IS barriers Vs. Hard wired safety circuit


Thread Starter

Tom D.

I have a hard wired series of safety interlock switches to be run through an explosion proof zone. Am I to run that circuit through IS barriers to a safety relay? Won't the IS devices break the hard wired circuit up? Is there a safety relay that would operate on an intrinsically safe current?

Ric Nazareno

correct me if I'm wrong, i assume the whole circuit is IS and you want to add another relay in the circuit to pick up another contact(s) for switching purposes? You can get an IS relay (hazard to hazard installation) which should enable you to install it without further re-certification (try RTK Linden, [email protected])

Hope the above helps.

I think your best bet is to use explosion proof safety switches (I infer you are talking about safety gate switches or guard interlocks) and run them back in conudit with the appropriate seals. I don't see how an IS circuit would qualify as control reliable.

The safety relay is in a non-hazardous area, the signal going out is 24vdc. That signal passes into the hazardous zone to the series of safety interlock switches. Because there are no safety relays on the market that send out an I.S. current, I would have to pass the 24vdc through I.S. barriers to step down the current. Because it is a safety circuit, it is required to be hard-wired, and my question is, is it acceptable to put an I.S. barrier (which is an electronic component) into a safety hard-wired circuit?

Johan Bengtsson

Ok, I have not studied the actual rules really to the letter but I think like this:

1. The rule to hard wire safety relays and such is that if they fail they are guaranteed to not produce a signal in the wrong situation.

2. Electronic devices are harder to make that guarantee. Especially if there are a micropocessor and a program involved in producing the logic.

3. However, an electronic device can not put out a signal if there is no voltage source into the electronic and there are no energy storage (battery, capacitor, coil, etc) inside.

4. The electronics inside an IS barrier are designed to limit current, I guess they are feed by the same signal they are designed to limit and does not contain any energy storage components.

5. If the 24V signal into the electronic is stopped, and that is the only energy source for the barrier, then no fault at the barrier can produce a signal output.

If all of the above are true you won't violate safety.

/Johan Bengtsson

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I think this where you need to carry out a Safety Integrity Level (SIL) assessment to allow you to determine if the loop meets your safety requirements. I believe there are threads within this site that discuss this issue.

Steve Smoker

I think maybe we're getting a little carried away.

A SIL assessment will only confirm if the process your trying to protect NEEDS a level of protection, and what that level is. Mitigating factors already present in the existing equipment can sometimes bring the risk down to within defined 'tolerable' levels.


What a SIL study wont tell you is if the design of the protection your putting in place is adequate!
Thats for the designers to come up with the failure data of the chosen components in the protection system and see if they match up with the level of protection (if any)required by the SIL study.


Personally I cant see why an IS barrier would compromise a safety circuit as (hopefully) to meet the SIL criteria of the system study the protection equipment has been designed 'fail safe'. I've seen a pharma installation with everything run through IS barriers and they were the least troublesome parts of the loops.

Its really for the safety system designers to purchase and fit good quality equipment with the correct failure mode to fulfil the functional safety requirements, not blame individual pieces of equipment as long as they are of reputable manufacture and proven use.


Fitting explosion-proof housings/conduit etc is another way I would probably think it more expensive and in the long run, being no more reliable.

Just as a note - in a UK industry survey of 34 accidents in safety systems, they found 44% were attributable to bad equipment specification, while only 6% were down to overall system design.

Food for thought: choose carefully I guess.

Hope this heps,

Steve Smoker