Barrier vs Isolator

8/12/2019 Barrier vs Isolator

1/17

Shunt-Diode SafetyBarriersand GalvanicIsolatorsa CriticalComparison

By

LC Tow le BScCEng MIMechE MIEE MInstM C

Technology Director

The MTLInstrumen tsGroup plc

Introduction

The discussion on the rela tive meritso fga lvanic isola torsand

shuntdiode sa fety ba rriershasbeen carried on for many

years.The ma jority ofo ther artic leson thissub jecthave a imed

to prove the superiority ofone technique over the other.This

paper bringstogether the illustra tionswh ich have accumula ted

over a considerable time in response to variousquestions

ra ised a tpresen ta tions.Ita ttemptsto make ba lanced

argumen tsso tha tthe reader can decide the mostsuitable

technique for a particular applic a tion .

Itendsw ith a score sheetwh ich migh tbe found usefulifa

decision isnotcompletely selfrevea ling .

The pa per hasevolved over a considerable time,however itis

unlikely tha titcoversa ll aspectsand w ill inevitably need to be

revised astechniquesdevelop.Ifyou have any commen tson

the conten tsor omissionsw ithin thisdocumen tthe author

would like to receive them.In thisway the documen tw ill

become more comprehensive and more va luable.

June 1996


2/17

Um: 250v

Separately or together

Intrinsically safe interface

SAFE AREAHAZARDOUS AREA

1

Certifiedhazardous

areaapparatus

Uncertifiedsimple

apparatus

Certifiedinterface

Uncertifiedsafe areaapparatus

eg:

Computer

Shunt diode safety barrier


Barriers are usually described in terms of their safety parameters: 28V 93mA 300R is a

common barrier safety descriptionwhere VO= 28 volts Current Limiting Resistor (RO) = 300R

CLR

VO

IntrinsicSafety earth

IO

RO

3

Barriers

Concept 1958

Redding

Approved 1961

Gresham Kent

Isolators

Switch

Analogue

Telemechanique 1950'sPepperl & FuchsEvershed & Vignoles 1953MTL 1974

History 2

Figs1 and 2The basic function ofan intrinsica lly-

sa fe inter face isto remove the necessity

for cer tifying the sa fe-area equipment.

The equipmentin the sa fe area is

usua lly complex ,needsto be flex ible

and con ta insconsiderable power.

Therefore itcan injectsign ifican tleve ls

ofenergy into the haz ardousarea ,

par ticular ly under fau ltconditions.

An ide a linter face a llowsthe norma l

low energy sign a lto passw ith assma ll

a vo ltage drop asp ossible and w ith

very little attenua tion .Ifhowever a fau lt

deve lopsin the sa fe-area equipmen t

then the interface changesitstra nsfer

characteristic and restric tsthe energy

tra nsferred to the haz ardousarea to a

sa fe leve l.

Shunt-diode sa fety barrierswere

deve loped in the late 1950sas

processcon tro lcomputerswere more

wide ly applied to the chemica lindustry

and are genera lly regarded asb eing

the older techn ique.However,

intrinsica lly-sa fe re lay isolatorsfor

switch inputshave been ava ilable for

many years(qu ite how many Ihave notbeen able to establish)and ana logue

isolatorswere ava ilable in 1953 .The

recentgrowth in the use ofb oth typ es

ofisolatorsh asresulted fromimproved

performance and lower cost,notto any

change in fundamenta lprinciples.

Fig 3 and 4

Fig 3 illustra teshow a shun td iod esa fety barrier isconstructed so asto

limitthe currentand voltage ava ilable

fromthe haz ardous-area termina ls.The

fuse restric tsthe fau ltpower,the zeners

restric tthe voltage and the current

limiting resistor [CLR]restric tsthe

current.


