Intrinsically Safe Instruments Design

Making instrumentsintrinsically safe need notseem like a nightmare. Here,the basics of intrinsic safetycircuit design are discussed.

Paul S. Babiarz

Intrinsic safety prevents instrumentsand other low-voltage circuits inhazardous areas from releasingsufficient energy to ignite volatilegases. Although it is used widely inEurope to safely install and operateinstrumentation circuits inhazardous areas, it has causedmuch confusion in North Americanmarkets. Many users have heard ofit and want to know more; however,most feel uncomfortable applyingintrinsically safe products. Onereason is that intrinsic safety hasbeen a part of Section 504 of theNational Electric Code only since1990. In addition, the number ofdifferent products on the market andseemingly endless calculationsmake applying intrinsic safety seemlike an engineer’s nightmare.

This is the first of a series of shortarticles that explain how to makethe most common field devices(thermocouples, RTDs, contacts,solenoid valves, transmitters, anddisplays) intrinsically safe. We willbegin with an introduction to thepractical side of intrinsic safetycircuit design.

Start With The Field DeviceAll intrinsically safe circuits havethree components: the field device,referred to as the intrinsically safeapparatus; the energy-limiting

device, also known as a barrier orintrinsically safe associatedapparatus; and the field wiring.When designing an intrinsically safecircuit, begin the analysis with thefield device. This will determine thetype of barrier that can be used sothat the circuit functions properlyunder normal operating conditionsbut still is safe under fault conditions.

More than 85% of all intrinsicallysafe circuits involve commonlyknown instruments. Figure 1 showsthe approximate use of intrinsicallysafe apparatus in hazardous areas.

An intrinsically safe apparatus (fielddevice) is classified either as asimple or nonsimple device. Simpleapparatus is defined in paragraph

3.12 of the ANSI/ISA-RP 12.6-1987as any device which will neithergenerate nor store more than 1.2volts, 0.1 amps, 25 mW or 20 µJ.Examples are simple contacts,thermocouples, RTDs, LEDs,noninductive potentiometers, andresistors. These simple devices donot need to be approved asintrinsically safe. If they areconnected to an approvedintrinsically safe associatedapparatus (barrier), the circuit isconsidered intrinsically safe.

A nonsimple device can create orstore levels of energy that exceedthose listed above. Typical examplesare transmitters, transducers,solenoid valves, and relays. Whenthese devices are approved asintrinsically safe, under the entityconcept, they have the followingentity parameters: Vmax (maximumvoltage allowed); Imax (maximumcurrent allowed); Ci (internalcapacitance); and Li (internalinductance).

The Vmax and Imax values arestraightforward. Under a faultcondition, excess voltage or currentcould be transferred to theintrinsically safe apparatus (fielddevice). If the voltage or currentexceeds the apparatus’ Vmax orImax, the device can heat up orspark and ignite the gases in thehazardous area. The Ci and Livalues describe the device‘s abilityto store energy in the form ofinternal capacitance and internalinductance.

Figure 1. Current use of intrinsically safeapparatus in hazardous areas.

Figure 2. Barrier circuit

Safe Area Intrinsically Safe Barrier Hazardous Area

FuseCurrent Limiting

Resistor

ZenerDiodes

InputVoltage

FieldDevice

Instrinsically SafeGround

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Intrinsic Safety CircuitDesign

Intrinsically Safe Intrinsically SafeApparatus Applications(% )

Switching 32.0%mechanical switches 85.0%proximity switches 15.0%

2-wire transmitters 22.0%Thermocouples & RTDs 13.0%Load cells 8.5%Solenoid valves 4.5%Potentiometers 2.5%LEDs 2.0%I/P transducers 2.0%Other devices 13.5%

Total field devices 100.0%

Limiting Energy To The FieldDeviceTo protect the intrinsically safeapparatus in a hazardous area, anenergy-limiting device must beinstalled. This is commonly referredto as an intrinsically safe associatedapparatus or barrier. Under normalconditions, the device is passiveand allows the intrinsically safeapparatus to function properly.Under fault conditions, it protectsthe field circuit by preventing excessvoltage and current from reachingthe hazardous area. The basiccircuit diagram for an intrinsicallysafe barrier is shown in Figure 2.

There are three components to abarrier that limit current and voltage:a resistor, at least two zener diodes,and a fuse. The resistor limits thecurrent to a specific value known asthe short circuit current, Isc. Thezener diode limits the voltage to avalue referred to as open circuitvoltage, Voc. The fuse will blowwhen the diode conducts. Thisinterrupts the circuit, which preventsthe diode from burning and allowingexcess voltage to reach thehazardous area. There always are atleast two zener diodes in parallel ineach intrinsically safe barrier. If onediode should fail, the other willoperate providing completeprotection.

A simple analogy is a restriction in awater pipe with an overpressureshut-off valve. The restrictionprevents too much water fromflowing through the point, just likethe resistor in the barrier limitscurrent. If too much pressure buildsup behind the restriction, theoverpressure shutoff valve turns offall the flow in the pipe. This is similarto what the zener diode and fuse dowith excess voltage. If the inputvoltage exceeds the allowable limit,the diode shorts the input voltage to

ground and the fuse blows, shuttingoff electrical power to the hazardousarea.

When conducting the safetyanalysis of the circuit, it is importantto compare the entity values of theintrinsically safe apparatus againstthe associated apparatus. Theseparameters usually are found on theproduct or in the control wiringdiagram from the manufacturer (seeFigure 3).

Will The Circuit Work?It also is important to make surethat the intrinsically safe circuit willwork under normal conditions. Withthe current-limiting resistor, avoltage drop will occur between theinput and output of the barrier. Thishas to be accounted for in yourcircuit design. In the subsequentarticles in this series, a step-by-stepexplanation will be given on how tocalculate these voltage drops andmake sure that the circuit is safe.

Determining Safe EnergyLevelsVoltage and current limitations areascertained by ignition curves, asseen in Figure 4. A circuit with acombination of 30 V and 150 mAwould fall on the ignition level ofgases in Group A. This combinationof voltage and current could createa spark large enough to ignite themixture of gases and oxygen.Intrinsically safe applications alwaysstay below these curves where theoperating level of energy is about1 watt or less. There are alsocapacitance and inductance curveswhich must be examined inintrinsically safe circuits.

The purpose of this series ofarticles is to simplify the applicationof intrinsic safety. Consider theignition curves to demonstrate apoint about thermocouples.

Z

Associated Apparatus Apparatus(barrier) (field device)

Open circuit voltage Voc ≤ VmaxShort circuit current Isc ≤ ImaxAllowed capacitance Ca ≥ CiAllowed inductance La ≥ Li

Figure 3. Comparison of the entity values ofan intrinsically safe apparatus andassociated apparatus

Figure 4. Ignition curves – resistance

Ign

itio

n C

urr

ent

(A)

Open Circuit Voltage (V)

Groups C and DMethanePropaneEthylene

Groups A and BHydrogen

1010 mA

20 mA

50 mA

100 mA

200 mA

500 mA

1A

2A

5A

20 30 40 50 100 200

Z-132

A thermocouple is classified as asimple device. It will not create orstore enough energy to ignite anymixture of volatile gases. If the energylevel of a typical thermocouplecircuit were plotted on the ignitioncurve in Figure 4, it would not beclose to the ignition levels of themost volatile gases in Group A.

