THE ARTOF

INTRINSIC SAFETY

INTRINSICSAFETY

Contents

1. Combustion Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2. Area Classification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3. Explosion Prevention Methods . . . . . . . . . . . . . . . . . . . . . 5

4. Ignition Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

5. How Intrinsic Safety Works . . . . . . . . . . . . . . . . . . . . . . . 11

6. Installation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

7. Certification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

8. Definition of Barrier Terms . . . . . . . . . . . . . . . . . . . . . . . 18

9. Barrier Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

10. Advanced Barrier Selection. . . . . . . . . . . . . . . . . . . . . . . . 22

11. Ronan’s Philosophy on the Intrinsic Safety Barrier . . . . 24

Appendix A: Ignition Graphs . . . . . . . . . . . . . . . . . . . . . . 26

1

We need to start our discussion onIntrinsic Safety with some backgroundabout ignition, combustion, and explo-sions. Since our goal is to prevent an explo-sion, we first need to know how anexplosion can occur.

An explosion requires the properenvironment in order to exist. By eliminat-ing those conditions an explosion isimpossible!

The three ingredients necessary for anexplosion (fuel, oxidizer and ignitionsource) make up the Ignition Triangle (seeFigure 1-1). All of the triangle’s compo-nents must be present in the correct pro-portions for ignition to occur.

Once ignition occurs, if the conditionsare right, a combustion wave can growfrom the ignition. An explosion is any un-controlled combustion wave.

THE COMBUSTION PROCESSAn explosion starts when the fuel and

oxidizer are present in the correct propor-tions, and an ignition source (i.e. spark)occurs. Energy then proceeds along a nar-row combustion zone, and is conducted tounburnt gas ahead of the flame front aswell as to burnt gas behind the flame.

Initially, most of the energy is supplied bythe spark. If more energy is supplied bythe spark (through the combustion zone)than is lost to the surrounding gas, theflame sphere will grow. As the diameter ofthe sphere increases, the wave begins toresemble a planar wave. Eventually, theamount of energy produced by combustionis sufficient to continually supply energy

COMBUSTION

1. The elementscome together.

2. And form anincipient flamesphere whichgrows.

3. Energy incombustion zoneis lost to unburntgas ahead of it,and burnt gasbehind it.

4. If the initial energy islarge enough, thecombustion wavebecomesself-propagating.(Explosion)

Figure 1-2: Incipient Flame Sphere.The Incipient Flame Sphere must reach a critical diameter if combustion is to become a self-propagating explosion.

Figure 1-1: The Ignition TriangleIn order for an explosion to occur, the ignition trianglemust have all the ingredients in the correct proportions.

FuelGas, Vapors,

or Powder

Ignition SourceThermal or

Electric

OxidizerAir or

Oxygen

Combustion Theory

2

to the unburnt gas ahead of the flame,thereby causing the wave to self propa-gate. Figure 1-2 shows the beginningstages of combustion. The larger the in-itial spark is, the easier it is for the waveto propagate.

If the initial spark is not large enough,(i.e. not enough energy), the combustionzone will not have enough energy to selfpropagate. It will just collapse upon itself,and fizzle out. This means a small sparkcan occur in a potentially explosive air/gasmixture, with no danger of an explosion.The maximum amount of energy of this“safe” spark varies with the specificair/gas mixture present.

GAS GROUPINGFigure 1-3 illustrates the effect of con-

centration on ignition energy. Everyair/gas mixture has a certain concentra-tion which is the most easily ignited con-centration (MEIC). When trying toprevent an air/gas mixture from explod-ing, the safest approach is to assume themixture is always at its MEIC. Therefore,the maximum energy you may safely al-low must be less than the minimum igni-tion energy (MIE) of that specific air/gasmixture.

Many gases have similiar ignition char-acteristics. Instead of having thousands of“Concentration vs. Ignition Energy”

curves, gases with similar curves aregrouped together. There are four differentgroups (or curves), two of which are shownin Figure 1-3. The actual curve shown,representing the group, is the most easilyignited gas of the group.

LEGEND

MEIC - Most Easily Ignited ConcentrationMIE - Minimum Ignition EnergyLEL - Lower Explosive LevelUEL - Upper Explosive Level

Figure 1-3:The Effect of Concentration on Ignition Energy.

Ing

itio

nEn

erg

y(m

J)

Volume Concentration (%)

Propane-Airat 1 ATM

Hydrogen-Airat 1 ATM

MIE MEIC

LEL UEL

6020 4030 5010 70

3

CLASSESHazardous materials (such as gas or

dust) are classified by their generic typeas follows:

Class I - Flammable Gases or Vapors.

Class II - Combustible Dusts.

Class III - Fibers or Flyings (particlesnormally suspended in air).

CLASS I: GASES AND VAPORS

Materials are grouped according to theirlevel of explosion hazard. The followinggroups are listed in order of the most tothe least easily ignited.

Group A - Acetylene (has a tendency toform copperacetylides which are easilyignited by friction).

Group B -Hydrogen,HydrogenMixtures.

Group C -Ethylene, mostEthers, someAldehydes.

Group D - Alkanes (Butane, Ethane,Methane, Octane, Propane), HydrocarbonMixtures (Diesel Oil, Kerosene, Petro-leum, Gasoline), Alcohols, Ketones, Esters,Amines, Alkenes, Benzenoids.

CLASS II: COMBUSTIBLE DUSTS

Group E - Metallic Dusts (Resistivity<100kohms/cm).

Group G - Non-Conductive Dusts, whichinclude agricultural, plastic, chemical andtextile dusts. (Resistivity>100 kohms/cm).

Note: Since 1974, a dividing line of 100kohms/cm separates groups E & G.

DIVISIONSLocations are classified by division ac-

cording to the probability that an explo-sive concentration of hazardous materialmay be present.