3/17

Galvanic isolation

SAFETY

SEGR

EGATION

Hazardousarea

circuit

Safearea

circuit

Certifiedcomponent

Safe areaconnection

Certifiedtransformer

Power

Hazardous

area

connection

Energy-limiting

HAZARDOUS AREA SAFE AREA

4

Galvanic Isolator under fault condition

Isolated internalcomponents

Field mountedinstrument

Plant bond

Instrumentsystem

5

0V

Barrier under fault condition

Isolated internal

components

Field mountedinstrument

Plant bond

Instrumentsystem

L

N

E

BarrierBusbar

X1 X

6

The ga lva nic isolator illustra ted in Fig 4

breaksany d irectconnection between

sa fe-and haz ardous-area circuitsb y

interposing a layer ofinsula tion

between the two .The power tra nsfer is

usua lly via some formo ftra nsformer

and the return sign a lv ia an

op tocoupler,tra nsformer,or relay.The

fina lpower limita tion isachieved byusing a d iode resistor networkvery

similar to thato fa shun t-diode barrier.

Figs5 and 6Since the haz ardous-area circuitfrom

an isolator isnotd irectly connected to

the sa fe area circuit,itisusua lto

regard the fundamen ta laction as

effective ly block ing the excessive

energy atthe layer ofinsulation .In

practice the 0V ofthe instrumen tsystem

isnorma lly r eturned to the neutra lstar

po intfor interference avoidance and

s

afe

ty rea

s

ons

.The res

ult

antfau

lt

currentisthusre turned to the neutra l

star pointin the usua lw ay,rupturing

the protective fuse and remov ing the

fau lt,in a rela tive ly shor ttime .

The conventiona lfau ltconsideration of

the shun t-diode barrier isillustra ted in

Fig 6 where the fau ltcurrentisre turned

to the neutra lstar pointw ithin the sa fe

area in much the same way.The

importan td ifference isthatthe tra nsien t

vo ltage difference be tween the barrier

busbar and the neutra lstar point[X1X]

isnow tra nsferred to the haz ardous

area and hence mustbe restra ined to a

low leve l[lessthan 10V].In

consequence the busbar to neutra lstar

po intbond on the shun td iode sa fety

barrier mustb e oflow resistance and

be secure ,since itiscritica lto sa fety.


4/17

Comparisons

Barriers

Simple

Isolators

Complex (low MTBF)

Isolator has more components, comparison should considertotal functional components.

Isolator less vulnerable to supply variations

(70yrs)(35 yrs with relay)

(18yrs)

8

Comparisons

BarriersSimple

Versatile

Low dissipation

Loop powered

Tightly controlled supply

Restricted voltage available in hazardous area

Higher packing density

Safety earth fundamental

Imposes a reference 0 volt on system

Circuit must be isolated from earth in hazardous areaAccuracy and linearity higher (0.1%)

Lower cost

Good frequency response [100KHz]

Encapsulated , irrepairable

IsolatorsComplex low MTBF

Application specific

High dissipation (2VA)

Separate power supply

Wider range power supply

Higher voltage (power) available in both hazardous area andsafe area

Lower packing density

Safety earth reduced significance

Isolation between signals

Circuit may be earthed at one point in hazardous areaLower accuracy and linearity (0.25%)

Increased cost

Limited frequency response

Can be repaired

Vulnerable to lightning and other surges.

Acceptable solution in most parts of theworld

Less vulnerable to lightning and other surges.

Preferred solution in marine installations and Germaniczones of influence

7

Switch with Zener Barrier: preferred solution

Field switch isnormally closed, open

on alarm

MTL 787SDiode Return

Barrier

24V+

Relaycoil

IS Bond


9

Fig 7Fig 7 liststhe relative meritso fisola tors

and barriersand the sign ific ance of

these factorsvar ieswith the particular

insta ll ation .The rema inder ofthe

documen texpandsthese po intso f

compar ison so tha tthey can each be

eva lua ted .

Figs8, 9 and 10In genera lthe lower number of

componentsand basic simplic ity ofthe

shunt-diode sa fety barrier meansthey

are considered to be more reliable.

A more accura te compar ison hasto

compare the reliability ofan isola tor

with the barrier plusadd ition a l

componentsrequired to accomplish the

s

amefunc

tion .