Is the thermocouple which isinstalled in a hazardous area(Figure 5) intrinsically safe? Theanswer is no, because a fault could

occur on the recorder which couldcause excess energy to reach thehazardous area, as seen in Figure 6.To make sure that the circuitremains intrinsically safe, a barrierto limit the energy must be inserted(Figure 7).

The next installment in this serieswill explain how the selection ismade for thermocouples and RTDs,which comprise about 13% of allintrinsically safe applications.

BEHIND THE BYLINEPaul B. Babiarz is marketing managerof intrinsically safe products for Crouse-Hinds. He holds a B.S. from theUniversity of Rochester, an M.S. fromthe University of Michigan, and anM.B.A. from Syracuse University. Hehas more than 13 years of experiencein working in hazardous areas and hasspecialized in intrinsic safety.Copyright Instrument Society ofAmerica. Intech, October, 1992.All Rights Reserved.

NON-HAZARDOUS SIDE HAZARDOUS SIDE

Recorder


Recorder

110 V FAULT explosionpossible

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Maximum 0.1 volt produced by thermocouple

Intrinsic Safety Circuit Design Cont’d

Figure 7. Thermocouple with barrier

Figure 6. Thermocouple with fault

Figure 5. Thermocouple installed in a hazardous area


RecorderInstrinsically

Safe Barrier

Z

GRD

+

–

ControlRoom

NON-HAZARDOUS SIDE HAZARDOUS SIDECLASS I,II, IIIDIVISION 1GROUPS A-G

Thermocouples are simple devices.Do not need approval

Use same type of wire

Use same type of wire

Note: Use consistent wiring

Safety BarrierParameters

VN: 2.5 V Ri: 71 Ω

V

V

RTD

Voltmeter

+-

Figure 1. Current use of intrinsically safe apparatus in hazardous areas.

Intrinsic Safety CircuitDesign–Part 2

Paul S. Babiarz

When thermocouples and RTD’s(resistance temperature devices)are installed in hazardous areas,barriers are required to make theircircuits intrinsically safe. Theseintrinsic safety barriers preventexcess energy from possible faultson the safe side from reaching thehazardous area. Without thebarriers, excessive heat or sparksproduced by the fault conditioncould ignite volatile gases orcombustible dusts.

Hundreds of different barriers areavailable from North Americansuppliers. The multitude of productscan give control engineersnightmares as they try to select theproper barrier for commoninstrumentation loops. The searchcan be simplified, however. Onetype of barrier can be selected tomake all thermocouples and RTD’sintrinsically safe so that polarityproblems are avoided andcalculations are not necessary.

Normally, the design of allintrinsically safe circuits centers onone of two approaches: theuniversal approach, which uses auniversal device that often isisolated so that a ground for safetyis not required; or the engineeredapproach, which uses groundedsafety barriers.

Isolated temperatureconverters. These universaldevices measure temperature inhazardous areas, but at a highercost. (Dispensing with the need for aground is a convenience that maycost two to three times as much asgrounded safety barriers.) Isolatedtemperature converters accept alow-level DC signal from athermocouple or 3-wire RTD andconvert it into a proportional 4-20 mA signal in the safe area.•They also are available with setpoints that trip an on-off signal to

Fault conditions in hazardous-area temperature sensors can be explosive without the properprotection.You can safeguard all of the devices in your application with one type of intrinsicsafety barrier.

Figure 2. Typical values of barrier in thermocouple circuit.

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the safe side when the temperaturereaches a designated level.These units must be approved asintrinsically safe.

Advantages of isolated temperatureconverters as compared togrounded safety barriers include:• Good signal response• No ground required for safety

• More versatile application• One product for all applications

Disadvantages include:• Larger in size• Requires calibration• More expensive• May not work with all

thermocouples and RTD’s

Intrinsic Safety Barrier

Grounded safety barriers.These are passive devices thatprevent all excess energy from afault occurring on the safe side fromreaching the hazardous area.Under normal conditions, thebarriers allow the circuit to functionproperly by allowing the signals topass between the field device andthe control room. In a faultcondition, the barriers limit voltageand current to levels that are notsufficient enough to ignite gases.For a more detailed explanation,refer to Part 1 of this series.

Advantages of grounded safetybarriers as compared to isolatedtemperature converters include:• Less expensive• Precise signal response• Very small (less than 1⁄2 in. wide)• Simple application • One barrier for all types of

thermocouples and RTD’sDisadvantages include:• Requires ground• Requires some engineering

Examine The BarrierParametersArticles in this series will focus onmethods to select the proper groundedsafety barriers. Before we analyzethermocouple and RTD circuits, weshould examine the functionalparameters necessary to select theproper barrier. These parametersare: polarity of circuit; rated voltageof barrier; and resistance of barrier.

Polarity. The circuit’s polaritymust be known in order to choosethe correct type of barrier. DCbarriers are rated either as positiveor negative. AC barriers can beconnected to circuits with either apositive or negative supply.SIGNAL & RETURN barriers areused for transmitter and switchingapplications. All of these barriersare available in single- ordouble-channel versions. However,because double-channel barrierssave space and money by beingconnected to two legs of a loop,they are becoming the standard.

V

V Voltmeter

A

B

C

Figure 3. 3-wire RTD bridge

V Voltmeter

A

C

BV

Intrinsic Safety Barrier

R1

R2

Note: When R1 = R2, bridge remains balanced

Figure 4. 3-wire RTD bridge with barrier

Rated voltage. Like anyelectrical device, safety barriershave a rated nominal voltage, Vn,referred to as working voltage. Thebarrier’s Vn should be greater thanor equal to the supply to the barrier,much like the rated voltage of alamp must be equal to or greaterthan the supply to it. If the voltagesupply to the barrier is much greaterthan its Vn, the barrier will sense afault. The protective zener diodeswill conduct, causing leakagecurrents and inaccurate signals onthe loop. Most barriers have a ratedworking voltage that guarantees aminimal leakage current from 1 to10 micro amps if it is not exceeded.If the supply voltage to the barrierbecomes too high, the zener diodewill conduct. The resulting highcurrent through the fuse will causethe fuse to blow. Excess supplyvoltage is the main reason whygrounded barriers fail.

Internal resistance. Every safetybarrier has an internal resistance,Ri, that limits the current under faultconditions. Ri also creates a voltagedrop across the barrier.This drop canbe calculated by applying Ohm’slaw, V=IR. Not accounting for thevoltage drop produces the mostproblems in the proper functioningof intrinsically safe systems.

Thermocouple SystemDesign Pointers Polarity. A thermocouple has twowires, each with a positive andnegative polarity. Two single-channelbarriers, each with the properpolarity, could be used. Problemswould occur if the positive leg to thethermocouple were connected tothe negative terminal of the barrier orvice versa. There are two possiblebarrier choices for thermocouplecircuits:

Thermocouple circuit with one positive and one negative lead

1 standard DC barrier, positive polarityand

1 standard DC barrier, negative polarityOR

1 double AC barrier

When barriers and thermocouplesare being installed, the technicianmay forget which wire is positive andwhich is negative. To avoid polarityproblems on the terminals, a doubleAC barrier should be used. Thewires can be connected to eitherterminal and the circuit will functionproperly as long as thermocouplepolarity is maintained throughout.