Division 1 - defines locations wherethere lies a high probability that an ex-plosive concentration is present duringnormal operation. To be classified Divi-sion 1, there has to be a minimum of100 hours/year, or 1% probability thatan explosive material is present. InEurope, Division 1 is subdivided intoZone 0 and Zone 1. Zone 0 has the high-est probability of an explosive concen-tration being present (greater than10% probability). Zone classificationsare not presently made in North Amer-ica, but may be in the future.

Division 2 - defines locations wherethere lies a low probability that an ex-plosive mixture is present during nor-mal operation (10 hours/year, or 0.1%of the time).

Area Classification

4

There are three different philosophies orapproaches which have been used to pre-vent explosions. All deal with the ignitiontriangle (Figure 1-1) differently.

PERMIT IGNITIONThis method allows all three elements of

the ignition triangle to be present, but con-trols the location of the flame or explosion.Examples of this include a pilot light on agas stove, a flare stack, and an explosion-proof housing. The key safety feature hereis the physical device which controls orlimits the explosion location. Permittingignition is normally costly and requireshigh maintenance.

CONTROL ATMOSPHEREThis method segregates the hazardous

fuel/air mixture from the ignition source.If you have a hazardous mixture in oneroom, you can put your potential ignitionsources in another room. The key safetyfeature here is the physical device whichcontrols the segregation. See Figure 3-1for one example. Another more commonexample is purging or pressurization. Con-trolling the atmosphere can be a very com-plex and costly procedure.

ELIMINATE THE IGNITIONSOURCE

The simplest method! Since all three ele-ments of the Ignition Triangle are neces-sary for an explosion, why not eliminateone or more of the elements?

By the very nature of a hazardous proc-ess, the fuel can not be removed. Noticethat in a general purpose environment,the fuel is the absent element, with oxi-dizer and ignition source safely present.

Rarely is the oxidizer (air) removed. Thehalon fire extinguisher is an example of anabsent oxidizer. It works by replacing oxy-gen with halon, thereby suffocating thefire.

The ignition source can be removed. Byinserting an energy limiting barrier, anypotential ignition source can be elimi-nated. This method is the only true com-bustion prevention method. The otherpossible solutions are actually combustionprotection.

METHODS FOR SAFE CONTROL INHAZARDOUS AREAS

Taking these three explosion preven-tion philosophies and implementing themproduces a variety of techniques. Theseinclude:

Explosion Prevention Methods

Figure 3-1: One way to separate the fuel/air mixture from the ignition source.

5

Pneumatic (Air) Fiber Optics (Light) Explosion-Proof Enclosures (Electric) Purging or Pressurization (Electric) Encapsulation (Electric) Oil Immersion or Powder Filling

(Electric) Intrinsic Safety (Electric)Let’s review each of these in a littledetail.

PNUEMATIC SYSTEMPnuematic systems are a safe means of

combustion control because they are pow-ered by air. This type of system is practi-cal in operations where clean compressedair is available. A typical pneumatic fielddevice is a control valve. Pneumatics canbe used in only a limited number of opera-tions, for limited distances and have aslow reaction time.

FIBER OPTICS OR PHOTOVOLTAICSPhotovoltaic loops provide a safe means

of control because they are powered bylight. A light beam goes out to the hazar-dous area and is either reflected back tothe safe area or is interrupted. This haslimited distance capabilities, is affected bydust or mist, and is limited to contact sens-ing applications.

Fiber-optic loops are also a safe means ofcontrol because they are powered by light.When the light reaches the instrumenthowever, it will probably need to be con-verted to electricity. In these cases, theelectrical safety methods discussed nextmust be used. Fiber optics has some poten-tial for contact sensing but special con-tacts need to be used, and are not yetreadily available.

EXPLOSION PROOF ENCLOSURESThe use of explosion-proof enclosures is

a method which allows ignition to occur,but prevents any significant damagingresults from the ignition.

The explosion-proof enclosure containsthe explosion so that it does not spread tothe surroundings and cause damage to theplant and the personnel. There are bolt-onenclosures (shown) and screw-on enclo-

sures available. Both types do not allowthe explosive energy to completely escapethe enclosure.

Care must be taken not to mar theflanged surfaces. Also, all bolts must betorqued down correctly. One missing orloose bolt or scratched surface could pro-vide a path for the explosion to reach theoutside world. Explosion-proof enclosuresare designed to resist the excess pressurecreated by an internal explosion. They arebuilt with a variety of materials, includingcast aluminum, cast iron, welded steel andstainless steel.

PURGING (PRESSURIZATION)Purging prevents explosions by main-

taining a protective inert gas within theenclosure at a pressure that is greaterthan the external atmosphere. Thismethod prevents the dangerous air/gasmixture from entering the enclosure. Pres-sure may be preserved with or without acontinuous flow of the inert gas. Pressureloss or opening of enclosure cause the sys-

Figure 3-2: Explosion Proof Enclosure.

Figure 3-3: Purging.

6

tem to alarm or power down depending onthe type of purge.

There are three types of purgingmethods, each of which reduce the areaclassification within an enclosure. Type Xreduces classification from Division 1 toNon-Hazardous. Type Y reduces classifica-tion from Division 1 to Division 2. Type Zreduces classification from Division 2 toNon-Hazardous.

Certain criteria must be met in order fora purging process to get rated as Type X,Y or Z (and thereby reduce the areaclassification accordingly). These criteriaare listed in Figure 3-4.

ENCAPSULATIONThe encapsulation method encloses the

hazardous operation with a resin pottingmaterial that prevents accidental shortingof electrical components. This method of-fers effective mechanical protection thatworks well as a complement to other pre-vention processes. If a spark does occur, itcan not escape through the potting mater-ial into the hazardous environment.

MISCELLANEOUSPROTECTION METHODS

The intent of this chapter is to give thereader a background of all the various ex-plosion prevention methods available. Themost popular methods are: explosion proofhousing, purging, and intrinsic safety. Therest of this book will concentrate only onintrinsic safety. Before we do that, how-

ever, let’s review a few miscellaneousprotection methods. These methods arenot used very often, but they provide an in-teresting insight to explosion prevention.

