ECT156Construção Quadros BT

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n° 156

dependability andLV switchboards

E/CT 156, first issued, May 1997

Olivier Bouju

After graduating in 1989 as anengineer from the Institut NationalPolytechnique de Grenoble and theInstitut Administratif de l'Entreprise,Olivier Bouju joined Schneider in1990 where he specialised independability studies in the Low

Voltage Switchboard Department ofthe LV Power Equipment Division.He is currently responsible forTechnical Management of LowVoltage Power Equipment.



Cahier Technique Merlin Gerin n° 156 / p.2

glossary

HV, high voltage.

IEC, International ElectrotechnicalCommission.

IP, degree of protection of low voltageswitchboards.

LV, low voltage.

MCC, Motor Control Centre,LV switchboard grouping the controland monitoring elements of a numberof motors.

MLVS, Main Low Voltage Switchboard.

MTBF,Mean Time Between Failures.

MTTR,Mean Time To Repair.

MV, medium voltage.

PE, protective conductor.

TTA and PTTA, Type-TestedAssemblies and Partially Type-Tested

Assemblies: LV switchgear andcontrolgear assemblies defined bystandards imposing certain serviceconditions, construction requirements,technical characteristics and tests.

UPS, Uninterruptible Power Supply.Bus (serial), a communicationsnetwork over which all data elements,including those related to monitoring,are transmitted one after another.

Communicating component, devicesuch as a circuit breaker or relay thatis capable of transmitting a wide rangeof information such as trip unitsettings, currents, overloads, causesof tripping and insulation-resistancevalues.

Intelligent switchboard, an assemblyincluding communicating components

(circuit breakers, relays, etc.) and acentral unit (processing capacity),connected by a communicationsnetwork or bus. It can function alone oras part of a supervision system.

Protocol, set of rules ensuringcooperation between entities, generallyseparated by a certain distance,particularly in order to establish andmaintain the orderly exchange ofinformation between them.

Switchboard Central Unit, aprocessing unit within the switchboard,used to organise digital informationforwarded by the communicatingcomponents, automate electricaldistribution functions and communicatewith the installation’s supervisionsystem.




dependability and LV switchboards

contents

1. Introduction p. 4

2. Switchboard functions The switchboard and p. 5its functions

The switchboard's functional p. 8

guarantee3. Optimum dependability Dependability characteristics p. 9

Industrial dependability concepts p. 11

Required dependability levels p. 18

4. Future perspectives for switchboards Power management p. 20

Power management for greater p. 20dependability

The technology p. 21

The "intelligent" switchboard p. 21

5. Conclusion p. 23

Appendix: bibliography p. 24




1. introduction

This Cahier Technique publicationdeals with the dependability ofcommercial and industrial low voltageelectrical installations. Its aim is toanswer the question "which installationbest satisfies our growing needs interms of electrical power availability?".

The subject is dealt with forLV switchboards and focuses on thefollowing problems:

c which switchboard functions guardagainst failure of the LV distribution

system?

c how should they be used?c with what components?c in what power system environment(number of sources and loads, type ofsystem earthing)?

The reason for this focus is thatLV switchboards are vital links in anypower distribution system.

This document is intended to helpoperators and designers of electricalinstallations to:v determine the points which must be

considered. These points are related to

the technical choices dealt with in thesub-chapter entitled "industrialdependability concepts" . Thediscussion is based on reliability levelscalculated on concrete cases andyields solutions in terms of equipmenttype. A summary is given in the sub-chapter entitled "required dependabilitylevels".v realise the increasing influence ofpower management systems onLV switchboard dependability.




2. switchboard functions

The switchboard is a key part of anyelectrical installation. It incorporatesdevices designed to:v distribute electrical power and protectcircuits,v protect persons,v control and monitor the installation.

Recent developments in this controland monitoring function have made theswitchboard even more vital to theinstallation. The dependability of theentire installation is largely determined

by the dependability of the switchboard.Moreover, the lasting viability of theassociated industrial or commercialactivity depends on the capacity of theswitchboard to keep pace with futureneeds.Dependability of electrical distributionmeans:c a very low probability of failure(reliability),c no dangerous failures (safety),c the ability to operate at any giventime (availability),c fast repair (maintainability),

... throughout the entire lifetime of theinstallation.These notions of dependability must betaken into consideration right from theswitchboard design phase.

Today, dependability requiresdecentralised management of theinstallation. For instance, loadshedding/reconnection and sourcechangeover automation systems,measurement instruments andprotective devices, are placed as closeas possible to the application to ensure:v optimum modularity,

v greater reliability (a local failure doesnot paralyse the entire installation),v operating flexibility with local controland monitoring possibilities atswitchboard level in addition tocentralised supervision. The dialoguebetween the various distribution levelsis considerably simplified by the use ofdigital communications networks.

As a result of this decentralisation, partof "intelligence" is integrated in thevarious LV switchboards of theinstallation which house the mainelectrical components used betweenthe transformer and the load devices(see figure 1).The result is a switchboard systemincluding:v the Main Low Voltage Switchboard,v the switchboards specific to motorcontrol (MCCs - Motor Control Centres),

v the subdistribution switchboards,v the final distribution enclosures.

the switchboard and itsfunctionsThe implementation of the functions ofa switchboard involves various aspects.

c the LV installation architecture,broken down into various switchboards,

enclosures, etc. at various locations,forming the installation layout.The switchboards are further dividedinto a number of zones for:v components,v busbars,v connection,v auxiliaries.The minimum clearances and safetydistances must be satisfied.

c the functional units, providing theelectrical functions needed by the user.

Each unit includes the componentsdesigned to cover a given function, forexample protection of a feeder or a setof feeders, motor control, incomingprotection, etc.

c the enclosure, providing:- protection of the electrical equipmentagainst external influences,- protection of persons against electricshocks (direct and indirect contact).

fig. 1: intelligence is now distributed and integrated in LV switchboards. Our example shows a

circuit board in an LV assembly (Digibloc board - Schneider).




v protection of electrical equipmentinside enclosures against penetrationby solid bodies and liquids;v protection of persons provided by:

- interconnection of all metal parts(frames, enclosures, including thedoor) which are earthed usingprotective conductors (PE),- reduction of openings (ventilation,cable entries, etc.) to prevent access tolive parts either directly or via tools(e.g. screwdrivers),- possible use of barriers to avoidcontact with live parts when the door isopen;v degrees of protection (IP)IEC 529, HD 365 and NF C 20-010standards define the various degrees

of protection of persons (against directcontacts) and equipment (see figure 2)using two numerals and two letters.Impact strength is characterised by a

separate IKx index as in Europeanstandard EN 50102;v adaptabilityThe enclosure must be adapted both to

the volume of the components to behoused and to the size of the premisesand means of access.Connections are made from the top orbottom, front or rear, as required.

c internal partitions.For increased dependability, thecubicles can be divided up by partitionsand barriers (metal or not). The variousequipment items are installed andcabled in the switchboard in such amanner that they do not interfere witheach other, for example throughelectromagnetic fields, vibrations or

arcs. Partitioning is a solution for mostof these phenomena and suitableventilation solves the associatedthermal problems.