For examp

leFig

s

9 and10 show the usua lswitch con tact

tra nsfer using a barrier relay

comb ina tion which should be

compared with the more complete

functionso fthe isolator.Thisreduces

the apparentsuperiority ofthe barrier.


5/17

Switch with Galvanic Isolator

Field switch is normallyclosed, open on alarm

Line faultdetect resistors


Load, alarmor shutdown

circuits

Supply

Plant reference potentialDigital Input

10

Comparisons

Barriers

Versatile

Isolators


Isolators usually designed to solve a particular problem withlimited flexibility

Barriers: more than one application, but require applicationalanalysis.

Compatability trials desirable in both circumstances

11

Figs11 and 12 and 16In genera lbarriers are more versa tile

than isola tors.For example the

MTL78 7S barrier ofFig 16 isidentic a l

with tha tused in the switch applica tion

ofFig 9 .

Ifthe flex ibility ofbarriersise xp loited

to solve a new app lic ation then an

an a lysistaking into accoun tp ossible

res

istive drop

s

andleakage curren

ts

is

desira ble ,asind icated in Fig 16 .

Whenever a d ifferentun tried

combination ofan intrinsica lly sa fe

interface and field moun ted equipmen t

isproposed ,itisadv isable to try an

experimenta linterconnection under

laboratory conditions.A sa tisfactory

tria lincre asesthe probab ility ofthe

fina linsta ll ation working .

Fig 12 sho wsan isola tor for use w ith a

convention a l4 to 20mA tra nsmitter.

The isolator isd esigned for use with this

type oftra nsmitter and isnotusefulfor

any other function .

Isolator for 2-wire transmitters


Load

Powersupply20-35Vdc

Typicallyup to 800R

Isolation

300R

28V17.5V4-20mA

12

Powersupply


6/17

Fig 13Isola torsrequire addition a lpower

wh ich u tilisesspace,d issipa tesmore

he a t and increasescost.The use ofa

we ll ventila ted (possibly force

ven tilated )cabine tb ecomesa necessity

ifisola tors are closely packed .

Itmustbe remembered thatfor both

barriers and isola torsthe maximum

permitted ambien ttemperature isthe a ir

temperature immed iately adjacen tto

the appara tusi.e.the temperature

inside the enclosure .Thistemperature

may be ra ised by other ad jacen t

electrica le qu ipmen t.

Figs14, 15, 16, 17, and

18Barriersmay have over-vo ltage

pro tection to coun teractlarge sup ply

va riationsbutthe protection absorbs

line volts.

Convention a lbarriersrestric tthe

av a ilable haz ardous-area line volts.For

ex ample,the conventiona ld iode return

barrier tra nsmitter combina tion ofFig16 hasa maximumline voltage drop of

0 .9V. By compar ison the MTL3041

circuito fFig 17 has5 .5V ava ilable for

line voltage which readily permitsthe

use ofloop powered loca lind ica torsas

shown in the d iagram.

Comparisons

Barriers

Low dissipation

Isolators

High dissipation

Isolators require 1-2VA for added signal power

Temperature rise in large cabinets can be a problem

Barriers dissipate very little 500mW500 isolators yield 1Kw

30C rise in 2 meter high unventilated rack

13

Comparisons

Barriers

Loop poweredTightlycontrolled supply

Isolators

Separate power requiredWide range power supply

Isolator - higher power available for both hazardous and safe

area. Variable supply

Barrier - dissipates some power but can be used in existing

circuits

14

Comparisons

Barriers

Restricted voltage in hazardous area

Isolators

Higher voltage (power) available inhazardous area

Isolators inject voltage to replace losses

Barriers absorb available voltage

15


7/17

Active barrierssuch asthe MTL706

shown in Fig 18 permitmore line

vo ltage and are toleranto fsup ply

vo ltage varia tion .They consume

add ition a lpower,buto ffer an

a lternative to isolatorsifthere isa

strong preference for a barrier solution .