Oil Immersion - A piece of equipmentis immersed in oil. A spark can not passthrough the oil covering to ignite the sur-rounding atmosphere. This method issometimes used for large electricalapparatus such as transformers. Theelectrical apparatus may even havemoving parts since the oil will not impedethe movement.

Powder Filling - An enclosure con-taining electronics is filled with quartzpowder. The level and granulometry of thepowder is specified by an approval agency(CENELEC). A spark can not passthrough the quartz powder and ignite the

TYPE Z PURGEREDUCES ENCLOSURE FROM

DIVISION 2 TO NON-HAZARDOUS

TYPE Y PURGEREDUCES ENCLOSURE FROM

DIVISION 1 TO DIVISION 2

TYPE X PURGEREDUCES ENCLOSURE FROM

DIVISION 1 TO NON-HAZARDOUS

1. Label stating four volumes of purgegas needed before power.

2. Pressure of 0.1 inch water.3. Enclosure temperature < 80% of

ignition temperature of gas.4. Purge failure alarm or indicator. (No

automatic power-off necessary.)5. Warning nameplate.6. 1/4 inch tempered glass window.

1. Label stating four volumes of purgegas needed before power.

2. Pressure of 0.1 in water.3. Fused based on enclosure thickness

to ensure enclosure temperature< 80% of ignition temperature of gas.

4. Purge failure alarm or indicator. (Noautomatic power-off necessary.)

5. Warning nameplate.6. 1/4 inch tempered glass window.7. Equipment mounted within exclosure

must meet Division 2. (HermeticallySealed Switches, Relays, Contacts.)

1. Timer to allow 4 volumes of purgegas.

2. Pressure of 0.1 in water.3. Fused based on enclosure thickness

to ensure enclosure temperature <80% of ignition temperature of gas.

4. Power Disconnect on purge loss.(Pressure or flow actuated.)

5. Warning namplate.6. 1/4 inch tempered glass window.7. Automatic power disconnect switch

on door.

Figure 3-4: Purging Methods.

Figure 3-5: Encapsulation.

7

surrounding atmosphere. No moving partsare possible.

Non-Incendive - Also called Simpli-fied Protection Method in Europe, it iswidely used in England. This protectionmethod does not consider fault conditions.It assures you can not ignite a hazardousair/gas mixture during normal function-ing. Devices which may spark during nor-mal operation such as relays and switchesmust be hermetically sealed. This safetymethod only applies to Division 2 areas.

Increased Safety - Developed in Ger-many. Recognized by CENELEC. Takesinto account connections, wiring, compo-nents, distances and temperature changes.

Sealing (Limited Breathing) - Usesseals and gaskets. The enclosure is

tightened so gas accumulation does not in-crease above lower explosive level (see fig-ure 1-3) for a period longer than thepresumed presence of the hazardous gas/dust.

INTRINSIC SAFETYIntrinsic Safety (I.S.) removes the igni-

tion source from the ignition triangle. Theofficial definition is as follows: Intrinsi-cally safe equipment and wiring shall notbe capable of releasing sufficient electricalor thermal energy under normal or abnor-mal conditions to cause ignition of a spe-cific atmospheric mixture in its mosteasily ignited concentration.

Figure 3-6: Intrinsic Safety removes the ignition source from the ignition triangle. The official definition is as follows:Intrinsically safe equipment and wiring shall not be capable of releasing sufficient electrical or thermal energy undernormal or abnormal conditions to cause ignition of a specific atmospheric mixture in its most easily ignited concentration.

METHOD ADVANTAGES DISADVANTAGES

Pneumatic 1. Self-contained system2. Non-electric (air)

1. Limited distances2. High maintenance3. Limited number of operations

Fiber Optic orPhotovoltaics

1. Speed2. Non-electric (light)

1. Cost2. Special components (switches, etc.)3. Limited distance, environment & applications

Explosion-Proof Enclosures 1. Familiarity2. Proven applications3. High voltage

1. High cost2. High maintenance, no live maintenance3. Size, weight

Purging (Pressurization) 1. Lowers the area classification2. Can be used in applications where

nothing else can be used

1. High cost & maintenance, limited access2. Gas supply needed3. No live maintenance

Encapsulation,Miscellaneous

1. Good complement2. Mechanical protection

1. Not approved2. Throw-away

Intrinsic Safety 1. Low maintenance2. Low installation cost3. Many applications

1. New in North America

Figure 3-7: Comparison of Safety Methods.

8

Ronan’s Series X57 Intrinsic Safety Bar-riers eliminate the ignition source fromthe ignition triangle, thereby preventingcombustion. There are two basic types ofignition sources.

THERMAL IGNITIONThe first type of ignition source is ther-

mal. A hot surface can cause ignition byspontaneous combustion. This is whyevery hazardous area has a temperatureclassification (see Table 4-1 for a full list-ing of all the temperature classifications).

Let’s take one example of how tempera-ture classification works. A hazardousarea classified as T4 risks explosion if anysurface gets hotter than 135°C (275°F).Faulty motor windings, for example, cancreate such a high temperature veryquickly. Any piece of equipment used in-side this particular hazardous area mustnot be capable of generating a tempera-ture greater than 135°C. Only equipmentrated at T4, T5, or T6 would be allowed inthis hazardous area.

IGNITION BY SPARKThe second type of ignition source is the

spark. Matches and cigarette lighters fallinto this category. Many chemical plantsplace containers at the entrance wherepeople must put their matches or lightersbefore entering.

A spark can also be produced electroni-cally. Any electronic device which stores orproduces energy can cause a spark. Let’sdiscuss three different electronic ignitionmechanisms in more detail.

CLOSING OF CONTACTSIN CAPACITIVE CIRCUITSA capacitor stores energy in the electri-

cal field between its plates, and this en-ergy can present itself as a spark in thehazardous area.

When the switch in Figure 4-2 is open ina capacitive circuit, the capacitor chargesto a voltage “V”, and accumulates anenergy equal to one-half the capacitance(C) times the square of the voltage.

