structures. Part 4 ofthis standard will cover the subject of cables and input willbe required from the SC18A. Again, it would seem to me thatthe appropriate bodies need to be represented and broughttogether to achieve a standard that will assist in providing arange and number of cables that meet needs without intro-ducing more new designs. The future then would be that forthe offshore industry, at least, the LEE Green Book will nolonger be the reference work that it is today.

P Waterworth (Delta Crompton Cables Ltd) I am pleasedto say that someone did spot the deliberate mistakes I madein my paper and I congratulate Prof Stewart on this feat!

Taking his second question first, although a strong advo-cate of standardisation, especially in the technical field, theauthor recognises the problems inherent in this process. Theproblem in this specific instance is two-fold. First and fore-most, as stated, it is most regrettable that IEC SC 18A isdormant and inactive, despite having several individualmembers (the author being one) who want to see it up andworking, providing the co-ordination and leadership neededin this field. Hopefully this will be addressed in the not toodistant future.

The second point is that which bedevils almost all world-wide organisations. The cables, materials and technologyused by the low technology developing nations are thosewhich were replaced several years ago in the more devel-oped markets. Butyl rubber, long ago replaced in Europe byEPR, is still quite popular and frequently used, as in Chinawhich is a classic example.

The problem then facing the person writing the standardis how to combine the 'old' technology of the developingnations with the 'current' technology of more developednations, and also possibly allowing a degree of 'new' tech-nology. Certainly any group or action which will help re-duce the current range and varieties of cables availablewould be welcomed by the author and his industry.

This then brings me nicely onto your first, althoughrelated, question. Speaking now specifically about the UKthen, a much closer relationship is needed between yourorganisation, UKOOA, and that of the author's, ie TheBritish Cablemakers Confederation (BCMC). Only by meet-ing more frequently to exchange knowledge and experi-ences can UKOOA gain an appreciation of the effects uponcost that its requests have, and, conversely, the BCMC canlearn of your needs and practices to allow technically validsuggestions to be made as to the use of a particular cable forseveral applications, rather than the individual ones weappear to have now.

Off the record though, our marketing people may not likeit but ultimately anything that saves us both money willsurely be welcomed by our bosses 'up top'.

A B Tait (Atlantic Power & Gas Ltd) The author is to becongratulated on a very good historical presentation anddevelopment of electric subsea cables.

43

P Waterworth

44

Fig 2 Cable catenary configuration

Free hanging Chinese lantern

Lazy S Steep S

Lazy wave Steep wave

Of particular interest to me is the fatigue life associatedwith 11 kV cables for use subsea on floating production units(FPSO, semi-submersibles, etc).

The cable would run along the seabed and up to a mid-arch buoy, into a catenary arrangement and up to the 'floater'.This cable would be subjected to:

rise and fall of the floater;

possible rotational twisting;

sea currents;

horizontal movements with the vessel operating in itsanchor movements.

Could the author comment on this application.

P Waterworth (Delta Crompton Cables Ltd) I do notprofess to be an expert in the rather narrow specialised fieldof subsea electrical cables. For advice in this area I rely onexperts within AEI Subsea Cables; being a specialist subject,specific advice is outside the scope of this paper, howevergeneral guidance can be given and would be as follows.

In designing a subsea dynamic cable system for powersupply from a fixed structure or shore base to an FPSO, anumber of parameters over and above those for static subseacables must be taken into consideration. These would in-clude:

I. the choice of a wholly dynamic design versus a jointeddynamic/static design;

cable catenary configuration (see Fig 2 of the discus-sion);

fatigue life;

stiffness versus flexibility;

long service life versus ease of replacement of dynamicportion.

The typical life expectancy of a static submarine cableunder normal loading conditions, and without third party

Trans IltlarE, Vol 107, Parr 1, pp 27-45

damage, is about 20 years. As you will be aware some are stillserviceable after 60 years or so.

It is believed that the longest surviving medium voltage3 phase ac system operating in dynamic mode is about 4-5years old. This is operating from a floater in the South ChinaSea where the platform has to be regularly abandoned dueto seasonal typhoons.

To prolong dynamic life to that of static, the fatiguecharacteristics of all the cable components should beoptimised, with particular emphasis on the metallic ele-ments.

Once the optimum cable design is established, computeranalysis should be carried out to determine the optimumconfiguration of the dynamic part in terms of:

optimum 'wave' catenary;

distance from floater to touchdown;

position and number of flotation devices;

tension and bending moment distributions.

Finite element analysis can be carried out with respect toa number of different sea states. In an ideal case the abovecable analysis can be married to the vessel motion analysisfor system optimisation. Once the configuration has beenoptimised then ancillary items can be taken into considera-tion to minimise specific risks. These would include:

bend atstrictors/ limiters;

additional cable protection against impingement fromrisers and FSPO mooring chains;

anchorage/bend restriction at touchdown position;

drag chain cables on floater deck;

joints.

The selection of the optimum solution should also takeinto consideration the overall cost of the marine installationsolution and the cost of repair or replacement of all or part ofthe system at a subsequent time.

Cat

'2.

'5.