Transmitter with diode return barrier (MTL 787S)


MTL 787S+

V+25.1V26.0V

250R

IS Bond

1-5V

0V

340R max

20R + 0.9V

4-20mAInstrument

12V

6.8V

1.3V

16

Loop-powered 4-20mA Indicator


12.73 C

16V

4-20mA

Power

supply +20-35V

dc.MTL 3041

Isolation300R

28V17.5V4-20mA12V

1V

17

15V 28V

300R

40mA max

50mAProtect

Regulate

Floatingsupply

4-20mA

250Rload

22V-35V

0VIS Bond

TX

MTL 706 schematic


18


8/17

Figs19 and 20The number ofinterfacesw hich can be

moun ted in a cabine tislargely

de termined by the size ofcabling to be

accommod a ted and the degree of

accessibility to be achieved .Faci lities

for crosswiring a lso use considerable

amoun tso fspace.

The numbersquo ted are those for

generously designed racksand higher

density can be achieved a tthe expense

ofaccessibility and temperature rise .

The figuresav a ilable do how ever

demonstra te the higher pack ing density

wh ich can be achieved with barriers.

The number ofmechan ica lvar ia tions

av a ilable in bo th barriers and isolators

isincre asing and each hasitsp ar ticular

merits.

The d iscre te use ofmultichannelun its

can a lso effective ly incre ase the number

ofchannelsper cab ine t.However

sing le loop integrity issacrificed and

some thoughtasto whether thisis

sign ifican tor notisessen tia l.

Figs21, 22, 23, 24 and

25There isnorma lly much emph asison the

necessity for correctbond ing ofthe

shunt-diode sa fety barriers0 volt

busbar.Ho wever the developmen to f

the required bonding systemfromFig

22 showing the requirementsfor

structure ,computer 0V and screen to

the sligh tly mod ified systemo fFig 23

illustra testha tthere isno d iff icult

requirement.To avoid sign ifican t

differencesw ithin the haz ardous area

the bond should have a low resistance

[0 .1!]and be ofh igh integrity.The

Comparisons

Barriers

High packing density

Isolators

Low packing density

19

Isolators physically larger than barriersFor some functions barriers require additional equipment

In practice rack size is affected by cable trunking, termination of screens

and armourTraditionally U.K. barriers were busbar mounted

DIN rail mounting for both barriers and isolators now commonplaceBackplane mounting, with plug in facilities available for both interfacesMultichannel interfaces save space, but sacrifice individual loop integrity

Interface Numbers in Cabinet

Cabinet

600 x 600 x 2100mm without crosswiring

20

MTL 700 400 unitsMTL 7000 650 units

MTL 3000 192 units allows for Power SupplyMTL 4000 256 units

Comparisons

Barriers


Isolators

Safety earth has reduced significance

Isolators - require consideration of bonding of screens or bond

for interference avoidance

Barriers - return path for fault current essential

21


9/17

Computer

Standard bonding practice 22

24V

0V

Computer

Shunt diode safety barrier bonding system

0V

23

24V

SAFE AREAHAZARDOUSAREA

possib ility ofinsta ll ing a second lead

(asind ica ted by the broken line)for

mon itoring purposesshould be

considered. However the presence and

integrity ofthe bond isessentia lin a ll

typ eso finsta ll a tion .

The isolator bonding systemillustra ted

in Fig 24 isidentica lw ith tha to fthe

non-hazardoussystem,butthe 0V bond

ha ssome sa fety implications.

The overa ll e ar thing systemillustra ted in

Fig 25 emph asisestha tthe instrument

systembonding isonly a parto fthe

tota lsystem.The requiremen tsare only

marg ina lly a ffected by whether barriers

or isola torsare used .

Galvanic isolator bonding system 24

24V

Galvanic Isolator

0V



10/17

Plant bonding 25

Barrier0V

Interfacecabinet

System cabinets

Screen

Armour

Plant bond(structure + conductors + soil)

0V Powerbusbar

0V Cleanbusbar

Electricaldistribution

busbar

Motor

Plant Control room Sub stationtransformer

Computer0V

Powersystemearth

Lightningearth

Luminaire

Comparisons

Barriers


Isolators

Isolation between signal and power supply

Isolators usually have 3 port isolation. Interaction betweencircuits reduced. Fault mechanisms easier to define