E = 1/2 CV2

When the contacts close in a capacitivecircuit to discharge a capacitor, a sparkcan form just before the contacts touch. Be-cause the contacts are almost touching,the size of the spark is small.

The switch shown in this example canalso represent field wiring which is acci-dently shorted.

Figure A-1, in Appendix A, compares theignition voltage to capacitance.

Ignition Mechanisms

Figure 4-2: Closing of contacts in capacitive circuits.

Maximum Surface Temperature

EuropeUSA/

CanadaFahrenheit Centigrade

842 450 T1 T1

572 300 T2 T2

536 280 T2A

500 260 T2B

446 230 T2C

420 215 T2D

392 200 T3 T3

356 180 T3A

329 165 T3B

320 160 T3C

275 135 T4 T4

248 120 T4A

212 100 T5 T5

185 85 T6 T6

Figure 4-1: Temperature Classifications.

9

OPENING CONTACTSIN INDUCTIVE CIRCUITS

The energy stored in the magnetic fieldof an inductor can be released in the formof a spark in the hazardous area.

With the switch in Figure 4-3 closed,the inductor L stores an energy equal toone-half the inductance times the squareof the current.

E = 1/2 Li2

When the switch opens, the inductor“tries” to keep current flowing from A to Bas it had been before the switch wasopened. The voltage which attempts to con-tinue this current flow is determined bythe equation

V = L(di/dt)The term “di/dt” is the change in cur-

rent with respect to time. This numbercan be very large because the current willchange from its previous value to zero, the

instant the switch is opened. If the induc-tance is large, then the voltage developed(which is equal to L * di / dt) is large, anda spark could appear between the twoelectrodes formed by the switch opening.

The switch shown in this example canalso represent field wiring which is acci-dently cut.

Figure A-2, in Appendix A, comparesignition current to inductance.

OPENING AND CLOSING OFCONTACTS IN RESISTIVE CIRCUITS

In this instance, shown in Figure 4-4,the ability to ignite the hazardous atmos-phere depends on the open circuit voltage(V = VOC), and the short circuit current (ISC

= V/R). Energy from the power supply couldbe released in the form of a spark at thepoint of the circuit opening or shorting.

Figure A-3, in Appendix A, compares theignition current to voltage for resistivecircuits.

Figure 4-4: Opening and closing of contacts in resistivecircuits.

Figure 4-3: Opening contacts in inductive circuits.

10

I.S. BARRIER INTERCONNECTIONAs illustrated in Figure 5-1, an intrin-

sic safety barrier is inserted in the non-hazardous, or safe area between theinstrument and the field device. The bar-rier blocks dangerous energy from beingtransmitted from the instrument to thehazardous area. This energy could be froma power supply, stored in capacitors,stored in inductors, or some combinationof the three. This energy could be releaseddue to some combination of faults (opencircuits, shorts, grounds, etc.) occurring inthe system.

The field devices used in any hazard-ous area must be one of two types:

Simple Apparatus - Barriers may beused with devices which qualify as “simpleapparatus” without specific approval forthe simple apparatus. A simple apparatusis a field device which meets the followingrequirements: Device does not store more than 1.2Volts or 20µ Joules. Device does not draw more than 100mAmps. Device does not dissipate more than 25mWatts.

Some examples of simple apparatus in-clude: thermocouples, RTD’s, switches andLED’s.

I.S. Certified Apparatus - Any devicewhich does not fall into the category ofsimple apparatus must be I.S. Certified.This includes transmitters, current to pres-sure convertors, solenoid valves, et. al.Certification may come from FM, CSA,BASEEFA, or the regional qualifyingagency (see chapter 8 for more details).

An intrinsic safety barrier is used toconnect a non-certified piece of electricalequipment in a safe area to a certified orsimple field device in a hazardous area. Abarrier cannot be used to make an uncer-tified device safe in a hazardous area. If afield device is uncertified, it can have inter-nal energy storing components. These com-ponents, in the case of a fault, may causea spark, which in turn may start the com-bustion process. The barrier only protectsthe non-certified device in the safe areafrom transmitting dangerous energy to thehazardous area. In summary, the barrieris an energy limiting device placed on theelectrical wires between the safe and haz-ardous areas.

How Intrinsic Safety Works

Figure 5-1: The Intrinsic Safety Barrier. Notice the intrinsic safety barrier is always located in the safe area.

11

There are two basic varieties of intrinsicsafety barriers: the zener barrier and theactive barrier. Let’s review each of them ina little detail.

ZENER BARRIERThe zener diode barrier works by di-

verting potentially dangerous energy toground before it can reach the hazardousarea.

The zener diodes limit the fault voltageto the hazardous area. There are two such

diodes for redundancy. The series resistorlimits current to the hazardous area, andis considered an infallible component. Thearrow in Figure 5-2 shows the resultantpath if excess current enters the barrier asa result of excess voltage input from the in-strument.

Since a zener barrier is powered by theloop and it has a current limiting resistor,it has a voltage drop across it. This bar-rier, in addition to protecting against

Figure 5-2: Zener Barrier Fault Path.

Figure 5-3: Will the resistance of a zener barrier adversely affect this 24 V, 4-20 mA transmitter loop? Not if it is lessthan 340Ω.

Resistance Available in Loop R LOOP = 24 Volts / 20 mAmps = 1200ΩResistance Used Before Barrier R FIELD APPARATUS = 12 Volts / 20 mAmps = 600Ω

R LOAD = 250ΩR LEADS = 10Ω

Resistance Available for Barrier R MAX FOR BARRIER = 1200Ω - (600Ω + 250Ω + 10Ω) = 340Ω

If the barrier resistance is greater than 340Ω, a full scale signal will never reach 20 mA, it will be a lower value.For instance, if the barrier resistance is 500Ω, the maximum mA output will be: 24 Volts / ( 500Ω + 250Ω + 5Ω+ 5Ω + 600Ω ) = 17.6 mA. In order for this loop to function properly, the maximum end-to-end resistance of thebarrier (through both signal paths) must be less than or equal to 340Ω.