Barriers and partitions also contribute to:v protection against contact with liveparts belonging to the adjacentfunctional units,

v limitation of the probability of initiatingarc faults,v protection against the passage ofsolid foreign bodies from one functionalunit to another.The corresponding levels ofdependability are evaluated further onin this document.These partitions are often related to theswitchboard architecture and thusdelimit the various zones intended forcomponents, busbars, connections andauxiliaries.The separation of the various switch-

board elements and functions(see figure 3) is defined in standardsIEC 439-1 paragraph 7.7 andNF C 63-410.

element numerals meaning for the protection meaning for protection ofor letters of equipment persons

first characteristic against ingress of solid foreign bodies: against access to hazardous parts with:numeral 0 (non-protected) (non-protected)

1 diameter u 50 mm back of hand2 diameter u 12.5 mm finger3 diameter u 2.5 mm tool4 diameter u 1.0 mm wire5 dust-protected wire6 dust-tight wire

second characteristic against harmful ingress -numeral of water:

0 (non-protected)1 vertically dripping2 dripping (15° tilted)3 spraying4 splashing5 jetting6 powerful jetting7 temporary immersion8 continuous immersion

additional letter - against access to hazardous parts with:(optional) A back of hand

B fingerC toolD wire

supplementary letter supplementary information specific to: -(optional) H high voltage apparatus

M motion during water testS stationary during water testW weather conditions

fig. 2: elements defining a degree of protection IP as in standards IEC 529, HD 365 and NF C 20-010.




v form 1: no separation,v form 2: separation of busbars fromfunctional units,v form 3: same as form 2 plus

separation of all functional units, butnot of their terminals for externalconductors, from one another,v form 4: same as form 3 plusseparation of the terminals forexternal conductors which are anintegral part of the functional unit.

c internal electrical connectionsConsisting of conductors (busbarsand cables) within the enclosure, theycarry and distribute the currentaccording to the installation diagram.v their cross-sections and numbervary according to the nominal

currents.

fig. 3: "the forms" defined by standards

IEC 439-1, EN 439-1 and NF C 63-410

delimit the various zones in a switchboard. fig. 4: the various possible installation methods for components in an LV switchboard.

However their characteristics alsodepend on other parameters, forexample the rated short-circuitwithstand current of a switchboard,

equal to the root mean square of thecurrent that can be withstood by theswitchboard for one second(see standard IEC 439-1).v their supports must in turn withstandthe corresponding electrodynamicforces and thermal stresses, and alsocomply with minimum creepagedistances.v as regards control circuits, theircoexistence with power circuit isachieved by running them separatelyand using appropriate connections.Likewise, the auxiliaries (for form 3

separation or higher) are isolated fromthe other units and are thus subjectedto a less restrictive environment inthermal and electromagnetic terms.

c component connectionsThe way a component is connected orinstalled influences availability and

maintainability. Component installationmethods include fixed, withdrawableor disconnectable.

Reminder:v a device is said to be fixed whentools are required to separate it fromthe main circuit,v a device that is withdrawable from abase or frame (for a heavy device) canbe moved to a position for which anisolating distance is achieved betweenits upstream and downstreamconnecting elements,v a disconnectable device has awithdrawable upstream connection anda fixed downstream connection.

Likewise, these installation methodsare linked to switchboard technology

which may fixed, drawout (racks) ordisconnectable (see figure 4).

For example: a withdrawable assemblycan be either a switchboard containingfixed devices in drawout units orwithdrawable devices (on base orframe) on a fixed panel.

form 1 form 2

form 4form 3

+

+

+

+

+

+

+

elements:

component

LV switchboard

component

technology

fixed

switchboardtechnology

disconnectable

subassembly

drawout

fixed

withdrawable

on base

withdrawable

on carriage

or frame

disconnectable




the switchboard’sfunctional guaranteeSwitchboard design refers to standards

governing the entire low voltage domain,and, more specifically, to standardsrelating to assemblies (cubicles,desks,...).Compliance with these standards is theminimum guarantee of a level of qualityand dependability.Standards IEC 439-1, EN 60 439-1 andNF C 63-410 define the constructionrequirements, technical characteristicsand tests for "type-tested" and "partiallytype-tested" assemblies.c assemblies manufactured inaccordance with established types are

known as "Type-tested assemblies"(TTA),c assemblies derived from type-testedarrangements (e.g. by calculation) areknown as "Partially type-testedassemblies" (PTTA).The standards are discussed inCahier Technique n° 145 that deals withthermal studies of LV switchboards.

Heat exchange must be controlledwithin a switchboard to avoidoverheating the equipment installedinside. This requires proper ventilationand in some cases a careful choice of

installed components to ensure asuitable level of reliability.Moreover these thermal studies are partof work currently conducted by theSchneider technical sections and aimedat optimising the technicalcharacteristics of LV switchboards,particularly as concerns:c power connections(definition of a certain number ofparameters as a function of currents),c short-circuit mechanical andthermal withstand described above(using computer models),

c control and monitoring installation(using studies and tests),c dependability of low voltage distri-bution systems through switchboards.

In addition to the above work, theswitchboards undergo numerous tests(see above-mentioned standards) tovalidate the theory and guaranteeoperation of the resulting assembly.These tests include verification of:c temperature-rise limits,c dielectric properties,

c short-circuit withstand strength,c continuity of the protective circuit,c clearances and creepage distances,c mechanical operation,

c degree of protection.

Likewise, in order to meet customerneeds and ensure the durability of therequired quality level, the design,industrialisation and manufacture ofLV switchboards must comply with theQuality directives, methods andcontrols (see figure 5).

fig. 5: an LV assembly under test... "to meet customer needs and ensure the durability of the

required quality level" (LV Power Equipment division, Schneider).