Barrier 0 volt and computer 0 volt usually interconnected.Two channel barriers create pseudo 'earth free' circuits

Barrier circuits bonded at barrier '0' volt, hence must be isolated

elsewhere.Isolation necessary for transfer across a hazardous area or toremote situation

26

SAFE AREAHAZARDOUS AREASAFE AREA

MTL 3058 (RS232C)or MTL 3059 (RS422A)

RS232 or RS422link to

process controller

20-35V dc Common

Tx

Rx

Tx

Rx

RS232 or RS422link to

process controller

20-35V dc

MTL 3058 (RS232C)or MTL 3059 (RS422A)

Serial data transfer across a hazardous area 27

Figs26, 27 and 28

Where sign a lshave to be tra nsferred

acrossa haz ardousarea using

intrinsica lly-sa fe cabling ,the preferred

technique isto use ga lva nic isolation at

bo th ends.Fig 27 illustra tesa system

frequently used between ana lyser

houses and con tro lro om.

Ifa sign a lh asto be tra nsferred to a

remote loca tion in the sa fe area ,Fig

28 ,the use ofan isolator atthe

inter face r emovesconcern over

possible po ten tia ld ifferencesb etween

ear th mats.The extra voltage needed to

drive the interconnecting cablesisa lso

useful.For these reasonsisola torsare

the preferred solution ,in these

par ticular circumstances.


11/17

Plant Bond

PlantMainsSupply

PlantSupply

Plant DataCollection

Centre Distance > 200m

RemoteComputer

Centre

0V

Signal transfer to remote computer


28

Hazardous area isolation

Barriers

Circuit must be isolated from earth in thehazardous area

Isolators

Circuit may be earthed at one point inhazardous area

Requirement of intrinsically safe circuits

Earthed at one point only Elsewhere insulated to 500V

If sensor already earthed eg. bonded thermocouple, best to use

an isolator Universally accepted

Barriers can be used with potential equalising conductor in some

countries (eg UK), but messy

29

Fig 29

There isa genera lrequirementtha t

intrinsica lly-sa fe circuitsshould be

ear thed atone pointonly and

elsewhere isola ted to w ithstand a 500V

insula tion test.Itfollowsthatwhere a

sensor or some other field appara tus

cannotb e insula ted (eg .a pH sensor)

then the preferred solution isto use an

isola tor.In some coun triesthe code of

practice permitsthe using of

equipo ten tia lbond ing conductorsbut

thisisnota universa lly accep ted

practice and should be avoided ifa ta ll

possible.

Comparisons

Barriers

Accuracy (0.1%) and linearity higher

Isolators

Accuracy and linearity lower (0.25%)

Isolator process for analogue signals has several conversions.

hence lower accuracy

Barrier does not distort current signals. Leakage currents very

small

Digital signals do not lose accuracy

30


12/17

Figs30, 31, 32 and 33

A lthough the changing techno logy of

isolatorsh asincre ased the ir accura cy,

they are in genera llessaccura te tha n

barrier systems.Usua lly tra nsfer

accura cy isadequa te butthe

temperature coe ff ic ien trema ins

sign ifican t.

Figs31 and 32 show comparable load

cell systemswhere every e ffortismade

to achieve maximumaccura cy.The

barrier load cellsystemcan achieve

0 .05%w ithoutd iff iculty, and the

isolator systemw ould achieve 0 .25%if

the amb ienttemperature varied by

25C .

Fig 33 showsa Smar ttra nsmitter which

when used with a 4 to 20mA ana logue

sign a lw ould expectto genera te a

0 .2%error a tthe interface.If,

however, the d igita lrepresenta tion of

the measurementw asused then the

isolator would no tintroduce a ny error.

Itfollowstherefore thatwhere isola tors

are the preferred solution for use w ith

Smar ttra nsmittersand the highest

accura cy isrequired then the dig ita l

sign a lshould be used .