12

dangerous energy entering the hazardousarea, must allow the loop to functionnormally.

The example in Figure 5-3 shows a24 V, 4-20 mA transmitter loop.In this example, the sum of the voltage

drops around the loop can be calculatedand verified that they are less than 24 V.Instead, since the maximum resistance ofthe barrier is specified (RMAX END TO END),Ohm’s law is used (V = IR) to calculate thetotal resistance available in the loop.Either method (loop voltage or loop resis-tance) works identically. What we are try-ing to decide is whether or not the resis-tance of the barrier will adversely affectthe loop.

ACTIVE BARRIERAn active barrier uses transformers,

opto-isolators or relays to provide isolationbetween the hazardous area and the non-hazardous area. It does not require an in-trinsically safe ground connection. Eitheror both the hazardous area or the non-haz-ardous area signals may be grounded.

This may be the most logical barrierchoice if a high quality I.S. groundingpoint is not available.

Figure 5-4 shows a diagram of anactive barrier. The barrier requires a24 Vdc power supply, and can typicallydrive larger loads than a loop poweredzener barrier.

Figure 5-4: Active Barrier. An active barrier is sometimes called a galvanically isolated barrier.

13

REQUIREMENTS FORINTRINSIC SAFETY

Intrinsically safe systems should satisfythese requirements:

u Ensure that there is a positive sepa-ration of intrinsically safe and non-intrin-sically safe circuits. This prevents theignition capable energy from intrudinginto the intrinsically safe circuit.

u Separate conduit, panduit, cabletrays, etc. should be used for I.S. wiring tokeep it separate from the non-I.S. wiring.If conduit is used, it must be sealed perthe applicable code. This is to prevent theconduit from becoming a means of conduct-ing flammable material from the hazard-ous to the non-hazardous location. I.S.wiring must always be identified. This canbe performed by tagging the wires, label-ling the junction boxes, and/or color-codingthe I.S. wires light blue.

u Ensure that the entity parameters,upon which approval of the system isbased, match up correctly.

ENTITY CONCEPTThe entity concept has been approved

by Factory Mutual since 1978. Theapproval agency specifies the maximumenergy a barrier can ever deliver, as wellas the maximum energy a field device canever receive and still be safe. The entityconcept allows interconnection of I.S. bar-riers to I.S. field devices not specificallytested in such combinations. It also allowsmixing of equipment from differentmanufacturers without additional ap-provals.

Certification of barriers and fielddevices is based on the following condi-tions. Barrier entity parameters (fromFM) must state the maximum voltage andcurrent delivered under fault conditions. Afield device entity parameter must statethe maximum current and voltage which

the hazardous area device can safelyreceive.

An I.S. barrier can safely be used with afield device if it has a maximum voltageand current less than or equal to thatwhich the field device can safely receive.All these parameters are defined by the ap-proval agency.

In addition to voltage and current, theentity parameters also include two moreterms to ensure that the energy storingdevices in the field do not store up adangerous amount of energy. These termsdeal with the total capacitance and totalinductance of the system.

A barrier has parameters which statethe maximum capacitance and maximuminductance which can be safely connectedto it. A field device has parameters whichstate the equivalent capacitance and in-ductance of the field device. Since wiringhas capacitance and inductance, this alsoneeds to be considered. The capacitance ofthe field wiring must be added to thecapacitance of the field device to deter-mine whether the total capacitance for the

Installation

Entity Equationsfor Capacitance and Inductance

Capacitance + Capacitance ≤ Capacitance(Field Device) (Field Wiring) (I.S. Barrier Max)

(Field Device) (Field Wiring) (I.S. Barrier Max)Inductance + Inductance ≤ Inductance

Entity Equations for Current and Voltage

ITOTAL (Barrier) ≤ IMAX (Field Device)

VTOTAL (Barrier) ≤ IMAX (Field Device)

14

field exceeds the maximum allowed forconnecting to the I.S. barrier. The sameprinciple applies to the inductance.

GROUNDINGWhen using zener barriers, the danger-

ous energy is diverted to ground. There-fore, it is important to ensure a highquality intrinsic safety ground. In fact,because this grounding is so critical, twoseparate ground connections from eachbarrier are recommended, each with a re-sistance of less than 1 ohm.

The following rules should be adhered tofor I.S. grounding:

Since zener diode barriers work by di-verting fault currents, they must have agood quality ground. The impedance of the conductor fromthe barrier ground bus to the ground con-nection should be less than 1 ohm. Follow local codes. Ground cables should be insulated. Ground cables should be protected me-chanically if there is a risk of damage. Ground cable should be an uninteruptedrun to the nearest high integrity earthpoint. Redundant cables are advisable.

15

Intrinsically safe products and systemsare certified or approved as safe byvarious agencies. These same agenciesalso approve explosion-proof devices,enclosures and accessories.

Why doesn’t a customer just buy safetyproducts from a vendor, connect them up,and pronounce the system safe? Why doeshe need an approval agency?

The approval agency does all the safetyanalysis for the end user. The user nowdoes not have the responsibility for deter-mining whether or not the system is safe.Most users do not have the extensive igni-tion testing facilities found at the approvalagencies. For a user to evaluate all theequipment he uses would require an enor-mous investment.

Another important reason for goingthrough an approval agency is that theuser reduces his liability by specifyingonly approved devices (including simpledevices) for use in hazardous areas. Mostinsurance companies require approveddevices in the hazardous areas. Large com-panies that are self-insured typically re-quire all approved devices as well.

The certification agencies in the U.S.A.are Underwriters Laboratory (UL) andFactory Mutual Research Corporation(FM or FMRC). Typically, FM does most ofthe intrinsically safe approvals. UL hassteered more towards handling approvalsfor systems rated at 115 Vac and higher,which have no application in intrinsicsafety.