Rated voltage. A thermocoupleproduces a very small voltage (lessthan 0.1 V). It is connected to avoltmeter which has a high impedanceand which requires a very smallcurrent. Since the thermocoupleproduces such a small voltage,choose a double AC barrier with ahigher rated nominal voltage (Vn). Asurvey of most double AC barriers onthe market shows that they are ratedat low nominal voltages from 1 V andhigher.Select one between 1 and 10 V.

Internal resistance. Since themV signal has a very small currentand is going to a high-impedancevoltmeter, the resistance of thebarrier will not influence circuitfunction. A simple rule of thumb isthat when a signal is going to ahigh-impedance voltmeter, aninternal barrier resistance of lessthan 1000 ohms will not affect themV signal. It usually is goodpractice, however, to select a barrierwith a low resistance in case thecircuit is modified later.

Barrier selection. For properoperation of thermocouples inhazardous areas, select safetybarriers based on the followingparameters:• Barrier type: double-channel AC

barrier to avoid polarity problems• Rated voltage: Barrier Vn > 1V• Internal resistance: barrier with lowest

resistance (less than 110 ohms)

Safety and installation check.Since the thermocouple is a simpledevice, it does not need third-partyapproval. Make sure that the barrierhas the proper approvals forhazardous area locations. Thethermocouple wires will be different

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Z

from terminal connections on thebarrier. Always use consistent wiringfrom the thermocouple to the barrierand then to the control room. Thiswill cancel any thermocouple effectcaused by the dissimilar metals onthe barrier connection.

RTD System DesignPointersRTD’s come in 2-, 3-, and 4-wireversions. The 3-wire RTD is used inmore than 80% of all applications.The 2-wire version is not asaccurate and is used mostly in theheating, ventilation, and airconditioning industry for set-pointtemperature measurements. The4-wire RTD provides the mostaccurate signal, but is moreexpensive and requires one moreextension wire to the process area.

Understanding RTD accuracy isessential in selecting the correctbarrier. Many RTD measurementsare in the form of a Wheatstonebridge, whose output voltage is afunction of the RTD resistance. Thebridge requires four connectionwires, an external source, and threeresistors that have a balancedtemperature coefficient. The RTDnormally is separated from thebridge by a pair of extension wires.

With a 2-wire RTD, the impedanceof the barrier in series with the RTDwill cause an imbalance on thebridge and will affect the accuracyof the temperature reading. This

GRD

Currentloop

signal

NON-HAZARDOUS SIDE

Item 1

Item 2

HAZARDOUS SIDECLASS I,II, IIIDIVISION 1GROUPS A-G

RTD’s are simple devices.Do not need approval.

RTD

This channel can be used to serve part of loop #2.

Safety BarrierParameters

VN: 2.5 V Rj: 71 Ω

GRD

effect can be minimized by using athird wire to measure the voltage(refer to Figure 3 for this discussion).If wires A and B are perfectlymatched and if the resistance inboth channels of the barrier is thesame, the impedance effects willcancel because each is in anopposite leg of the bridge. The thirdwire, C, acts as a sense lead to thevoltmeter.

Current loop A & B: Polarity. Thecurrent loop to the RTD has apositive and a negative polarity.Possible solutions are similar to thethermocouple:1 standard DC barrier, positive polarity

and1 standard DC barrier, negative polarity

OR1 double AC barrier

Select the double AC barrier toavoid polarity problems. Because itis smaller, it is also less expensive.

Current loop A & B: Ratedvoltage. The constant currentamperage sent to the RTD typicallyis in the micro amp (10-6) level. Themaximum resistance of the mostcommonly used RTD, Pt100 is390 ohms at 1560°C. The voltagedrop across the RTD will be in mV,so the Vn of the RTD loop is similarto the thermocouple. To be safe,select a barrier with a Vn greaterthan 1 V, similar to the Vn of thethermocouple barrier.

Current loop A & B: Internalresistance. The constant currentsource will have a rated maximumload or burden (resistance load itcan drive). Assume that thismaximum load is 500 ohms and themaximum resistance of the RTD atthe highest temperature is 390ohms. Knowing this information, theRi of the barrier can be calculated:

control room ≤ barrier + RTDresistance resistance resistance500 ohms < Ri ohms + 390 ohmsRi < 110 ohms

Current loop A & B: Barrierselection. Use the same barrier thatwas used for the thermocouplecircuit.

Leg C to the voltmeter: Barrierselection. The RTD leg going to thevoltmeter (C) is a millivolt signalsimilar to the thermocouple circuit.The rated voltage, Vn, and internalresistance, Ri, of the barrier willhave the same parameters as thebarriers used in the thermocoupleand current loop of the RTD.Selecting the correct barrier tomake all thermocouples and RTD’sintrinsically safe is not difficult. Usea double-channel AC barrier with arated voltage greater than 1 voltwith the lowest internal resistance.The double-channel barrier is thelowest cost solution. The AC versionwill avoid any polarity problems. Abarrier with a rated voltage between1 and 10 volts will provide a wideselection which have a lowresistance and are approved for thehazardous areas where thetemperature sensors are located.This single barrier can then be usedto make all thermocouples andRTDs intrinsically safe. And don’tforget, all thermocouples and RTD’sare simple devices, so they do notneed third party approval to beintrinsically safe. When they areconnected to an approvedintrinsically safe barrier, the circuitsare intrinsically safe.

Many temperature sensors areattached to 4-20 mA temperaturetransmitters, which comprise 22%of all intrinsically safe applications.The next article in this series willshow how to make thesetransmitters intrinsically safe.

Copyright Instrument Society ofAmerica. Intech, December, 1992.All Rights Reserved.

Z-136

Paul S. Babiarz

The 80/20 Rule actually is five rulesthat are based on the fact thatcertain practices prevail 80% of thetime, and 20% of applications aremore difficult. This article focuses onhow to choose intrinsically safebarriers when the transmitters areinstalled in hazardous areas for boththe 80% standard category and theremaining 20% more difficultapplications.

The most common way to processand send analog signals in theinstrumentation industry is via4-20 mA transmitters. Transmitterscan be one of the simplest devicesinvolving barriers. However,improper selection of intrinsicallysafe barriers in loops with 4-20 mAtransmitters can introduce too muchimpedance on the circuit and causethe transmitters to functionimproperly at the high end near the 20 mA reading.

Before selecting barriers, examinehow 4-20 mA analog circuitsfunction. Transmitters convert aphysical measurement such astemperature or pressure into anelectrical signal that can be sentwithout signal modification to a

control system over a long distance.The brains of the system, the DCS,interprets the electrical signal intothe physical measurement. Becausethese analog signals are sent to aDCS, 4-20 mA circuits are calledanalog inputs or A/I. Usingtemperature as an example,examine the function of thetransmitter (Fig. 1).