3. optimum dependability

Reduction in the number of failures andof the resulting shutdown timesincreases safety and productivity incompanies.What is more, users today demand a"tailor-made" level of dependability, i.e.an installation adapted to their needs.The notion of optimisation is thus vital,meaning just the right level ofdependability in order to ensure thebest price.If this is to be possible, manufacturers,

installers and specifiers must masterthe dependability parameters of theirinstallations.

dependabilitycharacteristicsc dependability parametersThe notions involved in dependability(reliability, maintainability, availabilityand safety) are all linked. Three ofthese notions can in particular beassociated by their representativequantities which are:

v for reliability, the failure rate (λ) or itsreciprocal (1/ λ), the MTBF (Mean TimeBetween Failures).The failure rate of a transformer is forexample 6 x 10-7 h-1 whichcorresponds to a mean time betweenfailures of 195 years, or to 1 device outof 195 failing on the average each year.v for maintainability, the value MTTR(Mean Time to Repair) is used. Thistime covers detection of the failure, thetime required to supply the spare partsand the actual repair time.v for availability, the quantification

depends on the combined aspects ofreliability and maintainability.The opposite of availability, which isobviously unavailability (ID) is

fig. 6: example of an electrical distribution system.

expressed (for most systems) by:ID = λ MTTRwhere λ represents the reliability andMTTR the maintainability.For a transformer, if 12 hours elapsebetween the failure and resumption ofpower, its unavailability is= 6 x 10-7 x 12 = 7.2 x 10-6, which isequivalent to 4 minutes of unavailabilitya year (i.e. 7.2 x 10-6 x number ofminutes in a year).Remember that for a given installation

architecture, availability is

characterised by a combination ofgood reliability and efficientmaintenance.

c dependability applied to assemblies.

To calculate dependability, the failuretree method must be applied to the LVelectrical distribution system studied(see Cahier Technique n° 144).

Analysis

Let us consider the availability ofelectrical power of application U1,

shown in figure 6.

T1

m1

Q1

v1

B1B1

m3

Q3

v3

C1

R1

application U1

B1

m4

Q4

v4

B1

v2

Q2

m2

B1

Q5

incomer 2incomer 1

T2

main busbars B1




fig. 7: failure tree associated with the diagram in figure 6.

and

or

or

Q3 circuit-

breaker faulty

absence of power on

application U1

fault on

busbars B1fault on

cable C1

down-

stream discon-

necting contact

v3 faulty

load

R1 faulty

Q5 repairs

-> incomer

de-energised

HV utility

failure

transformer T1

faulty

MV supply

failurecircuit-breaker

Q1 faulty

upstream

disconnecting

contact m3

faulty

downstream

disconnecting

contact v1

faulty

upstream

disconnecting

contact m1

faulty

"fixed maintainability"

unavailability

unavailability of incomer

(upstream of the main busbars)

unavailability between main

busbars and the application

unavailability due to short-

circuit on another feeder

or

no voltageon incomer 1 no voltage

on incomer 2

or

transformer T2

faulty

MV supply

failurecirucit-breaker

Q2 faulty

downstream

disconnecting

contact v2

faulty

upstream

disconnecting

contact m2

faulty

or

short-circuit

on

circuit-breaker

Q5

short-circuit

on upstream

disconnecting

contact m4

or

short-circuit

on

circuit-breaker

Q4




The undesirable event at the top of thefailure tree is thus absence of power onU1. This event is broken down into fourmodules as shown in the chart in

figure 7.

c unavailability of incomer.Each incomer can alone supply theentire LV distribution system on whichthe application depends. The twomedium voltage (MV) incomers areassumed to be taken from two differentsubstations, which virtually reduces thecommon failure mode to theunavailability of HV (high voltagetransmission).

The circuit-breaker failure modesconsidered for calculation of incomer

unavailability are:v spurious tripping,v refusal to close,v internal short-circuit,v temperature rise.The failures of HV system components,MV incomers, transformers anddisconnecting contacts have beenconsidered together.

c unavailability between the mainbusbars B1 and the application.This sums the unavailabilities of theelements encountered from the mainbusbars to point U1. Each failure is

broken down as finely as possible andresults in different repair times. Forexample, for the busbars:v loosening of the busbar supports dueto strong vibrations may cause the barsto break when they are subjected to ahigh electrodynamic force. Theresulting repair time is several hours(part replacement).v an object falling on these bars whenenergised, although highly unlikelygiven the construction arrangementschosen (form, IP,...), often results inarcing and in a repair time of several

working days.c unavailability due to short-circuit onanother feederThe clearing of a short-circuit occurringupstream of the first protective deviceon a feeder parallel to the feederconsidered results in de-energisation ofall the feeders.

We thus have to add up all the short-circuit probabilities by descending oneach parallel feeder up to the firstprotective device.

Downstream, a short-circuit affectingU1 is possible only if there iscombination of a short-circuit andfailure of a protective device to react,the combined probability of which isnegligible.

c "fixed maintainability" unavailability.Fixed maintainability is the term usedto indicate that the repair timedepends on the installation method(fixed or withdrawable) and affects useof the other feeders.Examples (see figure 6): applicationU1 is affected by repair of Q5 which,as it is fixed, requires shutdown of theincoming supply, whereas repair ofQ4, withdrawable, can be carried outwith the busbars energised and thuswithout affecting application U1.

The results

The following results are thosecorresponding to the usual reliabilityand MTTR values encountered for thevarious system components.Unavailability of the load is 6.4 x 10-5,i.e. 33 minutes a year. An examinationof the relative importance of the

different aspects gives the followingbreakdown of unavailability:v incomer 45%v between busbars andapplication 51%of which:- cable and load 32%- rest upstream 19%v short-circuit on another feeder 1%v fixed maintainability 3%The various points to be examined arederived from this analysis and will bedealt with in the next chapter.

industrial dependabilityconceptsAs defined above, the installation mustbe designed to meet the customer’sspecific requirements. In all systems, just one small element can often jeopardise overall dependability. So if

you do not want to end up "pushing aPorsch", the importance of the varioustechnical choices must be evaluatedwith regard to dependability.

These choices include:c the diagram (incomer, finalapplication, system earthingarrangement),c the connections,c the electric arcs,c the switchboard options (form,connection, fixed or withdrawablecomponents, IP...),c the motor feeder units,c the control and monitoringauxiliaries.

Dependability in relation to the

diagramTwo elements are of criticalimportance to dependability:v the incoming diagram,

v the final applications.A third element, the system earthingarrangement, also has great influence.

c the incoming diagramAs availability of the incomer affects allapplications, whether or not they arecritical, it is important, if at all possible,to choose an incoming configuration inkeeping with the downstream need.The chosen solution will depend on

the environment studied. For example:v in isolated regions, it may be hard toobtain an MV line with good availabilityand even harder to obtain twoseparate MV lines. In this case thestudy must consider independentenergy production such as by anengine generator set.v some sectors of industry (chemistry,petrochemistry, paper-making)generate energy (often in the form ofsteam) through their manufacturingprocess which they use to driveturbogenerators. The public

distribution system is then used as abackup source.

NB: if, despite this, the availability ofthe incomer is insufficient, a UPS(Uninterruptible Power Supply) mustbe placed as close as possible to thecritical applications.




v calculation of unavailability due to theincomerOn the example in figure 8 (2 parallelincomers, 20 motor feeders),

unavailability of the application is roughly1/2 hour a year, 50% of which is due tofailure of the incomer. From this weconclude that unavailability of theincomer, although not alwayspreponderant, may nevertheless accountfor a large part of total unavailability. Weshall see later on that the incomerunavailability percentage rangesbetween 7% and 90% according to themeasures taken to ensure reliability ofthe rest of the system.The incomer has two main critical points,namely:

- the high voltage transmission line,- the medium voltage line;Transformers, circuit-breakers anddisconnecting contacts are 100 to 1000times more reliable than these twosources of failure.