FM has been around for well over 100years. FM started when a group of insur-ance companies formed a lab to testproducts used in the properties theyinsured. This agency approves a widevariety of equipment, materials, and ser-vices for property conservation. A few ofthese products are fire extinguishers, auto-matic sprinkler systems, buildingmaterials, shut-off valves, and electricalequipment used in hazardous areas. Such

electrical equipment would includeexplosion-proof, non-incendive, and intrin-sically safe devices.

Equipment rated as intrinsically safe isrecognized by article 500 of the NationalElectrical Code as safe for use in hazard-ous locations without special enclosures orphysical protection that would otherwisebe required.

The Factory Mutual approval standardfor intrinsic safety is number 3610, datedOctober, 1988. Any intrinsically safeproduct, whether it is located in the haz-ardous area or it connects to a device inthe hazardous area, must meet thisapproval standard. After January 1, 1992,any device which was not examined or notre-examined under this new standard cannot be sold as an I.S. device. This newstandard attempts to get rid of all the var-ious I.S. products on the market approvedto old standards. In the past, companieswould get a “grandfather clause” and sellnew devices approved to old standards.

The sharp customer will want to verifythat any I.S. product he purchases (i.e. I.S.field device, I.S. barrier, or product with in-tegral I.S. barrier) is approved to the newstandard. He will also want to know whatapplications the approval is for: e.g., canyou use the product with two barriers con-nected in parallel, or one barrier con-nected to two field devices? Additionally,he will want to know the entity parame-ters for the product. The customer can askthe vendor for a copy of the approval aswell as for the appropriate installationdrawings.

The design standard, which both FMand UL design their approval processes tomeet, is ANSI/UL 913 (formerly NFPA493). The installation standard isANSI/ISA RP12.6, “Installation of Intrin-sically Safe Instrumentation Systems inClass I Hazardous Locations.” Thisdocumentation should be used for guid-ance on I.S. installations. The National

Certification

16

Electrical Code, section 504, also discussesI.S. installation.

In Canada, I.S. product approvals areobtained through the Canadian StandardAssociation (CSA). This group developsthe standard and also certifies devices tothe standard.

When obtaining FM and CSA approv-als for intrinsic safety, the equipmentmust also meet the requirements for gen-eral purpose use. For FM, this means youmust also meet the requirements of the

Factory Mutual Standard for ElectricalUtilization Equipment. This ensures apiece of equipment is not a shock or firehazard.

When someone specifies FM approval,they should specify which type of FMapproval they require. If it is generalpurpose (G.P.) approval, a device which isapproved for I.S. or G.P. can be used. Ifthe approval is for I.S., only I.S. approveddevices may be used.

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VOLTAGEOpen Circuit Voltage (VOC) - The maxi-mum voltage a barrier can deliver to thehazardous area in the case of a fault. Thisvalue is assigned by Factory Mutualbased on the configuration of the barrierapplication. This entity parameter istested and assigned by Factory Mutual.Basic Safety Specification (V) -Approximate maximum voltage a barriercan deliver to the hazardous area. Thedesign engineer will want to use VOC in-stead of this term.Working Voltage (VWK) - The maximumdc voltage (or peak ac voltage) that can beconnected between the safe area ter-minals and ground, with the hazardousarea terminals open circuited. This is themaximum voltage the barrier can safelypass with no (or very little) leakage cur-rent through the zener diodes to ground.Maximum Voltage (VMAX) - The maxi-mum peak voltage which can be sustainedwithout opening the fuse. The hazardousarea terminals are open circuited for thismeasurement. If they are not, then thiscurrent will reduce VMAX. This is ameasure of zener diode breakdowntolerance, and fuse characteristics. This

I.S. barrier term should not be confusedwith the VMAX entity parameter of a fielddevice.

RESISTANCEBasic Safety Specifications (Ohms) -The value of the terminating resistor.This resistor performs the current-limit-ing function, and is a wire wound (infal-lible) resistor.Maximum End-to-End Resistance(Ohms) - The total resistance of the bar-rier between the safe and the hazardousarea connection terminals, including allresistors and fuses.

CURRENTShort Circuit Current (ISC) - The maxi-mum current a barrier can deliver to thehazardous area in a fault situation. It isbased on the configuration of the barrierapplication. This entity parameter istested and assigned by Factory Mutual.Basic Safety Specification (mA) - Theapproximate maximum short circuit cur-rent available in the hazardous areaunder fault conditions. This value is calcu-lated from the basic safety specificationsfor voltage and resistance (V/R).

Definition of Barrier Terms

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Intrinsically safe barriers are designedto be application oriented. If you want abarrier for any application, an RTD, for ex-ample, you can select one of the few barri-ers a company has designed for RTD’s.You may have a choice of a zener barrierto work with a two wire RTD, anotherzener barrier to work with a three wireRTD, and yet another zener barrier towork with a four wire RTD. You may alsohave a choice of an isolating active barrierand even a barrier which takes a threewire RTD input, performs signal condition-ing, and gives an isolated, linearized, 4-20mA output.

In general, once you know the applica-tion, you can choose from a small subsetof a manufacturers’ large variety of bar-riers. There are certain steps that youneed to go through to select the properbarrier.

We are now ready to go through theprocess of selecting an I.S. barrier for anapplication. We will go through all thesteps required so you can become familiarwith them. Two examples follow, bothwith the same application.

The first example is basic barrier selec-tion. It takes into account voltage and cur-rent. The second example is advancedbarrier selection and is shown in Chapter10. In this example, we take the basic bar-rier selection and expand it to includecapacitance and inductance. These fourparameters: voltage, current, capacitance,and inductance help form the entity con-cept which allows users to safely “mix andmatch” barriers and field devices from dif-ferent manufacturers.

These examples show two considera-tions the user needs to look at when select-ing a barrier. The first is that the barrierallows the loop to function during normaloperating conditions. The second is thatthe loop is safe during fault conditions. Inorder to impress certain points, these ex-amples do not review the voltage dropthat will occur across the barrier when

current passes through the barrier resis-tance. This must be considered, becauseduring normal operating conditions, we donot want the voltage drop across the bar-rier to adversely affect the circuit. Thiswill not affect the safety of the loop, it willjust not allow the loop to function prop-erly. These examples do not attempt tochoose between an active or a zener bar-rier. The benefits of each were discussedearlier. The examples work equally wellfor both active and zener barriers.

