EEx 'd' or IS

M

Thread Starter

MJ

In the oil and gas, I am often faced with deciding to chose between an intrinsically safe circuit with a barrier and an ATEX certified EEx 'd' or EEx 'e'.

To me they've always been interchangeable considering:

- IS barriers add an extra point of failure

- EEx 'd' are expensive and have onerous testing, repairing, re-certifying etc.

but this is just a pro/con list. What am I missing? How else does one decide between the two?

Thanks
 
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Bruce Durdle

If it's mandatory to use all Zone 0 instrumentation, the Ex ia is your ONLY option - Ex d is not acceptable in Zone 0.

Regarding the original post, (which makes no mention of Zone) - the first factor to consider is the electrical rating of the circuit. Intrinsic safety is restricted to low levels of voltage and current (typically < 30 V, 100 mA) and cannot be used to deliver high power levels. If the load requires more than these levels, Ex e or Ex d are the usual options. Ex e has fewer restrictions than Ex d, but cannot be used for equipment where internal sparking may occur.
 
If one is thinking ahead, maintenance on IS circuits can often be carried out with a cold work permit, provided you have IS certified test equipment, whereas Ex d would require a hot work permit.

Geir
 
Just think two decades ahead: Ex d will be limited to a fewer cases (high power loads) and Ex ia will dominate in field instrumentation, mainly for economical reasons. In example, have a look in the new 2-wire (Ex ia) line of coriolis meters from E&H, traditionally restricted to 4-wire (Ex d).

So, IMHO choose Ex ia where possible (more safety, less maintenance effort and, mainly, less costs).
 
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