How can the dependability of theincomer function be improved?There are many possible solutions, andthe context is a decisive factor. Greaterreliability can be obtained byconcentrating on the following points:v redundant incomersTwo medium voltage lines from twodifferent source substations, used inparallel, solve the problem ofunavailability of the medium voltagelines. The unavailability of the incomerfunction is now virtually reduced to thatof the high voltage system alone whichis roughly 17 minutes a year, comparedto 10 hours for the MV system.Availability can also be increased byadding one or more generator sets (seeCahier Technique n° 148 “Highavailability electrical power distribution”).v splitting into priority and non-priorityfeedersThe search for increased availability ofelectrical power nearly always results(depending on installation size) individing applications into two types:- priority,- non-priority.In the event of an overload or failure ofthe main source, non-priority loads arethen shed, while priority loads continue

fig. 8: unavailability of an incomer may account for a large part of total unavailability, in this

case 50%.

M M M M

incomers

20 feeders

load

b) causes of unavailibility

on a feeder

c) unavailability on a feeder as a function

of the system earthing arrangement

1

1/2

0ITTT TN

unavailability

(hrs/yr)

failure of incomer, of which:

98% due to public HV failures,

2% due to MV failures, roughly due 0% to circuit-breakers.

failure of the final

load devices (cables and motors).

a) diagram

type of

system

NB: for IT systems, unavailability is

calculed considering repair to be

compulsory, on the first fault.

50%

20%30%

failure of LV distribution

and of control devices.




to run on a secondary source(second MV incomer, generatorset,...).v source changeover systems

If a failure occurs, circuits can betransferred to backup sources notused in normal operation or to thesources of non-priority feeders, withload shedding of the latter.Three types of changeover systemsare possible:-synchronousThe main source and the replacementsource are or have the possibility ofsynchronising, thus ensuringchangeover without loss of loadsupply.This process is used in installations

requiring a high level of dependability.- delayedThis is the most common type ofsource changeover system. Withtransfer times ranging from 0.4 to30 seconds, its use is widespread forindustrial and commercial applications.- pseudo-synchronousA fast-acting switching device (60 to300 ms) is used for the sourcechangeover. This system is found, forexample, in the following sectors:- chemistry,- petrochemistry,

- thermal power plants.c loadsUnavailability due to the load devicesis illustrated in the diagram in figure 8and concerns for instance the motorsand the cables supplying them fromthe switchboard. Reliabilitycalculations show for example thatwhen using a motor M, 30% of itsdown time due to failures is causedeither by the cable or by the actualmotor. It is thus necessary to clearlydefine the technical characteristics ofthe loads as regards conditions of use,

as well as the maintenanceprocedures intended to preventfailures.Most electrical failures in motors aredue to phase/earth faults occurring onmotor startup.Insulation monitoring before starting amotor, particularly using the VigilohmSM 20 developed by Schneider,enables:

fig. 9: choice of system earthing arrangement directly affects the dependability and reliability of

the installation.

system TT TN IT

action during an immediate immediate continuation ofinsulation fault de-energising de-energising operation

fault tracking preparation beforede-energising

magnitude of several dozen several several dozenfault current amps kiloamps milliamps(determines damage (short-circuit (1st fault)to installation) current)

v preventive maintenance to beprogrammed,

v irreversible motor damage to beavoided.

c system earthingThe three system earthingarrangements are (see figure 9):v TT system (earthed neutral andearthed protective conductors),v TN system (earthed neutral andprotective conductors connected toneutral),v IT system (unearthed neutral andearthed protective conductors).The system earthing arrangementaffects availability and maintainability inthat the circuit must be broken on afirst fault for TN and TT systems butnot for IT systems. In addition, themagnitude of the earth fault currentdepends on the system earthingarrangement and determines theextent of damage caused to theinstallation and in particular to theloads.

The results of a reliability study areshown on the histogram in figure 8.The IT system, with an automaticsystem for fast locating of the first fault,is the one offering the best availability,as it ensures that:v

operation is not interrupted (continuityof the production cycle in progress),v the fault can be repaired when theinstallation is not in operation,v servicing can be prepared duringproduction, resulting in increasedmaintainability.The IT system is recommended in thefollowing cases:

v presence of loads sensitive to highfault currents,v high risk of fire,v installations with generator sets (to

prevent damage to the generator by aninternal fault),v need for a high level of dependability(availability + safety), for example inoperating rooms in hospitals.

NB: in the IT system, the probability ofde-energising due to a second fault (ifthis fault occurs before the first faulthas been located and cleared) is lessthan in the TN and TT system as thesimultaneous presence of the first andsecond faults is necessary on differentphases.

We saw earlier that the system earthingarrangement must be selected withgreat care. Once this choice has beenmade, the equipment (switchboard andcomponents) can be chosen, and acertain uniformity sought in thereliability of the different links in thechain making up final unavailability.

Dependability and connections

A switchboard is made up of a largenumber of connections and it istherefore important to consider thefailures they cause.

A connection fails when it ceases to

convey the electrical power for which itwas designed. A local temperature risethen occurs which may causeirremediable damage to the device and/ or the cables.The importance of good connections isillustrated by the example of aninstallation with two separate incomerssupplying 20 feeders.




The results of the reliability study (seefigure 10) show that 88% of totalunavailability is due to various failures(incomer, components,...) and 12% to

connections.A distinction should be made betweenfactory connections and those made onsite, as statistics show that the latterare more prone to failure.

In practice, dependability can beconsiderably increased by:c properly sized contact surfaces(overlapping),c proper surface finish (flat and clean),c a tightening torque suited to thematerials.