Lastly, these examples do not considermultiple channel barriers, connecting mul-tiple field devices to one barrier, or con-necting multiple barriers to one fielddevice. This gets a little more complicated.Any barrier must have an approved instal-lation drawing showing that it is ap-proved for the particular connection youare considering.

BASIC BARRIER SELECTIONWhen selecting a barrier, you must ask

the following questions: What is the hazardous area classifica-tion?

Barrier Selection

19

What is the input voltage to the barrierwith respect to ground (AC, +DC, or -DC)?This is called VLOOP.What is the maximum voltage and cur-rent which can be applied to the fielddevice (VMAX and IMAX)? These are theentity parameters of the field deviceassigned by FM.

You will want to follow these guidelineswhen selecting a barrier: Select a barrier that is approved for usein the hazardous area where it will beinstalled. VLOOP VWKG (barrier) VOC (barrier) ≤ VOC

ISC (barrier) ≤ IMAX

VWKG = working voltage

VOC = max voltage delivered to the hazardous area

ISC = max current delivered to the hazardous area

VMAX = max voltage which can safely be applied to the field device

IMAX= max current the field device can safely draw

Figure 9-1 shows a barrier connected be-tween an instrument in the safe area andan I.S. approved field device. The barriercan be safely used here and has full ap-proval. During normal operating condi-tions, this instrument loop operates at+24 Vdc. The zener diodes in the barrierdo not start to conduct any voltage toground until that voltage reaches +25.5Vdc. Thus, under normal operating condi-tions, the barrier is transparent to theloop.

During fault conditions, the maximumvoltage the barrier can ever deliver to thehazardous area is 31.2 V. The field deviceis approved safe for that hazardous area ifit never receives more than 35 V. The bar-rier ensures this. The barrier can neverdeliver more than 90 mA to the hazardousarea during fault conditions. The fielddevice is safe as long as it never receivesmore than 110 mA. The barrier ensuresthis as well. Therefore, because the volt-age and current entity parameters match

Figure 9-1: Basic Barrier Selection. In order to simplify the understanding of barrier selection, we will not consider thecapacitance or inductance of the field wiring and the field device in the basic example. We will add this to the advancedexample. The basic barrier selection example satisfies the three equations, therefore it is an approved loop. In addition tothe three equations, the field device must be approved for use in that particular hazardous area (CL1, DIV1, GPC, T4).The barrier must also be approved for connection to that hazardous area (CL1, DIV1, GPC). All the critical safety ap-proval parameters can be found in the installation drawings done by the manufacturer of the barrier and the field device.

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up correctly, this loop is approved intrinsi-cally safe.

This was a little bit simplified, and didnot consider the capacitance and induc-tance entity parameters. In reality, all theentity parameters (voltage, current,capacitance and inductance) must matchup correctly in order for the loop to beapproved intrisically safe. The entityparameters are all given by the approvalagency and obtained through the manufac-turer. These are shown in an installationdrawing. There is one installation draw-ing from the barrier manufacturer andone from the I.S. field device manufac-turer. Both of these should be obtained bythe user to ensure correct design. Andkeep in mind, just because the loop is safedoes not mean that it works!

The normal operating conditions aregiven by the manufacturer and are notsubject to approvals.

Let’s take the next step and discusscapacitance and inductance.

CABLE CAPACITANCEAND INDUCTANCE

A cable (or wire) is an energy storingdevice. A cable stores capacitive energyand inductive energy. From an intrinsicsafety standpoint, this cabling needs to beanalyzed to ensure safety at the systemlevel. What good does it do to limit theamount of energy which can be passedthrough the barrier if potentiallydangerous energy can be stored in thecable?

Figure 9-2 diagrams the relationship ofcapacitive energy to cable length and of in-ductive energy to cable length. Capacitiveenergy stored in cabling increases as cablelength increases.

The maximum inductive energy whichis stored in the cable is related to the ratioL/R, and is independent of the cablelength.

The L/R parameter is the unit induc-tance of the cable divided by the unit resis-tance of the cable. This parameter is usedin the example in Chapter 10.

Figure 9-2: Cable Capacitance and Inductance.

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Let’s go through our first example asecond time, taking into considerationcapacitance and inductance. In addition tothe questions asked in the basic selection,we need to ask the following:u For the field device: What is the

value of the capacitance and inductanceappearing at its terminals? (This is calledCEQ and LEQ and is specified by FM.)

u For the cables: What is thecapacitance and inductance of the cableused [CCABLE (in pF/ft.), and LCABLE (inµΗ/ft.)]? The L/R ratio of the cable can beused instead of the inductance/ft.,simplifying the calculations.u For the barrier: What is the amount

of capacitance and inductance which maybe safely connected to it? This is calledCA and LA and is specified by FM.)

Advanced Barrier Selection(Includes Cabling)

Figure 10-1: Advance Barrier Selection. In this example, we take the basic example and also consider the capacitanceand inductance of the field wiring and the field device.

22

The following simple equations need tobe satisfied:

CEQ + CCABLE ≤ CA

LEQ + LCABLE ≤ LA

Figure 10-1 illustrates the previous ex-ample with capacitance and inductancetaken into consideration. The maximumallowable cable capacitance, and thereforemaximum allowable cable length is calcu-lated as follows:

CEQ + CCABLE ≤ CA

CCABLE = CA - CEQ

= 0.4 µF - 0.2 µF= 0.2 µF max

Length = 0.2 µF / 40 pF / ft= 5000 ft max

Due to Capacitive Energy

LEQ + LCABLE ≤ LA

LCABLE = LA - LEQ

= 2.9 mH - 2.1 mH= 0.8 mH max

Length = 0.8 mH / 0.3 µH / ft= 2667 ft max

Due to Inductive Energy

Since capacitive energy increases perunit length, 5000 ft. is the max. cablelength permitted due to capacitive energy.However, inductive energy calculationsresult in a maximum length of 2667 ft.

