Thanks for all the good information. I have a question regarding the use of an Ex d enclosure located in a Zone-1 area. Within the Ex d enclosure is a data logger system that will sample several IS sensors remote to the enclosure connected by suitable rated cables and glands. Is it considered acceptable to install IS barriers within this Ex d enclosure? (assuming that all separation distances are maintained). Thereby implying the interior of the Ex d enclosure is a 'safe' area. Note that the data logger itself is not IS rated, and the power to the Ex d enclosure will be unprotected.
I have seen this done, and seen it approved by seemingly knowledgeable 3rd party authorities. In one case the inspector insisted a stainless steel chassis be placed over the barriers...
The practice seems illegitimate and unsafe to me in that in the event that the Ex d enclosures does its job and contains an internal explosion or fire, there would be a high likelihood of the IS barriers being damaged and thereby possibly allowing dangerous voltage/current combinations to be supplied to the remote IS sensors.
This is a question I have wondered about myself. This is an area where common sense and an in-depth understanding of the real issues involved take precedence over the letter of the law.
Clause 12.2.1 of IEC60079.14 does apparently allow installation of [Ex i] apparatus in Zone 1 inside an Ex d or Ex e box - "Associated apparatus should preferably be located outside the hazardous area or, if installed inside a hazardous area, shall be provided with another appropriate type of protection in accordance with Clause 5 which is suitable for the ignition sources which the associated apparatus may present."
However, note that the assumption made here is that the [Ex i] associated apparatus is the only source of ignition considered. The possible flow-on effect of ignition of the gas inside the enclosure on the performance of the apparatus is nowhere mentioned. If the enclosure held only Ex i associated apparatus, I would be reasonably comfortable as there are restrictions on the maximum fault level of the supply to barriers etc. However, with other equipment (possibly with elevated surface temperatures) and unlimited supply, the situation is a bit different.
There is also the issue that the general principle of the Ex d approach is that any ignition of the gas inside the enclosure cannot result in ignition of the surrounding atmosphere. While, strictly speaking, protection is aimed at the external ignition coming about through hot products of combustion escaping from the enclosure, you could be a bit more pedantic and state that any event that could cause ignition outside the enclosure has to be avoided.
Ignition of the internal atmosphere could damage insulation and result in effective paralleling of protected outputs - giving an excessive current flow. If the rules regarding segregation of wiring inside the enclosure have been followed, there should be little chance of a high-power circuit shorting to an IS one. An ignition event is going to be transient as combustion will stop as soon as the box fills with combustion products, so it's not likely that the barriers themselves will be damaged significantly.
On balance, I think that it is probably reasonably safe, especially bearing in mind that the likelihood of ignition inside an Ex d enclosure is pretty small anyway. (Note the use of lots of bet-hedging fudge words in the above sentence!) However, the suggestion that all will be well if a partition is fitted inside the box breaks one of the main rules of Ex d enclosures - don't do anything to create sub-sections that could result in pressure piling.
In short, definitely raise the possibility and let the proposer place his or her neck on the chopping-block - but it's acceptable under the standard provided reasonable precautions are taken. A better solution would possibly be to use an extra enclosure rated Ex e as an intermediate box - most if not all barriers will comply with Ex e requirements for creepage and clearance distances if not actually rated as Ex e. Look for a barrier rated as Ex e.
Best of all is to put the whole lot in the safe area where it belongs.
Thanks for your thoughts and the comments. I certainly concur with your closing sentence. However, I also acknowledge that it is sometimes not possible or at least not practical. It never occurred to me that one might place appropriately rated IS barriers into an Ex e enclosure separate from or external to an Ex d enclosure. While I can imagine this perhaps being feasible in a Zone 2 or Div 2 area with the right barrier (R. Stahl 9000 series barriers seems to allow this), I tend to doubt such a thing is acceptable in a Zone 1 or Div 1 area. A more acceptable idea might be that of placing IS barriers within an Ex p protected enclosure separate from the Ex d, then again if one can create an Ex p solution, why bother with the Ex d in the first place...
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