Dependability and arcing

c

unavailability due to arcingA number of events can result in thecreation of arcs in the switchboard, forexample intrusion of small animals(rodents or reptiles), objects forgottenin the switchboard during maintenancework, a temperature rise or depositionof conducting dust.The damage caused by electric arcs isfrequently serious and leads toshutdown times of up to severalhundred hours for an “ordinary”switchboard, i.e. 11% of its total

unavailability (see figure 10). Incomparison, for an "improved"switchboard, this percentage isnegligible as its shutdowns are limited

to the time required to put the distri-bution system back into working order(cable tightening, cleaning ofcarbonised surfaces...), i.e. roughly onehour.To prevent this unavailability, thefollowing three points should beconcentrated on:v risk of arc occurrence,v arcing time,v propagation of electric arcs in theswitchboard.These actions aim at reducing bothrepair times and the extent of the

damage caused by arcs.

c preventing arcingIt is better to prevent a problem than tocure it, in other words to take action onthe cause of electric arcs. Note that:v arcing due to dielectric breakdowndoes not occur if:- materials are properly chosen,- creepage distances and clearancesare complied with.

v introduction of objects or foreign

bodies, including conducting dust, and

intrusion of small animals, are thecause of numerous electric arcs in LVcubicles.To prevent arcing, considerable care

must be taken with enclosure design:- form,- choice of IP,- addition of filter...v when breaking occurs (on a short-circuit or overload), pressurised ionisedgases are given off by the protectivedevice and may cause arcing, forexample on a nearby busbar. This riskcan be eliminated by a carefullydesigned architecture and/or barriers.v a faulty connection can often result increation of an arc. To avoid this,connections must be correctly tightened

(see section on "dependability andconnections").

c limiting the arcing timeDamage caused by arcing can belimited by minimising the duration of thearc. Possible solutions are:v setting the “short-time delays” (short-circuit protection) to the minimum valuethat will still provide discrimination.These short delays, designed toimplement time discrimination, delaycircuit-breaker tripping on a short-circuit, thus prolonging the arcing time.Note that when zone selective

interlocking can be implemented, it isthe best solution as it allows absolutediscrimination with minimum delays forall distribution stages(see Cahier Technique n° 18).v using limiting devices which quicklybreak short-circuit currents, thuslimiting the fault current. Arcing time isthus reduced and thermal effectslimited.v choosing a protective device thattakes past transient short-circuits intoaccountThe peculiarity of the arc is that it is a

somewhat furtive phenomena, for tworeasons:- due to switchboard layouts, an arc isquickly extinguished. However theionised gases that it generates maycause restrikes on other live parts. Anumber of extinguishing and restrikesequences are therefore possible.- moreover, the impedance of the arcvaries according to the speed at whichit moves and the obstacles that itcomes across.

fig. 10: unavailabilities due to arcing and connections account for roughly 20% of causes of

system unavailability.

incomers

20 feeders

12% of unavailability

due to connections.


due to arcing.


due to:

MV incomer,

transformer,

components...


due to various failures.




However, each time arcing occurs, theequipment is subjected to a number ofstresses which can be cumulated.The solution to the problem is to

provide protection systems whichintegrate the fault over time, i.e. when afault appears and disappears (or dropsbelow the trip threshold before theprotective device trips), this time andcurrent information must be stored inthe protective device to obtain tripping ifthe fault or brief high current valuesrapidly reoccur. Thus LV circuit-breakers can be designed to storetransient short-circuit information inmemory and only gradually return totheir initial tripping characteristics(see figure 11).

c preventing propagation in theswitchboardThe laws of physics cause the arc tomove quickly away from the source. Tolimit its consequences, the arc must notbe allowed to spread through the entireswitchboard. It is essential to controlthe arc throughout its duration by:

v partitioning the various switchboardzones; insulated bushings andpartitions prevent the arc and itsionised gases from spreading;

v enhancing arc extinction, for instanceby implementing- insulation shrouds around thebusbars,- busbar geometries that lengthen thearc.

Dependability and the switchboard"options"

The form, type of connection (front orrear), device installation method (fixedor withdrawable) and the degree ofprotection are all possible options whenmanufacturing and/or purchasing an LV

switchboard.The example in figure 12 shows theeffect of these choices on availability atfeeder level.

c form (see figure 3)Consider form 1 with "unsealedopenings" compared to form 2 with"cable access openings sealed".

fig. 11: a type B Masterpact circuit-breaker (delayed) equipped with an ST608 control unit

temporarily stores short-circuit information in memory (Schneider).

fig. 12: unavailability times depend on

switchboard technology and particularly on

its connection type (the chart corresponds to

the diagram in figure 10).

0

1

2

3

4

unavailability

(hrs/yr)

unavailability dur to:

front

conn.

rear

conn.

front

conn.

rear

conn.

form 1 form 2

cable access

sealed

incomers.

fixed components.

withdrawable components.

The abbreviated expression "cableaccess openings sealed" means thatthe user has passed the cablesthrough a bottom plate equipped with acable bushing.

NB: this arrangement is used forform 2 and above.This example clearly shows that awise choice of form increasesavailability (see figure 12), as it affects:- likelihood of fault occurrence (rodentintrusion impossible),- arc propagation (presence ofpartitions).

For good availability, LV switchboardsshould be partitioned (form 3),including the terminals for external

conductor connections (form 4), since,as already pointed out, theseconnections are the cause of mostfaults (see paragraph on"dependability and connections").c front or rear connectionThe space reserved for electricalequipment when designing premises




frequently determines the type ofconnections used. This constraint has acertain effect on availability. Access toa switchboard with front connections

only is often difficult, resulting in lengthyrepair times compared withswitchboards offering dual accessibility(see figure 13).

Note that the unavailability of aswitchboard with front connections iseven higher if fixed components areused that require tools for dismantling.To increase maintainability of aswitchboard with front connections,designed to stand against a wall, asmall servicing clearance should beprovided at the rear.

c fixed or withdrawableAvailability can be improved by

choosing withdrawable devices (seefigure 12). In this way, maintenance isfaster and does not affect adjacentfeeders.

Since withdrawal takes place off-load(with the circuit open) but with poweron, breaking is not necessary upstreamand interruption of supply to the otherfeeders parallel to it is not required.

However the withdrawable option maynot offer any great advantage forinstallations subject to a high levels ofunavailability elsewhere (unreliable

source, single incomer presentingrisks,...) or when excellentmaintainability does not affect otherfeeders.However, in the case of a form 2switchboard with front connections, theadvantage of using "withdrawable"circuit-breakers is clear (see figure 12).In this instance unavailability is dividedby 3 compared with the "fixed" solution.

c degree of protection (see figure 2)Only the first two characteristic digits ofthe IP (ingress of solid bodies andliquids) are examined in this section.

The first numeral gives the maximumsize of objects or particles likely toenter the switchboard, thus limiting thesize of the access points to live parts.This numeral (1 to 6) increases as sizedecreases.The second numeral concerns liquidsand describes protection obtained by:v canopies, covers or baffles protectingagainst vertical and/or horizontal liquidsplashing and jetting,

fig. 13: a good compromise between maintainability and floor space can be obtained using a

switchboard with front connections and a small servicing clearance at the rear.

v seals and suitable devices protectingthe enclosure even in the event ofimmersion.

In conclusion, the higher the first twocharacteristic numerals of the IP index,the better the protection.

However, all electrical devices produceheat and most of them have a thermallimit.Excessive imperviousness is contraryto proper switchboard ventilation andmay thus affect operation of thecomponents. Heat extraction and/or asuitable choice of devices is thusnecessary.The degree of severity of theenvironment and the qualification of

switchboard operators determine thechoice of degree of protection. Thenecessary protection levels, for eachtype of premises, are reviewed infigure 14.

