Intrinsic Safety


Thread Starter

B. Wills

I need to interface volt free status contacts in the field ( hazardous area ) to my DCS through I.S barriers. The contacts in the field are auxiliary contacts from motor starters and valve limit switches. Can these contacts be wired straight to the I.S barriers or should the field devices to which these contacts belong also have I.S certification ?

Also what are the special features of field JBs for I.S circuits ?

Pham Duc Hai

You can visit MTL website at "": . There is a lot of information about I.S barrier and other related stuff there.

Good luck.
Best regards,

Pham Duc Hai
Sales Manager
Resident Representative office in Hochiminh-City
111 Nguyen Thai Binh Street, District 1, Hochiminh City, Vietnam
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Bob Peterson

If your area is classified by US norms (Class I, Division 1), likely the aux contacts on the motor starters are in an explosion proof or otherwise
protected enclosure so they would probably not require IS barriers. Just the normal seals in the conduit would be adequate. If the valve limit switches are explosion proof, again only seals would be needed and some limit switches
come with built in seals. If the limit switches are not explosion proof, you can run IS wiring back to your barriers, in a seperate conduit. You still need conduit seals but only at the area classification transition points. It is simplest not to put JBs in unless you really have to, but they can be normal terminals with no special requirements. Your IS wiring (downstream of
the barriers) must be segregated from non-IS wiring, and normally is put into blue colored wire duct/cover to distingish it from non-IS wiring inside enclosures. Some special grounding and labeling requirements also exist at the IS barriers and the conduits that carry the IS circuits to the field devices. As for the field devices themselves, if they are "simple" devices,
they don't have to be certified. Simple devices are things like dry contact switches. Anything with energy storage capability in it (such as capacitors or inductors) or energy generation capability requires certification to use as IS. There are also some limits on cable length (due to the capacitance and inductance of the cabling), but I have never found this to be an issue, but in long runs it could become one. The IS barrier literature will tell you the maximum allowed capacitance and inductance. The cable manufacturer can tell you the amount of capacitance and inductance per foot of cable. If
you use multiconductor cabling for your IS circuits, each pair must have a seperate shield. An overall shield is not aceptable. I have always used an individual cable for each field device because it is usually simpler that way.

If you area is classified according to European standards (zone 0/1), its a different ball game. All the terminals and JBs (and everything else for that matter) have to be rated as EExE, which is something they refer to as "increased safety". I am not exactly sure what the difference is between EExE and non-EExE rated stuff, but its a requirement. The junction boxes
have to be inspected by a certified inspector after assembly to insure they have been assembled properly and only EExE rated components have been used. Generally this means you buy the JBs with the terminals already assembled and inspected by someone who is already certified, but you can also get an inspector to come out and inspect your own work if so inclined, but I am told its a nuisance and pricey to boot.

Keep in mind there are also some special grounding requirements in hazardous areas. You will need to get someone who knows this to do the installation work.

This is a very brief coverage of a "not as simple as it looks" situation, so if you are unsure, get in touch with someone who knows what he/she is doing.

Bob Peterson

Bruce Durdle

First, you need to check your local rules -

but, generally speaking, a switch contact that is not part of a component carrying other voltages can be treated as "simple apparatus" and does not
need to be specifically certified as IS. This would apply to valve limit switches if they are distinct items and not part of a positioner.

The contactor auxiliary contacts are another story because of the potential for a break-down between the IS components and the non-IS circuits. At the least you will need a clear physical barrier (preferably earthed metal) between the IS circuitry and any other circuits. But surely you will have other means of protection for any motor starters in the hazardous area? eg explosion proof.

There are no special requirements for field JBs in IS systems - they are also specifically regarded as "simple apparatus".

This is an area where I see/saw most errors. There are three problem scenario's:

a) If the input-sensor circuit passes thru the Classified area, i.e., unclassified - classified - unclassified, then barriers are required at
each interface.

b) If the input-sensor circuit starts at an enclosure in the classified area, even if x-proof, and there is a high voltage circuit in close proximity to the input-sensor circuit then a 2nd barrier is required at the enclosure. NOTE: the Rule-of-Thumb (sorry, it just came out)
definition of proximity is 2" with an impervious mechanical barrier, and 4" without. CAUTION: the local Approval Authority has final say.

c) Although not part of the original query, the same precaution is required for output-circuits. For example... Remote Interface Relay (RIR) located in the starter.

Phil Corso, PE
(Boca Raton, FL)


Field JB's & Instruments should have the certification based on the Hazardous area classification and It also should be instrically safe. So you can connect the cable directly from the Instrument to the I.S barrier. Please note that based on the hazardous area classification the cables also can be Intrinsically safe. All depends on the area classification.

Engineer - I & C.
TCE Consulting Engineers Ltd.
Responding to Bob Peterson's (Fri, Jul1, 11:26am) reply:

Bob, you're mixing oranges and apples. The seal and barrier have different purposes. The seal's purpose is to: a) prevent the products of an internal explosion from passing into the Div 1 area; b) minimize the passage of potentially hazardous vapors and liquids from entering the enclosure.

The barrier's purpose is to limit the amount of ignition energy that can enter the Div 1 area. I agree that excess current/voltage levels can be ruled out by the barrier dealing with the source of energy for the field device. However, there could be inadvertent contact with non-intrinsic circuits within the enclosure, or even thru specific combinations of conducting materials used in the field-devices' construction. Also, while IS and non-IS circuits are usually kept segregated, it must be done in perpetuity. Violation of tis pinciple, at least in Europe is grounds for criminal prosecution. In addition, the field device must/should be approved for IS-circuit use... often left up to the discretion of the Approval's Authority.

Phil Corso, PE
(Boca Raton, FL)
Gentlemen , Thankyou for all the very trenchant and useful inputs to my query. It has really helped to clarify things.

McGinnis, Patrick W


Not to be rude, but I hope that no reader answers your question in a positive "get a brand XYZ model ABC-123" manner. Your question shows a lack of Intrinsic Safety understanding that can (literally) be dangerous.

IS design is much more specific. You don't even mention if you are dealing with an input or output device, analog or discrete, voltage/current levels, Fieldbus, Profibus, etc...

I suggest you contact an experienced IS Interface supplier and discuss your situation. I'm certain they will be glad to talk about 120 units...

Make sure that they ask you about all the parameters I just mentioned, plus some... I also suggest that you study IS wiring and grounding practices... Essential for a circuit to qualify IS.

As a start, I'd contact Pepperl+Fuchs (I have no business interest in that company).

Good luck.


********* The opinions expressed herein are mine, not necessarily in agreement with those of my employer. Caveat emptor. *********
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