A power source in the DCS usuallysupplies 24 VDC to the transmitter.The transmitter converts thephysical measurement into anelectrical current signal. Transmittercurrent ranging from 4-20 mA issent back to the DCS. Currentsignals are used to avoid potentialvoltage drops or electricalinterference associated with voltagesignals. However, because the

controller reads a voltage signal, aconversion resistor (most commonly250 ohms) converts the 4-20 mAcurrent range into a voltage signalon the DCS input channel. ApplyingOhm’s Law of V = IR, the controllerhas a 1-to-5 V signal (Table 1).

Assume the temperature span to bemeasured is from 0°C to 100°C. Thetransmitter is calibrated so that a 4mA signal equals the low reading of0° and a 20 mA signal equals thehigh reading of 100°. The DCS thenruns the signal through a conversionresistor which can be placed eitheron the supply (+) or return (-) leadof the circuit, converting the signalback to a voltage reading.

There are three types of barriers forintrinsically safe transmitterapplications: ungrounded repeaters,grounded repeaters, or groundedsafety barriers. Each has itsadvantages and disadvantages(Table 2).

Ungrounded repeater barriers, alsoknown as galvanically isolated ortransformer-isolated barriers, areused more frequently in Europe

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Temperature → converted to x multiplied by = converted to mA signal ohm resistor a voltage reading

in the DCS

0°C (min) → 4 mA (0.004 A) x 250 = 1 V

100°C (max) → 20 mA (0.020 A) x 250 = 5 V

Use The 80/20 Rule In Intrinsic Safety Circuit Design

Part 3 of this series on intrinsic safety circuit design describes how to select barriers forintrinsically safe 4-20 mA transmitters. Use the 80/20 Rule to simplify this process.

+24V

250 Ω

DistributedControlSystem

4 – 20 mA2-Wire

Transmitter

SENSOR

4 – 20 mA

Conversion

+

–

+

–

Figure 1. 4-20 mA 2-wire transmitter.

Table 1. Conversion of physical measurement to electrical signals.

than in North America. Repeaterssuit most transmitter applications,but at a higher cost. Grounded orungrounded repeaters supply aconstant regulated voltage of 15 to17 V to the transmitter from a 24 Vsource. The return channel is thenrun through the barrier, whichrepeats it without any appreciableloss in signal. For example, if atransmitter sends 19.6 mA throughthe barrier, it is repeated in thebarrier without any loss so that19.6 mA reaches the control room.Repeaters act like mirrors byretransmitting, or repeating, theanalog signals.

When budget constraints or controlpanel space are importantconsiderations, grounded safetybarriers may be a better choice.

80/20 Rule #1: In North America,most analog circuits areprotected by grounded safetybarriers because of lower costs.

Define the hazardous areawhere the transmitter is located.In North America, these areas aredefined by the National ElectricCode as classes, divisions, andgroups. The class defines the typeof materials that are in thehazardous area. Class I —flammable gases and vapors; ClassII — combustible dusts; Class III —fibers and flyings. Hazardous areasare further broken down into twodivisions. Division 1 means normallyhazardous; Division 2 means notnormally hazardous. The group

designates the type of vapor or dustin the area. Group A — acetylene;Group B — hydrogen; Group C —ethylene; Group D — propane;Group E — metal dust; Group F —coal dust; Group G — grain dust.

Complex devices. Becausetransmitters can store energy, theyare considered complex devices,and must be approved asintrinsically safe. If they arethird-party approved, they haveentity parameters such as Vmax,

Imax, Ci, and Li (see Part 1 of thisseries).

Selection of safety barriers. Theproper barrier must be selected bytwo separate evaluations: one todetermine that the analog circuitfunctions properly at 20 mA, andone to determine that the circuit issafe under fault conditions.

Functional parameters:Type ofsafety barrier, voltage input (Vn),and internal resistance (Ri). Thetype of safety barrier is largelydetermined by the placement of theconversion resistor. If the resistor isplaced on the supply leg of thecircuit, a simple DC positive barriercan be used (Fig. 2).

80/20 Rule #2: Most transmittercircuits have the conversionresistor on the return channel.Use the double channel supplyand return barrier.

The supply channel is constructedlike the positive DC barrier; itprevents a fault on the safe sidefrom transferring excess energy tothe transmitter. The return channelhas two diodes in series which allowthe signal to pass only in onedirection back to the DCS, andprevent any excess fault energyfrom being transferred to thetransmitter. These diodes and thesupply channel have voltage dropswhich must be accounted for in theanalog circuit (Fig. 3).

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Z

Advantages Disadvantages

Grounded Safety Least expensive Requires ground

Barrier Precise signal response Requires engineering

Very small size

(<1⁄2 in. wide)

Grounded One product can be used More expensive

Repeater Can use transmitters with Requires ground

higher operating voltage Larger in size

Consumes more power

Ungrounded One product can be used Most expensive

Repeater No ground required Larger in size (1 in. wide)

Can use transmitters with Possible radio frequency

higher operating voltage interference

Isolation, if good ground May not be compatible

not available with smart transmitters

Table 2. Advantages and disadvantages of grounded safety barriers, grounded andungrounded repeaters.

+24V250 Ω


ConversionResistor

GRD


+ + + +

––––

IntrinsicallySafe

Transmitter

SENSOR

+

Figure 2. Positive DC barrier.

GRD


+24V

250 Ω


IntrinsicallySafe

Transmitter

SENSOR

ConversionResistor

+ + +

–––

+

–

Figure 3. Supply and return barrier.

Z-139

80/20 Rule #3:The supplyvoltage normally is 24 VDC.

Select the voltage input, Vn.One of the reasons that barriersfail is because the voltage supplyis too high. Use a regulated supplysource with a high end of tolerancethat does not exceed the barrierrating and a low end that is enoughto drive the circuit. A 24 Vdcsource ±1% usually is a goodchoice.

Determine the internalresistance, Ri (also referred toas end-to-end resistance) of thebarrier best suited for yourcircuit. The most criticalcomponent of the barrier selectionis the barrier’s internal resistance.If the resistance is too high, thetransmitter will not work near20 mA. As seen in Table 1 and thefollowing discussion, at 20 mA thevoltage drops across the barrierand the conversion resistor will bethe highest. If the internalresistance is too low, the barrier’sshort circuit current, Isc, mayexceed the transmitter’s entityparameter, Imax.

The easiest way to determine thebarrier’s permitted resistance is tocalculate the total voltage drop onthe circuit. To select the propertransmitter barrier, determine thefollowing:• Hazardous area Groups A-G or

C-G• Placement of the conversion

resistor on either the supply orreturn leg of the circuit

• Size of the conversion resistor(250 ohms is most common)

• Minimum operating voltage of thetransmitter (This figure, alsoreferred to as lift-off voltage, is inthe transmitter data sheet. Mostoperate at a minimum of 12 V orlower.)

• Entity parameters of approvedtransmitter

Case 1. Assume that conditionsare as follows:• Groups A-G• Supply• 250 ohms• 12 V• Vmax = 30 V, Imax = 150 mA, Ci = 0 µF, Li = 0 mHCalculate the maximum allowableresistance of the barrier underworst-case conditions when thetransmitter is sending a 20 mAsignal. The supply is 24 Vdc; thetransmitter requires a minimum of12 V; and the 250 ohm conversion

resistor requires 5 V at 20 mA.Simple subtraction leaves amaximum allowable voltage drop of7 V. Using Ohm’s Law, thisconverts to an internal resistanceof 350 ohms. Allow for a cableresistance of about 10 ohms. Thus,the circuit functions properly with abarrier having an internalresistance of 340 ohms.