Dependability and the drawoutmotor feeder unit

MCC drawout type switchboards areoften used in process industries(see figure 15).Good continuity of service is normallyrequired for motor control. Drawoutunits allows quick, easy maintenance:the faulty feeder is immediately

replaced by an identical device whilepower continues to be supplied to theswitchboard.A drawout unit corresponding to amotor feeder can be composed(see figure 16) of a fuse, contactor andthermal relay or of a circuit-breaker,contactor and thermal relay.In terms of availability, bothconfigurations are more or lessequivalent in normal operation, butdiffer considerably should the contactorfail.In actual fact some 20% of feeder

failures are due to contactors (contactssticking), with the added disadvantageof having to remove the contactor fromthe drawout unit. The power circuit thenhas to be opened, which is possiblewith the circuit-breaker/contactorcombination by opening the circuit-breaker. In the other case (fuse/ contactor combination) power must beswitched off upstream, thus making allthe other motor feeders unavailable.

small

servicing

clearance

servicing

clearance

a) switchboard

with front connections,

standing against the wall

b) switchboard with front

connections and small servicing

clearance

c) switchboard with rear

connections and the

necessary servicing

clearance




The consequences of this procedurecan be demonstrated for a diagramwith 20 motor feeders supplied by2 separate MV incomers, an example

illustrated by the results histogram(see figure 16).Two contactor operating rates can beidentified (low and high). Thelikelihood of failure of a drawout unitare linked to the operating rate of thecontactors. It is thus preferable to usea circuit-breaker rather than a fuse asa protective device if intensive use ismade of contactors (operating rate andalso utilisation categories of loadsAC3, AC4, operating voltage...).

Dependability and the control andmonitoring auxiliaries

Using the same example(see figure 16), the influence of thecontrol and monitoring auxiliaries ontotal availability can be determined.Their associated failures relate torelays, connections or to their powersupply.The individual wiring of non-standardised auxiliaries is a lengthyprocess and subject to errors by fitters,resulting in potential failures.

fig. 15: detailed view of an MCC type LV switchboard with drawout units (MB400 model -

Schneider).

fig. 14: examples of minimum degrees of protection (as in NF C 15-100 and practical guide

UTE C 15-103).

sectors of use examples IP degree

domestic premises bedroom 20

washroom 27technical premises electrical service 00

air conditioning washer 24

refrigeration chamber 33

boiler plants and associated fuel storage 20

premises (power > 70 kW) coal storage 50

boiler plant 61

garages and parking areas repair shop 20

(area > 100 m2) washing area 25

buildings for collective use offices 20

gymnasium 21

large kitchen 35

farms alcohol warehouse 23

hen-house 45fodder storage 60

industry electroplating shop 03

paperboard manufacturing 33

quarry 65

commercial and associated art gallery 20

premises hardware shop 33

bakery 50

cabine tmaker 60

fig. 16: comparison of levels of unavailability

for a 20-feeder drawout type switchboard

depending on type of components and their

operating rate.

high

rate

low

rate

high

rate

low

rate

non-standardised

auxiliaries

standardised

auxiliaries

unavailability

(hrs/yr)

1

1/2

fuse/contactor combination

circuit-breaker/contactor combination

auxiliaries




To avoid this, Schneider offersstandardised products for auxiliaryfunctions (Digibloc, Dialpact,...). Theseare boards or control and monitoring

modules connected by powerdistribution blocks or by standardiseddigital links. These elements centraliseinformation and can be used toimplement a wide variety of controlschemes.

Furthermore, these schemes can beeasily modified by setting boardparameters or by associating newmodules, with the followingadvantages:c time savings on implementation,c increased reliability by eliminatingwiring errors,c repair time reduced to the timerequired to replace the board ormodule,c open-ended solution.

The results of the reliability calculationson the histogram show that thesestandardisation efforts increase theavailability of control auxiliariesdepending on the operating rate (from30% at low rate to 60% at high rate).

required dependabilitylevelsA large number of technical options areavailable for LV installations, all offeringdifferent dependability levels. The rightchoices depend on the application andon the choices made at other levels.For example use of a form 4switchboard is advantageous providedthat the other major sources of failureson the installation have beenovercome.The right approach when designing anLV installation is not therefore tochoose and install at random a range of

effective, reliable devices in the hope ofgaining maximum "peace of mind".In actual fact, each application or sectorusing LV electrical power requires anappropriate level of dependability,

depending on operating imperatives(see figure 17):c the commercial and service sectorincludes both of small shops and

schools..., as well as supermarkets,shopping centres, large banks, officeblocks, hospitals.c industry comprises all types offactories (automobile, aeronautic,textile,...) and has special needs interms of distribution (power systemprotection and architecture) andprocesses (motor control, controlsystem), which are vital in continuousproduction applications such aspetrochemistry, cement works, foodprocessing...

In what way do these sectors represent

different needs? Accidents such asBHOPAL (December 1984),CHERNOBYL (April 1986) andPASADENA (October 1989) areevidence of the high risks run by peopleand the environment. Hence theunfailing question "is it dependable?".In fact this question is meaningless. Asthe possibility of failure is alwayspresent, however small, the rightquestion is rather: "is it dependableenough?".For all sectors this means choosing anacceptable level of probability of

dangerous failure (in safety terms) andof dependability (in economic terms):c in telecommunications, FranceTelecom has a probability ofunavailability of 1 hr/century fortelephone exchanges (λ < 10-6 h-1).c in air transport, two dependabilityconditions are laid down to ensure that:v all "overall catastrophic" failures areextremely unlikely(λ <10-9 h-1),v all "critical" failures are extremely rare(λ <10-6 h-1).This figure can be compared with the

likelihood (λ

<10-6

) of a human beingdying within the next hour.c in banks, power failures result in lostentries and recording of erroneousoperations. The costs involved in

tracking and recovering these errorsprovide the necessary referenceelements.c in hospitals, safety of persons can be

immediately affected by a failure.Operating theatres and reanimationwards are especially designed toensure a high level of dependability.c in industry, failures also considerablyaffect continuity of service. An articlewritten by Y. Lafarge and published in"Le Monde" quotes two examples:v for BSN (Danone), a 10 minuteshutdown causes a production loss of20,000 items,v for Peugeot, out of a production of1,650 vehicles a day, a one hourcomputer failure means 100 cars are

not manufactured, i.e. a loss of profit of4 million francs.It is thus easy to understand the impor-tance attached by firms to availability ofthe electrical power on which the entireactivity of the company depends.