The ideal intrinsic safety systemdesigner takes the capacitance andinductance into account at the beginningof the design, not as an afterthought toverify safety.

The designer can calculate that capaci-tance and inductance allowed in the cable.Next, knowing his longest cable length,he can calculate some cable requirements.The values used in the above example(40 pF/ft. and 0.3 µH/ft.) are typicalparameters and are usually not exceededin instrumentation cable. By definingmaximum cable parameters, the designeris leaving nothing to chance.

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Before designing the X57 Series, Ronaninvestigated many different factors thatshould go into the design of an intrinsicsafety barrier.

Several factors involved meeting therequirements of the various approvalagencies. In the development of the I.S.approval standards, these agencies took

Ronan’s Philosophy onthe Intrinsic Safety Barrier

Figure 11-1: Intrinsic Safety Barriers with Surface Mount Chassis.

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extra safety precautions with each step.Some of these measures include: assum-ing any mixture present is at its mosteasily ignited concentration, 1.5 safety fac-tor on the maximum current allowed, twoground wires for redundancy, and twozener diodes required per channel.

When these safety standards were re-leased, an alarming fact was discovered.Although no explosions have occurredwith intrinsic safety barriers properly in-stalled, explosions have resulted fromfaulty installations.

Further research and customer feed-back illustrated a major need for easy in-stallation, low maintenance and safetyassurance. Observation of I.S. loops in thefield showed additional areas where im-provement was needed.

Taking all of these factors into account,Ronan developed a chassis-based designwith modular (plug-in) barriers. Allwiring connects directly to the chassis, notto the individual barriers. Also, a metalcover plate is provided to insulate the bluehazardous area terminal blocks. Thisgives additional protection for the intrin-sically safe wiring. These design featureshave many advantages:u The chassis has FM, CSA, andBASEEFA (CENELEC) approvals.u Users wire up their system and canverify wiring accuracy before insertingany barriers in the loop. This minimizesthe risk of losing any barriers duringstart up.uIf a barrier needs to be removed orreplaced, wire removal is not necessary.The old barrier snaps out easily and thenew one snaps in.

This eliminates a major cause of intrin-sic safety problems: maintenance. Mostmanufacturers of barriers do not provideadditional safety in their mounting pack-age to reduce installation and main-tenance errors. Here’s a direct quote fromanother manufacturer of barriers concern-ing replacement of intrinsic safetybarriers.

“Should a safety barrier require removalfrom the intrinsically safe circuit please fol-low these guidelines.

Disconnect the wiring from the safetybarrier’s nonhazardous terminals priorto disconnecting wires from the intrinsi-cally safe terminals.Cover bare wire ends with tape or otherinsulating material, especially thoseconductors that are towards the hazard-ous (classified) location.Disconnect the safety barrier fromground. In most cases this step willalso remove the safety barrier from themounting hardware.Reverse this procedure to mount thenew barrier."

Obviously these guidelines leave muchto be desired.u The chassis is easily installed andready to use, unlike the flimsy “do-it-your-self” mounting arrangements availableelsewhere.u The zener barriers, which requireground, all connect to an FM approved in-ternal I.S. ground bus. This bus connectsto two external mounting studs which theuser connects to I.S. ground. In a typical20 position chassis, this reduces the num-ber of crucial ground connections by 95%.u The active barriers, which require 24Vdc to operate, all connect to an internalpower bus. This reduces the number ofpower connections by up to 95%.u Any barrier can connect to any positionin any chassis.

Keying of barrier and chassis positionsis available in order to prevent putting abarrier in the wrong position.

In short, Ronan took an acceptablestandard and went the extra mile to makeit better, developing a design strategythat does not rely on ideal situations.Ronan I.S. Barriers provide safety alongwith ease of installation and maintenancein the real world.

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The following are some ofthe many graphs which anintrinsic safety approvalagency uses when certify-ing a field device for use ina particular hazardousarea. These are the actualignition graphs for thevarious gases. They repre-sent the amount of electri-cal energy required to causethe combustion of anair/gas mixture at its mosteasily ignited concentra-tion. The area under thecurves represents an en-ergy which is too small tocause an explosion.

Figure A-1 applies toresistance-capacitance cir-cuits only. This graph com-pares the ignition voltage tocapacitance for a Group Bgas at its most easilyignited concentration.Various current limitingresistors are added inseries to the discharge pathof the capacitance. This isshown to lower the effectivecapacitance.

Appendix: Ignition Graphs

Figure A-1: Capacitance Circuits

C

V

COMPARISON OFIGNITION VOLTAGETO CAPACITANCE

FOR CIRCUITSIN HYDROGEN

ENVIRONMENTS

26

Figure A-2 applies to resistance-induc-tance circuits only. This graph comparesthe ignition current to voltage for specificvalues of inductance. The curves repre-sent the inductive energy required tocause the combustion of a Group B gas atits most easily ignited concentration.

Figure A-3 applies to resistive circuitsonly and shows the combinations of volt-age and current which will ignite air/gasmixtures for groups A, B, C and D. Whendesigning intrinsically safe products, a

safety factor of 1.5 is applied to the igni-tion current allowed.

The approval agencies apply safety fac-tors when using these graphs. You are notallowed to have a circuit which operatesjust below the ignition energy required fora particular gas. This would be too close toa potentially dangerous situation.

The selection of which intrinsic safetybarrier to use for a particular applicationdoes not entail the use of these graphs

Figure A-2: Inductance Circuits

COMPARISON OFIGNITION CURRENT TO

VOLTAGE FORVARIOUS INDUCTANCECIRCUITS IN GROUP B

Open Circuit Voltage VOC (Volts)

Min

imum

Val

ueof

I SC

for

Igni

tion

(Am

ps)

27

Figure A-3: Resistance Circuits

V

I

COMPARISON OFIGNITION CURRENTTO VOLTAGE FOR

RESISTANCE CIRCUITS

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