Next, to make sure the circuit issafe, verify that the barrier’s entityparameters match the transmitter’sentity parameters. This designoffers the lowest cost solutionwhere two transmitters can beconnected to one double channelbarrier. This circuit arrangementallows one common barrier to beused for most circuits (Fig. 4).

Case 2. Use the sameconditions as in Case 1, exceptchange the placement of theconversion resistor to the returnside, and use the supply andreturn barrier. Voltage drop onthe barrier occurs on both the

supply and return side. Voltagedrop on the return side diodes isabout 0.7 V. This leaves amaximum drop of 6.3 V on thesupply side or a maximumresistance of 305 ohms (allowing10 ohms for cable resistance).Again, verify the entityparameters of the barrier andtransmitter.

80/20 Rule #4:The two solutionsabove cover 80% of alltransmitter applications.

But what happens if the circuit fallsinto the 20% category? Groundedsafety barriers may not work inconditions where a loop-poweredindicator is connected, or wherethe transmitter requires a minimumvoltage greater than 12 V. In thesecases, the easiest solution is touse a repeater barrier. Repeatersprovide a regulated power supplyof 15-17 V to the transmitters andcan drive a conversion resistorload of 750 to 1000 ohms (Fig. 5)

GRD


+24V

250 Ω


IntrinsicallySafe

Transmitter

SENSOR

5 volt loss maximum7 volt loss

12 volt loss

VOLTAGE BALANCE:Transmitter = 12 volts

Resistor = 5 volts Barrier(+ line loss) = 7 volts

Total Supply = 24 volts

24 Supply

+

–

+

–

+

–

+

–

Figure 4. Voltage balance.

+24V

750 - 1000 Ωmax


IntrinsicallySafe

Transmitter

SENSOR

4 – 20 mA

+

–

+

–


RepeaterBarrier

15 - 17V

4 – 20 mA

+

–

+

–

Repeated

ConversionResistor

Figure 5. Repeater barriers.

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Z

If repeaters still are not the bestsolution, there may be other ways touse grounded safety barriers. Eitherthe impedance in the circuit must bereduced or the voltage must beincreased. If these alternatives areused, recheck the barrier andtransmitter entity parameters tomake sure the circuit is safe.

Reducing Impedance Case 1. Reduce the conversionresistor. As seen in Fig. 2, only twofixed sources of impedance can bereduced: the conversion resistor orthe barrier. One solution is to reducethe conversion resistor to 100 or50 ohms to obtain maximum voltagereadings of 2.0 to 1.0 V respectively.(Example: 20 mA (0.02 A) x100 ohms = 2 V.) This may bepractical for new installations, but itmay not be possible for cases whereadditions are being made to anexisting control system.

Case 2. Select a barrier withlower resistance.

80/20 Rule #5: Many hazardouslocations are classified asGroups C-G.

Ignition curves in Groups C-G allowhigher rated voltages and currentbefore gases ignite (see Part 1 ofthis series, October 1992). Barriersdesigned for hydrogen and othergases classified as Group A or Brequire higher series resistancethan barriers designed for only themore common gases in Groups Cand D. Thus, most intrinsically safeinstruments should have entityparameters (Imax, maximum shortcircuit current) that are higher forGroups C-G. (As a practical matter,most instrument manufacturershave not taken advantage of thisfact.) With the Group C-G ratinghigh-current barriers can be used,which have a lower internalresistance. These barriers havecorresponding lower voltage dropsbut higher Isc (Table 3).

Increasing Voltage SupplyIf the voltage supply is increasedtoo much, the barrier may sense afault and the fuse could blow,interrupting the circuit. Someallowance can be tolerated forincreasing the voltage supply onbarriers with a nominal ratedvoltage of 24 VDC.

Case 1: Resistor on the supplyside. When transmitters are firstenergized, they transmit 4 mA forcalibrating zero readings. Therealways is at least a 1 V drop acrossthe resistor before the supplyreaches the barrier. The voltagesupply could be increased to 25 to26 V without the barrier sensing afault condition. This would allow 1 to2 additional volts on the circuit.

Case 2: Resistor on the returnside. Since the resistor is on thereturn side, the barriers receive thetotal voltage supply. Since thiscircuit is more sensitive to voltageincreases, be careful aboutincreasing the supply above thebarrier's nominal rated voltage, Vn.

Before the zener diodes in thebarriers reach their rated voltage,there may be some leakage currentthat could affect the transmittersignals. Diode leakage currentvalues ranging from 1 to 10µA arelisted by the barrier manufacturers.In Case 1, this could mean that thecurrent signal could be deformed bya maximum of 0.025% at 4 mA(1 µA/4 mA).

When the resistor is placed on thereturn side, leakage current is onthe supply side and does not affectthe transmitter’s 4-20 mA signal.

Transmitters comprise 22% of allintrinsically safe circuits. The nextarticle will feature discrete inputs,also referred to as switching. Theserepresent 32% or almost one-thirdof all intrinsically safe circuits.

See SIB Series of Intrinsically Safe Transmitters

Copyright Instrument Society ofAmerica. Intech, March, 1993.All Rights Reserved.

Groups Internal Resistance Voltage Drop Short Circuit Open Circuit

(Ri) at 20 mA Current, Isc Voltage, Voc

Barrier #1 A-G 340 ohms 6.8 V 93 mA 28 V

Barrier #2 C-G 140 ohms 2.8 V 213 mA 28 V

Table 3. Typical values of barriers rated for different groups.

Z-141

Making Digital InputsIntrinsically Safe

Part 4 of this series describes how to make a switch intrinsically safe by using a switchamplifier or a grounded safety barrier.

Paul S. Babiarz

Digital inputs constitute almost one-third of all process signals. Theyalso are known as binary, on-off,0/1, or simple switching signalswhere a switch is either opened orclosed. The most commonexamples of these are mechanicalor reed contacts, transistors, limit,float, on-off, and pushbuttonswitches. As defined in paragraph3.12 of the ANSI/ISA-RP12.6-1987,switches are simple devices thatneither generate nor store morethan 1.2 V, 0.1 A, 25 mW, or 20µJ.Since switches are simple devices,they do not have to be approved asintrinsically safe. If they areconnected to an approvedintrinsically safe associatedapparatus (barrier), the circuit isdeemed to be intrinsically safe.

To make a switch intrinsically safe,the user may select a switchamplifier or a safety barrier. A switchamplifier is an intrinsically safe relaythat solves virtually all switchingapplications. However, if power isnot available in the control panel orif panel space is an importantconsideration, a grounded safetybarrier may be a better choice. Thereis not a significant cost savings ofone alternative over the other. Eachhas its own advantages anddisadvantages, as shown in Table 1.