Thus, in the commercial and servicesector and industry alike, failures mayhave economic consequences, causedamage or be a source of major risks.All of which may affect our everyday lifein which good service in 99% of cases(λ =10-2) would mean:c more than 140 new-born babies

would accidentally be dropped bydoctors and nurses each year;c no electricity or water for severaldozen hours each year;c your telephone and television wouldbe out of order for more than10 minutes a week;c 400 letters an hour would neverreach their destination.

These evocative images clearly showthe consequences of choice ofdependability level. The table infigure 17, although not complete, givesthe most important choices for an LV

installation and for the various sectorsof activity. To specify these choices, theneed must be defined and thedependability concepts examined in theprevious chapter must be implemented.




fig. 17: the sectors of activity and operating imperatives determine the system earthing arrangement and the solutions implemented depend on

the form used and on the degrees of protection required.

standardised(boards, modules

and connections)

non-standardised(individual wiring)

standardised

circuit-breaker/contactor combination

fuse/contactor combination

non-standardised

sectors of activity

commercial and service sectors industry

shops hospitals workshops plants process manufacturingthe problem to be solved:

types of incomingdiagrams

operating numerous mobile continuity of uncertain earth continuity of service continuity of serviceimperatives and portable loads, service for certain circuits (worksites), for certain for most of the

frequent changes sectors, supply by a public sectors, operation.to distribution risk of fire, power system. presence of backup risk of serioussystem, presence of generator sets. damage bysupply by a public generator sets. insulation faultspower system. (motors,

automation).risk of fire

recommended system TT IT TT IT ITearthing arrangements

numerous auxiliaries atmosphere and/or loads(machine-tools), corresponding to highloads with low insulation risk of insulation faults.resistance.

TN TN sub-system

solutions implemented:

component type fixed fixed fixedor disconnectable or disconnectable

or withdrawable or withdrawable

switchboard type fixed fixed with drawout unitsor with disconnectable subassemblies

or with drawout units

form F4 F4 F4 F4to to to to

F1 F2 F2 F2

degree of protection IP 5 5 5 5(first two numerals) to to to to

2 2 3 3

motor controlcomponents

low rate

high rate

technology ofcontrol and monitoringauxiliaries




4. future perspectives for switchboards

Modern switchboard technology hasbeen and continues to be greatlyinfluenced by the development ofpower management systems. We musttherefore look into the implications ofpower management on dependability.The reader will see that powermanagement provides the installationwith greater dependability byintegrating information processingelectronics in the LV switchboard whichthus becomes "intelligent".

Power managementPower management is already used inBuilding Management systems, whichhave gradually replaced morecentralised systems in industrial,commercial and even domesticapplications to supervise, monitor andcontrol the following standard functionsand facilities:c heating and air conditioning,c fire protection,c intrusion protection,

c access and worktime control,c lifts, lighting...,c energy tariff management.Power management is becoming moreand more decentralised for reasons ofavailability, user convenience andmodularity (already mentioned inchapter 1). Over and above thetraditional functions performed byelectrical equipment (protection,automation, transfer of loads to backupsources), a power managementsystem provides a number of functionsin the electrical control and monitoring

field.To cite a few examples:c automatic, progressive resumption offeeder supply after a fault,c alignment of consumption to energysupply possibilities at a specific time(load shedding and reconnection,generator startup and shutdown),c optimisation of sources according toconsumption to derive maximumadvantage from electricity contractswith differentiated tariffs,

c optimisation of capacitor bankoperation,c contribution to discrimination(coordination of protective devices).It also enables:c local and remote control andmonitoring (indications, alarms, controlsand setting modifications,...),c supervision (graphic representation ofsystem status, event logging andinstallation control).

The need for power management

increases with the need for availabilityand, more generally, for dependability.

Power management systems havebeen made possible by the introductionand widespread use of microprocessortechnology which at the same timeprovides an opening towards greater,distributed "intelligence".

power management forgreater dependabilityA power management system relies on

two principles when a failure occurs:c the electrical distribution system canremain as it is and is not at risk by failureof a management module. This issimplified by the use of bistable powercontrol devices such as switches,impulse relays and circuit-breakers.c the protection, control and monitoringsystems continue to be independentlyactivated, thus making operation incrippled mode possible. This principleensures the prime objective ofdependability even though certainfunctions of convenience aretemporarily lost. Thus even if thesupervision system fails, protectionfunctions will continue to fulfil their taskand the switchboard central unit willremain operational.

Moreover, power managementreinforces dependability of the LVinstallations in terms of:

c reliabilityv the power management systemreduces the major risk of failurerepresented by human intervention,

v complete information eliminates therisk of error in system management.

c maintainabilityReliability can be obtained by rigorousdesign, but product dependability alsorequires a high level of maintainability.There are two types of maintenance:v preventive maintenance is designedto anticipate problems and thus to limitthe risk of shutdown due to a fault (itprevents the fault from occurring),v curative maintenance is designed to

quickly restore the system to its"operating" condition (it locatesthe fault).Preventive maintenance takes priorityover curative maintenance as it avoidsproblems during operation. However itrequires sound knowledge of theproducts at all stages and the capacityto detect potential failures. Experimentsand tests on equipment can providethis knowledge and a powermanagement system can use it in anoptimum manner:v a preventive maintenance system is

established to reduce the number offailures, using the following:- operation counters,- insulation resistance measurementdevices...v a curative maintenance systemlocates the fault in the event of afailure.v two other systems, remotemaintenance and/or remotediagnostics, considerably enhanceswitchboard operation:- remote maintenance ensuressurveillance without the need for a

control room and a permanentmaintenance team on site. Remotetransfer of information on failuresmakes frequent inspections of thevarious electricity supply pointsunnecessary.- remote diagnostics enabletroubleshooting to be conducted on thebasis of quantifiable parameterstransmitted via a telecommunicationssystem. The reduction in maintenancetime is obvious, particularly when




outside suppliers are responsible formanagement and servicing of theinstallation. Remote diagnostics givethem the best chance of repairing the

failure on their first visit to the site.

c in terms of availabilityAvailability is naturally the result both ofreliability and maintainability, as wellas:v prevention of overloads with thesolution of load shedding andreconnection to prevent tripping,v management of sources (switching,coupling and startup of generator sets),v discrimination of the variousprotection levels which, as explainedabove, has an important role ininstallation availability.

c in terms of safetyv safety of persons is guaranteed byreflex protective devices (placed asclose as possible to the fault) which,although part of the managementsystem, can function independently if afault occurs.v maintenance operations are fewerand can often be scheduled, allowingpersonnel to work under less stress.v operating staff are guaranteedadditional protection by indication ofdevice status in maintenance areas,

and by warning of potential failures.

the technologyControl and monitoring "intelligence"must be organised with sufficient careto ensure a good level of dependability.It particularly calls for implementationof:v high-performance electronics,v communication networks usingreliable buses,v software of recognised reliability, foroverall control.