HAZARDOUS AREA

6-wire connection minimised error

SAFE AREA

SupplySupply

Sense

Signal

0V

End-to-endresistancevariation

No voltage drop due

to high impedancecircuits

Line loss variation

Constantexcitationvoltage

31

HAZARDOUS AREA

Intrinsically safe load cell power supply

SAFE AREA

Multiplecompensatedload cells

MTL3992

+20to 35V

0V

Intrinsically safe power supply

Certified

Interface

32

Plug-in isolator with transmitter


Smarttransmitter

+

HHC*

HHC*

I

I

MTL404120-35V dc

Load4/20mA

+

*Hand-held communicator

33


13/17

Comparisons

Barriers

Lower cost

Isolators

Increased cost

Isolators for analogue signals more expensive than barriers

Switch inputs cost difference small. Possibly lower cost per

channel with isolators

34

50

Costperchannel

Cost of switch interfaces

MT

L787S

Shuntdiodebarrier

Singlechannel

Isolator

Two

channel

Is

olator

Threechannel

Isolator

35

Figs34, 35 and 36

Costcompar isonsare a lwaysd iff icult

because there isvery rare ly a precise

coincidence offunction .There is

however a genera lp ercep tion tha t

shun td iode sa fety barriersare less

expensive than isola tors.

In practice for switch tra nsfer purposes

there islittle difference in costp er

channelb etween the two techn iques.If

ind ividua lloop integrity and minimum

faci litiesare accep table the

multichannelswitch isola torsare lower

in cost than corresponding barrier

solutionsasillustra ted in Fig 35 .

An a logue isola torsare more complex

than the corresponding barriersand the

costd ifference be comessign ifican tas

shown in Fig 36 .

However,e xcep tfor very large

insta ll a tionsthe costd ifference israrely

the decid ing factor.

50

Cost of analogue transmitter interfaces

MTL787S

Shuntdiode

barrier

MTL706

Acticeshuntdiodebarrier

MTL3041

Isolator

MTL3046B

Smartisolator

100

36


14/17

Comparisons

Barriers

Good frequency response

Isolators

Limited frequency

Isolators - limited frequency spectrum

Designed for specific frequency band

Barriers - 3dB at 100khz

Note: Diode capacitance is non linear with voltage

37

HAZARDOUS AREA

Isolating interfaces for smart transmitters

SAFE AREA

28V

MTL3046B

300R

15V

HHC* HHC*

Load

Power supply20-35V dc

4/20mA

Isolation

38

*Hand-held communicator

Comparisons

Barriers

Encapsulated

Isolators

Can be repaired

Isolators usually have fused primary and can theoretically be

repaired

Usually not economic

Barriers encapsulated, hence throw away item

39

Fig 37 and 38In genera lthe frequency response ofa

barrier isd etermined by the va lue of

the currentlimiting resistor and the

diode capaci tance .The d iod e

capaci tance isnon-linear with voltage

and hence some d istor tion ofany high

frequency sign a lsinevitably takesp lace

and a discussion on frequency

response can be mislead ing .Ifa system

frequency ish igher than 50kHz the

on ly solution isto try the system

experimen ta lly,attempting ifp ossible to

a llow for the effectso finterconnecting

cables.

Isola torsin genera lhave to be

designed to operate a tthe spe ci fic

frequency tra nsmitted .For example,

with Smar ttra nsmittersthe inter face

MTL3046B illustra ted in Fig 38 works

with the ma jority oftra nsmitters,butnot

a ll .Itistherefore essen tia lto check

compa tib ility be tween the particular

appara tusand the isolator to be used .

Fig 39

In genera

lrepa ir i

s

nota prac

tica

l

proposition .

Some thoughton the accep tability of

barriersw ith interchangeable fuses

needsto be g iven iffrequentd amage is

an tic ipa ted and cannotbe avo ide d.