Switch AmplifiersThe most common application isswitching through an intrinsicallysafe relay (Fig. 1). Relays, whichnormally are powered by 110 VACor 24 VDC, have a low voltage andcurrent which are safe at the contactin the hazardous area. When thiscontact is closed, the relay transfersthe signal from the hazardous

location to the non-hazardous side.A closed switch on the hazardousside operates a relay or optocoupleroutput on the non-hazardous side.The signals are electrically isolatedso that grounding is not required.

When proximity switches became apopular means of sensing thepresence of objects and materials,the NAMUR-style sensor wasdeveloped. Contrary to popularopinion, NAMUR is not an approvalstandard. It was organized by theGerman chemical industry todevelop operating standards forproximity switches. A NAMUR-styleproximity switch is a 2-wire DCsensor that operates at 8.2 V withswitch points operating between 1.2to 2.1 mA. This NAMUR standardlater was superseded by theGerman Standard DIN 199234,Measurement And Control:

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IntrinsicSafety Barrier








Figure 1. Switch amplifier — 2 channels.

Electrical Sensors Used ForIntrinsically Safe 2-Wire DCSystems. Because these switchesrequired a remote amplifier foroperation, most switch amplifiersstandardized on an intrinsically safevoltage of 8.2 V and current of 8 mAat the contacts in hazardous areas.This provided enough power tooperate NAMUR-style proximityswitches safely.

The amplifiers are sensitive enoughto detect closed contacts incorrosive or abusive areas. Despitethe fact that the intrinsically safevoltage and current at the contactsare very low, most modern switchamplifiers will detect a closedcontact when the resistance of thecircuit is less than 3000 ohms.Intrinsically safe switches can belocated a long distance from theswitch amplifiers and still functionproperly.

Switch amplifiers are available withtwo different output contacts to thesafe side, relays and optocouplers.The more commonly used relayversions are applied in slow speedswitching to operate smaller pumps,motors, or other electrical devices.Optocouplers are transistorsoperated by photo diodes to closethe output contacts. These outputshave lower contact ratings but analmost infinite switching capability.Optocouplers are used for switchingback to a DCS or for high-speedcounting operations up to thousandsof times per second (KHz).

Switching Through SafetyBarriersWhen a 110 V supply is notavailable in the control panel, safetybarriers frequently are used fordigital inputs back to a DCS. Thereare two methods of switching:current sourcing or current sinking.Both of these methods can use thesame types of barriers that were

used for transmitters (see Part 3 ofthis series).

The current sourcing method ofswitching in Fig. 2 could use thesame signal and return barrier thatwas used for 4-20 mA transmitters.The voltage to the switch is suppliedthrough the supply channel. Thesecond channel is used for signalreturn. A closed switch will close thecontact in the DCS. Most digitalinput signals operate with 24 V and10 mA. If the same barrier is usedfor switching as 4-20 mAtransmitters, there will be about a 3to 4 V drop across the barrier.

The barrier used for current sinkingswitching can be a single-channelDC barrier as seen in Fig. 3. Whenthe switch is open, the DCS inputwill sense 24 V. When the switch isclosed, the DCS will recognize alower voltage. This lower voltage iscalculated as a voltage dividercircuit.

Make sure the rated voltage of thebarrier, Vn, is equal to or greater

than the voltage supply. Since mostswitching uses 24 VDC, select abarrier rated at 24 V. The internalresistance of the barrier is not ascritical since the current in digitalinputs usually is very small.However, it always is good practiceto select a barrier with lowresistance. Check the approvals ofthe barriers to make sure that theyare rated for the proper hazardousarea group location.

Intrinsically safe relays, also referredto as switch amplifiers, can beapplied universally for all digitalinputs. However, if safety barriersare used, the same barriers used tomake analog inputs intrinsically safecan be used for either currentsourcing or current sinkingswitching.

The next article in this series willexplain how to make digital outputsintrinsically safe.

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Simple application Needs power supplyNo ground required Larger in sizeNo internal resistanceLEDs to indicate power and monitor operationsSensitive to detect closed contacts in corrosive areas


Smaller size Requires groundingDoes not require power supply Has internal resistance

Switch Amplifiers

Safety Barriers


ONLY THESE WIRESAREINTRINSICALLYSAFE

IntrinsicSafetyBarrier

Potential ofGround Loops



IntrinsicallySafe

Apparatus

DistributedControl

System

Figure 2. Current sourcing switching. Figure 3. Current sinking switching.

Table 1. Advantages and disadvantages of switch amplifiers and safety barriers.

Copyright Instrument Society of America.Intech, April, 1993. All Rights Reserved.

Paul S. Babiarz

Digital outputs refer to closedcontacts in a distributed controlsystem (DCS). They transfer avoltage to a process area to operatea field device. The two mostcommonly used digital output fielddevices, solenoid valves and LEDdisplays, can easily be madeintrinsically safe. For solenoidvalves, the same types of barriersare used that make analog anddigital inputs (transmitters andswitch contacts) intrinsically safe.LED’s may require a different barrier.

There is good news and bad newsfor making circuits (or loops)containing solenoid valvesintrinsically safe. The bad news isthat unlike transmitters which haveminimum operating voltages, valvemanufacturers often describe theirvalves with a nominal operatingcurrent or voltage. To select theproper barrier one needs to knowthe minimum operating characteristicsunder the most extreme conditions.Without these characteristics it canbe quite difficult to select a barrierthat will allow the circuit to functionproperly and still meet the entityparameters of the valves. Conditionsthat may affect the operatingcharacteristics are high ambienttemperatures, position of theactuator, and length of cable runs.

The good news is that there are onlya handful of approved intrinsicallysafe solenoid valves to choose from.For this article, manufacturers testedtheir intrinsically safe valves with themost common barrier used in analogand digital input circuits — the 24 Vdcbarrier with a resistance equal to orless than 350 ohms (Fig. 1).

To determine the correct barrier, startwith the basics. Since most digitaloutput circuits operate with 24 Vdcswitched on the positive side, use apositive DC barrier rated at 24 Vdc.Knowing the minimum operatingcurrent of the valve and the internalimpedance of the coil, you cancalculate the maximum allowableimpedance for the barrier and thecable.

Z-143

Intrinsically Safe OutputsMade Easy

Part 5 of this series explains how to make solenoid valves, LED’s, and I/P transducers intrinsically safe.

+

–

SolenoidValve

+

–

NON-HAZARDOUS SIDE HAZARDOUS SIDECLASS I, II, IIIDIVISION 1GROUPS A-G

GRD

+

–

Solenoid valves needentity approval

+

DigitalOutput

D/O

24 V

Typical Safety BarrierParameters

VN: 24 V Ri:≤ 350

VN = Rated voltageRi = Internal resistance

Typical Safety BarrierTypical Safety BarrierParameters Parameters

VVNN: 24 V R: 24 V Rii::≤≤ 350 350

VVNN = Rated voltage = Rated voltageRRi i = Internal resistance= Internal resistance

For example, assume a valve has aminimum operating current of 28 mAand a coil impedance of 400 ohms.The maximum allowable impedanceof the circuit is 857 ohms (24/.028 =857).If the internal impedance of thesolenoid coil is 400 ohms, theallowable impedance of the barrierand cable would be 457 ohms(857-400 = 457). The resistance ofone mile of #18 AWG wire at 60°Cis about 40 ohms (resistance of#18 AWG wire at 60°C is 0.00737ohms/ft.). This makes the maximumresistance of the barrier 457-40 =417 ohms.Selecting the barrier now is simple:1. Select a simple DC positive barrier.