c electronic components and circuitsare today increasingly reliable, drivenby developments in the aerospace,military, nuclear and general publicsectors. The reliability levels are easilycontrolled, since the statistical reliabilitylaws associated with components areperfectly applicable and reliabilitycalculations for assemblies wellcontrolled.Critical points are backed up byredundancy of all or some parts of the

fig. 18: general diagram showing control and monitoring of an electrical installation and its links

(BUS) supervision.

electronic modules or by usingcomponents with increased reliability.

c buses are responsible for thedevelopment of decentralised intelligentsystems and form the communicationbackbone. The serial links making upthe buses enable the transfer of data tomany points via a single cable (coaxialor twisted pair). Their reliability hasrecently been upgraded and it is nowpossible to isolate them from externaldisturbances of the electromagnetictype and by using protocols includingmonitoring of information exchanges.This subject is developed inCahier Technique n° 147 "Introductionto digital communications networks".

c

system dependability also dependson that of the software controlling the

system. In this case, rather than arevolution, we witness a systematicrace for rigour at all levels, from designto commissioning (specification and

development methods, special tools,highly sophisticated verification and testprocedures).

the "intelligent"switchboardThe "intelligent" switchboard includes alarge part of the power managementsystem (see figure 18), in particular:v the "intelligent" electrotechnicalcomponents,v specific systems(e.g. insulation monitoring),v the switchboard central unit

supervision

MV UPS generator set (GE)

LV switchboard

central unit

- intelligent protection

and control/monitoring

components

- classical components

(via Input/Output modules)

reactive

energy

compensation

system

insulation

monitoring

system

intelligent switchboard




v the digital communications buses.Use of microprocessors means thatintelligence is distributed right throughto component level (circuit-breakers,

switches,...). In addition to their basicfunction, they process a variety ofinformation and communicate with theswitchboard central unit, thus ensuring:c "sequencing" of actions (logic andtime sequencing),c capacity to calculate and processmany pieces of digital information sentby these devices, sensors and specificsystems,c remote transmission by “serial” busenabling the control and monitoringsystem to communicate with theoperator and/or the supervision system,

c control and monitoring, both local andremote, as well as supervision (ordersare transmitted by bus).

The switchboard can now be said to be

"intelligent": the intelligence integratedin the power management system willdepend on the degree of complexity ofthe installation to be managed. Thedistribution of electrical power in smallcommercial applications may onlyrequire the display of measurementsand status information on the frontpanel of the switchboard, whereas inlarge buildings, remote controlfunctions are required (lighting, sourcechangeover,...).

Power management is at presentimplemented in MV and LV by means

of various components. Thesecomponents, more and morestandardised and convenient to use byelectricians, will be available in

increasingly wide ranges.The various components in theintelligent switchboard are designed towork together. The consistency, both interms of hardware and software,guarantees easy implementation anduse.

An "intelligent" switchboard withappropriate overall design and made upof consistent, carefully designed andmanufactured products, opens the wayto efficient power management and themastering of electrical power willgreater dependability.




5. conclusion

The distribution of electrical powermust meet increasing requirements interms of:c dependability,c upgradability,c user friendliness.

Designers thus aim at producinginstallations which are "intelligent",independent, communicating, modular,reliable and easy to service.These criteria can all be achieved bydecentralisation. The basic functions

(protection and control) are performedas close as possible to the application,and only supervision has a "central"position, playing a vital role indistribution management as regardsthe man/system relationship.Decentralisation is a design feature ofeach product, both in their connections(defined between them) and in theiroverall architecture.

All these items (high powercomponents, control and monitoringdevices and electrical connections) areintegrated in the LV switchboard. Its

role is thus vital for distribution as awhole, given that it has to guaranteeoverall dependability.The following points should be borne inmind:

c the incoming diagram and thereliability of the final loads are thepoints which may most handicapdependability,c the system earthing arrangementaffects availability of final loads andmust therefore be chosen carefully,c connections seriously affectswitchboard reliability, thus calling forcareful design and implementation,c switchboard technology, form, degreeof protection, connection,... must be

adapted to the environment in whichthe equipment is installed (degree ofpollution of premises, qualifications ofoperators,...),c withdrawable components are usedwhen they provide the addeddependability required,c drawout motor feeder units areparticularly used in process industriesfor the flexibility and increasedavailability that they provide,c auxiliaries with standardisedconnections and implementationguarantee the reliability of installation

control and monitoring.Dependability is everybody’s job,including that of the designer (the rightchoices from the start), the installer(implementation in accordance with the

manufacturer’s recommendations andproper practices) and the maintenanceengineers (surveillance and preventivemaintenance of critical points).

This Cahier Technique shows howdependability objectives can beachieved and how, by choosing theright options, particularly in terms oftechnology, the required level ofdependability can be obtained.

The "intelligent" LV switchboard,

associated with power management,meets the criteria of dependability anduser convenience particularly well,providing a solution for both presentand future needs. The degree of built-in"intelligence" required depends on thecomplexity of the installation.

This intelligent switchboard, designedto ensure maximum standardisation,integrates power, control andmonitoring and communication viabuses. Should changes be made to thedistribution system, switchboardmodularity and simple parameter

resetting of the control and monitoringsystem ensure easy upgrading . Thereis no need to redo studies and tests foreach application as the product hasalready been thoroughly tested.



appendix: bibliography

Standards

c NF C 12-101: protection of workers inbuildings implementing electric currents.

c NF C 15-100: rules for LV electricalinstallations.

c IEC 529: classification of degrees ofprotection provided by enclosures.(NFC 20-010; NF C 20-011; HD 365);

c NF C 20-030: electric shockprotection, safety rules.

c NF C 20-040: creepage distances

and clearances in air.

c IEC 439-1: low-voltage switchgearand controlgear assemblies.

Schneider Cahiers Techniques

c Analyse des réseaux triphasés enrégime perturbé à l’aide descomposantes symétriques,Cahier Technique n° 18B. DE METZ NOBLAT

c Méthode de développement d’unlogiciel de sûreté,Cahier Technique, n° 117A. JOURDIL, R. GALERA

c Introduction to dependability design,

Cahier Technique n° 144P. BONNEFOI

c Etude thermique des tableauxCahier Technique n° 145C. KILINDJIAN

c Initiation aux réseaux decommunication numériques,Cahier Technique n° 147E. KOENIG

c High availability electrical powerdistribution,Cahier Technique n° 148A. LONCHAMPT, G. GATINE

Various publications

c Les automates programmables sont-ils plus fiables que les relais?Revue J3E - October 1990F. SAGOT

c Experience in critical softwaredevelopment,IEEE Fault Tolerant ComputingSymposium, 26-28 June, 1990.NewcastleC. SAYET, E. PILAUD (Schneider)

c

Risque et sécurité dans le domainedu transport,Revue Maintenance - November 1990J-C LIGERON

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