15/17

Comparisons

Barriers

Vulnerable to lightning and other surges

Isolators

Less vulnerable to lightning and othersurges

Transient surges caused by lightning and faults in electrical

apparatus cause potential differences across plant

Equipment in Zone O principal cause for concern

Barriers used with surge suppression units cause multiple earth

problems

40

Shunt diode safety barrier fault current


41

Vxy

THC

Zone O X Fault current

Protective conductor and structural bond Y

Instrumentation

ProtectionNetwork

Shunt DiodeBarrier

P E

SurgeSuppressionNetwork

10,000A

1000V

Zone O

100,000A

Instrumentation

90,000A

Protective

Network


Preferred isolator - surge protection solution 42

Figs40, 41 and 42

Where there isa sign ific an tprobability

ofp oten tia ld ifferencesacrossa plan t

due to severe electrica lfau ltsor

ligh tning then isola torshave grea ter

immunity to damage .

Fig 41 illustra tesa possible problem

with barriers.A fau ltcurrentfromthe

electric motor returnsv ia the plan tbond

generating a vo ltage between pointsX

and Y.Ifthisisthe case,breakdown

occursb etween the thermocoup le and

tankcre a ting a potentia lhaz ard.The

shun td iode barrier w ill sacrific ia lly

pro tectthe instrumen tation .

Intrinsica lly-sa fe isola torsare routine ly

tested to withstand 2 .5kV rmsan d

hence offer some degree ofp ro tection

to the instrumen ta tion system.

Figs42 sho wshow the use ofan

isola tor tog ether w ith a suitable surge

suppression networkcan preventan

unaccep table vo ltage d ifference

occurring w ithin the Zone 0 .


16/17

Overiding considerations

If installation already exists and is predominantly a shuntdiode safety barrier or an isolator installation then continuethe same practice

1) Training of technicians

2) Spares

3) Mounting probably cheaper and more convenient

43

Particular situations

Barriers

Acceptable in most areas of the world

Isolators

Preferred solution in marine installationsand German zones of influence

Marine regulations propose isolated circuits to avoid currents

in hull

Easier to use isolators than argue about 2

channel barriers

German attitude German engineers prefer galvanic isolation

particularly in Zone 0

Easier to to agree than convert

44

Fig 43The need to minimise the number of

techniquesused within an insta ll a tion is

ob vious.Usua lly therefore ,ifa plan t

a lready usesshun t-d iode barriersor

isolatorsin mostcircumstancesitis

be tter to ma inta in a consistentpra ctice.

Fig 44

H istoric a lly,the need to avoid

circula ting currentin shipshu llsh asled

to the preferred practice ofusing

isolated circuits.In genera lusing shun t-

diode sa fety barriersd oesnotlead to

any problemsbutthe argumen tsare

long and expensive .

Similar ly, G erman engineersare

tra ined to use and prefer ga lva nic

isolated circuits.

Figs45 and 46

The score sheetsprincipa lfunction istomake sure you consider each ofthe

con tributing factors.

Ifyou have a pre ferred techn ique then

pursue tha tline ,since either solution

will probably be sa tisfactory.

Score sheet

Allocate marks out of 10 if the requirement is important.eg. 8-2

Allocate marks out of 6 if the requirement is significant.

eg. 2-4

Allocate no marks if the requirement has no significance.

eg. 0-0

If total show a significant preference then follow the choice

If the argument is balanced then follow your personal preference

Whatever you do, use MTL equipment

45


17/17

Score sheet

BarriersPrefer Barriers

Existing system predominately barriers

Simple

Versatile

Low dissipationLoop powered

Tightly controlled supply

Restricted voltage available in hazardous area

Higher packing density



Circuit must be isolated from earth in hazardous area

Accuracy and linearity higher (0.1%)

Lower cost

Good frequency response [100KHz]

Encapsulated , irrepairable

IsolatorsPrefer Isolators

Existing system predominately isolator

Complex low MTBF


High dissipation (2VA)Separate power supply

Wider range power supply

Higher voltage (power) available in both hazardous areaand safe area

Lower packing density

Safety earth reduced significance

Isolation between signals

Circuit may be earthed at one point in hazardous area

Lower accuracy and linearity (0.25%)

Increased cost

Limited frequency response

Can be repaired

Vunerable to lightning and other surges.

Acceptable solution in most parts of the world

Less vulnerable to lightning and other surges.

Preferred solution in marine installations and Germaniczones of influence

46

Rating

Total

Rating

Total

Barrier vs Isolator

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