(The rated voltage should be 24 V.)2. Calculate the maximum allowable

resistance of the barrier as in theexample.

3.Confirm that the entity parameters of the solenoid valve match thoseof the barrier (refer to Part 1 of this series).

Associated Apparatus Apparatus(barrier) (field device)

Open circuit voltage Voc ≤ VmaxShort circuit current Isc ≤ ImaxAllowed capacitance Ca ≥ CiAllowed inductance La ≥ Li

LED’sLED’s (light emitting diodes) aresimple devices since they do notstore energy (capacitance orinductance); therefore, they do notneed to be approved. However, theystill must be used with safetybarriers to make circuits intrinsicallysafe. Typical LED’s are rated at 24,18, 12, or 6 V and operate at about

25 mA. Since there will always be avoltage drop across the barrier, thebest application is to choose anLED rated at less than 24 VDC. Usea barrier rated at 24 V, then subtractthe rated voltage of the LED. Thisdifference is the allowable voltagedrop on the barrier at the ratedcurrent. Use Ohm’s Law (V = IR) tocalculate the internal impedance ofthe barrier. Example:• LED rated at 12 V at 25 mA• Allowable voltage drop 12 V

(24-12 = 12)• Internal impedance of the barrier

= 480 ohms (12/.025 = 480)Choose a 24 V positive DC barrierwith an internal impedance of about480 ohms (Fig. 2).

Analog OutputsAnalog outputs refer to I/Ptransducers, also known as I/P’s(pronounced “Ida Pease”). An I/Ptransducer produces a pneumaticoutput proportional to the electricalcurrent input that it receives. Themore current that is applied to thetransducer, the more air pressure isallowed into the system to drive adevice. As opposed to a solenoidvalve which is either in an opened orclosed position, a transducer is aproportional valve. I/P transducersare referred to as analog outputsbecause a variable output, thecurrent signal, is sent from the DCSto the transducer.

I/P transducers need entityapproval. They act like resistors inthe circuit, so three facts must beknown to select the correct barrier:transducer impedance; maximumburden of the driver that sends thecurrent signal; and transducer entityvalues. Burden, rated in ohms,measures the maximum load theDCS can drive. To select the barrier,use the following characteristics:• Transducer impedance is 150 ohms• Burden of the drive is 1000 ohms

The barrier must have an internalresistance less than 850 ohms(1000-150 = 850). Verify the ratedvoltage of the barrier by calculatingthe voltage drop of the circuit. Forexample, use the same barrier andcable values as in the solenoid valveexample. The total impedance(impedance of barrier + transducer+ cable) of the circuit would be

Z-144

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+

–

LED Pilot Light


GRD

+

–

25 mA

12 volt drop across barrier(480 x .025)

12 V

LED pilot light is a simpledevice; does not need

approval

+

DigitalOutput

D/O

24 V

VN: 24 V Ri: 480 Ω


VN = Rated voltageRi = Internal resistance

VVNN: 24 V R: 24 V Rii: 480 : 480 ΩΩ

Typical Safety BarrierTypical Safety BarrierParametersParameters

VVNN = Rated voltage = Rated voltageRRii = Internal resistance= Internal resistance

Figure 2. LED pilot light.

+

–

VN ≥ 12 V Ri ≥150 Ω


VN = Rated voltageRi= Internal resistance

VVN N ≥≥ 12 V R 12 V Ri i ≥≥150 150 ΩΩ

Typical Safety BarrierTypical Safety BarrierParameters Parameters

VVNN = Rated voltage = Rated voltageRRii= Internal resistance= Internal resistance

I/PTransducer

+

–


GRD

+

–

+ 4-20 mA

I/P transducers needentity approval

AnalogOutput

A/O

Single Channel DC Barrier

Figure 3. 4-20 mA I/P transducer.

540 ohms (350 + 150 + 40). At themaximum current of 20 mA, thevoltage drop would be 10.8 V(540 x 0.20 = 10.8). Select abarrier rated equal to or higherthan 10.8 V. A barrier rated at 12 Vor higher with an internalresistance of 150 ohms also wouldbe a good choice (Fig. 3). Confirmthat the entity parameters of thebarrier correspond with those ofthe transducer.

This series of articles has shownhow the most commonapplications of temperaturemeasurements and analog ordigital inputs/outputs can be madeintrinsically safe with a few intrinsicsafety barriers. Selection is simple:

1. Determine if the field device is asimple or nonsimple (energy

storing) device that needsapproval and has entityparameters.

2. Select the type of barrierneeded to protect the individualungrounded lines of the circuit.Normally, temperature sensorsuse an AC barrier. For analoginputs and current sourcingswitching, use the supply andreturn barrier. The remainder(analog and digital outputs andsome switching circuits) requireDC barriers.

3. Select a barrier with a ratedvoltage equal to or greater thanthe voltage of the circuit.

4. Confirm that the internalresistance of the barrier willallow enough voltage for the fielddevice to operate properly.

5. Confirm that the entityparameters of the barrier matchthose of the field device.

Use Table 1 as a guide in selectinggrounded safety barriers. Therewill always be exceptions to theseguidelines, so verify your selectionwith the manufacturer of thebarriers or field devices.

The last installment in this serieswill discuss the general rules ofgrounding, installation, andmaintenance of intrinsically safesystems.

Z-145

Device Barrier Rated Internal NotesType Voltage Resistance (IT = INTECH)

Thermocouples AC >1 V <1000* Thermocouples are simple devices; do not need approval.

RTD’s AC >1 V <1000* RTD’s are simple devices; do not need approval.

Digital inputs switch Dry contacts are simple amplifiers devices; do not need approval.

D/I - current supply & 24 350** Dry contacts are simple sourcing return devices; do not need approval.

D/I - current DC 24 350** Dry contacts are simplesinking devices; do not need approval.

A/I supply & 24 350 Transmitters need approval.transmitters return Check entity parameters.

Conversion resistor of 250 ohms is on negative side.Minimum lift-off voltage of transmitter is 12 or less.

A/I transmitters DC 24 350 Same, except conversionresistor is on + side.

D/O solenoid DC 24 350 Solenoid valves need approval.valves Check entity parameters.

A/O transducers DC >12 >150 Transducers need approval.Check entity parameters andDCS burden.

* Select a barrier with a low resistance.

** Other barriers with a different resistance can be used. However, these barriers match those of the analog inputs.

Table 1. Guide to selecting grounded safety barriers.

Copyright Instrument Society of America.Intech, September, 1993. All RightsReserved.

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Installing Intrinsically SafeSystems

Part 6 of this series summarizes the major points of barrier replacement, wiring, installation,grounding, sealing, maintenance, and troubleshooting of intrinsically safe systems.

Paul S. Babiarz

We have shown how an intrinsicallysafe circuit is designed for mostcommon applications. Now theintrinsically safe system must beproperly installed and provisionsmust be made to maintain andtroubleshoot it. These proceduresare discussed in detail in Article 504of the National Electrical Code(NEC) and the ANSI/ISARP 12.6-1987 RecommendedPractice — Installation ofIntrinsically Safe Systems ForHazardous (Classified) Locations.

WiringIntrinsically safe circuits may bewired in the same manner ascomparable circuits installed forunclassified locations with twoexceptions summarized asseparation and identification. Thesewiring practices are simple andclear; however, they often areoverlooked and are the source ofpotential problems.

The intrinsically safe conductorsmust be separated from all otherwiring by placing them in separateconduits or by a separation of2 inches of air space. Within anenclosure the conductors can beseparated by a grounded metal orinsulated partition (Fig. 1).

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Figure 2 . Barrier installation.

Figure 1. Suggested panel arrangement using separate wireways.


ONLY THESE WIRESAREINTRINSICALLYSAFE


Intrinsically safe wiring may be lightblue when no other conductorscolored light blue are used. Theraceways, cable trays, open wiring,and terminal boxes must be labeledIntrinsically Safe Wiring to preventunintentional interference with thecircuits. The spacing between thelabels should not exceed 25 ft.

Barrier InstallationThe barriers normally are installedin a dust- and moisture-free NEMA4 or 12 enclosure located in thenonhazardous area. Only the barrieroutputs are intrinsically safe.Conductive dust or moisture couldlessen the required distance of 2 in.between intrinsically safe andnonintrinsically safe conductors(Fig. 2). The enclosure should be asclose as possible to the hazardousarea to minimize cable runs andincreased capacitance of the circuit.If they are installed in a hazardousarea, they must be in the properenclosure suited for that area.

GroundingFirst determine if the intrinsicallysafe barriers used in the system aregrounded or isolated. The isolatedbarriers normally are larger, moreexpensive, and do not require aground for safety. The groundedsafety barriers are smaller and lessexpensive, but require a ground todivert the excess energy. The mainrules of grounding intrinsically safesystems are:• The ground path must have less

than 1 ohm of resistance from thefurthest barrier to the maingrounding electrode.

• The grounding conductor must bea minimum 12 AWG.

• All ground path connections mustbe secure, permanent, visible, andaccessible for routine inspection.

• A separate isolated groundconductor normally is requiredsince the normal protectiveground conductor (green oryellow/green wire) may not be atthe same ground potentialbecause of the voltage drop fromfault currents in other equipment.

• For installations designed toCanadian standards, theCanadian Electrical Code(Appendix F) recommendsredundant grounding conductors.

A poor grounding system can

influence the function of the systemby creating noise on the circuit ormodifying the signals. Fig. 3 showsan improperly grounded system.The numerous grounding pointscreate ground loops which canmodify the signals and induce strayvoltages into the intrinsically safecircuits. The correct method ofgrounding is shown in Fig. 4 whereall the grounds are tied together atone single point in the system.

SealingThe requirements for sealingintrinsically safe circuits have beendiscussed by a panel of experts andpublished in “Seals for IntrinsicallySafe Circuits,” EC&M, September1992, pp. 48-49. The panel’sconclusion is that seals are requiredto prevent the transmission of gasesand vapors from the hazardous areato the nonhazardous area, not toprevent passage of flames fromexplosions. Explosion-proof sealsare not required as long as there issome other mechanical means ofpreventing the passage of gasessuch as positive pressure in thecontrol room and/or application ofan approved mastic at cableterminations and between the cableand raceway. Many expertsgenerally agree that a commerciallyavailable silicon caulk is a suitablemastic which would minimize thepassage of gases. This must,however, be acceptable to theauthority having jurisdiction.

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Potential ofGround Loops



Main Earth Ground

IntrinsicallySafe

Apparatus

DistributedControl

System

Figure 3. Unacceptable grounding.

Figure 4. Acceptable grounding.

Single Ground Point



Main Earth Ground

Distributed Control System Intrinsically

SafeApparatus

When barriers are installed inexplosion-proof enclosures, whichare located in the hazardous area,explosion-proof seals are requiredon the enclosure (Fig. 5). Sinceother conduits containingnonintrinsically safe conductorsbetween the hazardous andnonhazardous areas requireexplosion-proof seals, it is goodpractice to maintain consistency andinstall explosion-proof seals on theconduits containing intrinsically safeconductors also. The exception tothis would be where multiconductorshielded cable is used. This cablemay be difficult to seal in someexplosion-proof fittings. However, itwill be necessary to seal both thecable terminations and between thecable and raceway to minimize thepassage of gases, vapors, or dust.

MaintenanceNo special maintenance ofintrinsically safe systems is

required. Once a year the barriersshould be checked to ensure thatthe connections are tight, the ground wiring has less than oneohm of resistance, and the barriersare free from moisture and dirt.Check the panel and conduits forseparation and identification of theintrinsically safe wiring. Never testthe barrier with an ohmmeter orother test instrument while it isconnected in the circuit (Fig. 6). This bypasses the barrierand could induce voltages into theintrinsically safe wiring.

TroubleshootingIf the intrinsic safety circuit does notoperate properly once it iscompleted and energized, followthese troubleshooting guidelines:• Make sure the connections are

tight.• Check the wiring to the

appropriate terminals against thecontrol wiring diagram. A control

wiring diagram is defined by theNEC as “a drawing or otherdocument provided by themanufacturer of the intrinsicallysafe or associated apparatus thatdetails the allowedinterconnections between theintrinsically safe and associatedapparatus.” These diagrams areeasier to obtain than in the past.Make sure that one of themanufacturers provides not onlydiagrams which show theinterconnections between the fielddevice and barriers, but alsowiring diagrams whichdemonstrate that the circuitfunctions properly and is safe bycomparing the safety parametersof the field device and the barriers.

• Make sure the circuit is powered.• Check to see if the resistance in

the barrier is too high for thecircuit. As stated in the previousarticles in this series, circuits areanalyzed for the proper loopresistance (barrier and cable) andsupply voltages. If the circuit doesnot operate properly, check thecircuit against the design in thecontrol wiring diagram.

• Check for a blown barrier fuse.This is accomplished bydisconnecting the barrier from thecircuit and measuring theend-to-end resistance of thebarrier. If the ohmmeter registersan infinite resistance, the fuse inthe barrier is blown. The fuse hasopened because of a fault in thecircuit, so reevaluate the entirecircuit before reinstalling a newbarrier.

Barrier ReplacementIf the barrier’s fuse has opened, itusually is the result of excessivevoltage being applied to the barrier.This causes the diode to conduct,which results in high current in thefuse. After determining the cause ofthe excess voltage, the barrier mustbe replaced. The procedure is todisconnect the wiring from thesafety barriers in the proper order ofnonhazardous terminal first,hazardous terminals next, and theground last. Cover the bare wireends with tape, replace the barrier,and then reverse the procedure tomount the new barrier. Always installthe ground first and disconnect theground last.

Copyright Instrument Society of America.Intech, October, 1993. All Rights Reserved.

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IntrinsicallySafe Seal

Explosion Proof Seal


DistributedControl

System

IntrinsicallySafe

Apparatus

Explosion ProofEnclosure


Testmeter

NEVER DO THIS!

Figure 5. Mounting in a hazardous area.

Figure 6. The barrier should never be tested with an ohmmeter or other instrument while it isconnected in circuit.

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