THE ADVANTAGES OF EXe Junction over Ex d Junction Box

[chauhanrasikend]

Anybody is having the drawbacks of Ex-d junction box compare to Ex-e Junction box.

[jberge]

Ex-e (or more precisly Ex-em) junction boxes can be made from plastic where Ex-d are metal. This tells me Ex-e must be lower cost.

I have seen Ex-em junction boxes in the form of field mounted multi port safety barriers. The ones I have seen have separate compartments for the Ex-em side and the Ex-ia side meaning you can open the Ex-ia side while power is still on at the Ex-em side which remains closed by a second internal cover. That is, Ex-em allows connection/disconnection of spurs under power. I think that is advantegous and I don't see how you could do it using Ex-d box.

Jonas

[Mike ONeill]

Jonas is quite correct to comment on the fieldbarrier approach that generates Exi spurs, but of course that isn't of much use if you want/need to use Exd devices.
The fundamental differences between Exe and Exd junction boxes concerns the size/weight and accessability; Exe JBs are lighter, much lower cost and may even be GRP plastic. Access in Exe boxes is relatively simple because these units do not have to have lots of bolts and other secure fitttings. Cable glands are pretty much the same (practically everyone has dual approval glands - Exd/e) unless you come across someone who believes in the 2L rule for IIC and insists on barrier (epoxy-filled) glands.
One big problem with Exd boxes is that you may well put some kind of spur-protecting electronics inside, but you can't then access any live terminals without de-powering the whole segment. Friends at some user companies claim that opening live Exd boxes need three people: a process guy with a gas detector, an instrument guy with the screwdriver and an electrician to undo & redo all the bolts.
Luckily there are several solutions for using Exd fieldbus devices based either on live de-mateable plug/sockets (CEAG, STAHL) or on other means of interlocking the spurs in the off position (MOOREHAWKE).

[dearkd]

This is really quite informative.

[Peter]

MTL also offer the MTL951 connector, either for 240 V AC or instrument signals

Peter Inns, MTL Australia

[Mike ONeill]

If you are putting a Megablock in Zone 1, does that require an explosion proof box with these connectors, or is there a version which has Exe-approved terminals, or some other mechanism by which you can install a unit with unshielded pluggable terminals in Zone 1?

[mikestrauser]

There are versions of Megablocks that are Ex em certified (part number have a "-XE" suffix). They are certified by an ATEX notified body (KEMA) to be safe in a Zone 1 area when used in an Ex e rated enclosure which as has been previously stated are typically plastic. For those who are familiar with the Zone 2 principle of "non-arcing", Ex e is "non-arcing" for Zone 1. By imposing more strict requirements on things like the connectors on the Megablock, the risk of arcs and sparks is reduced to a level that offsets the increased risk of an explosive mixture of gas being present. This method makes a lot of sense and looks like a great alternative to Ex d installations. It is too bad that it hasn't been generally accepted in North America yet.

[Mike ONeill]

Thanks, Mike.

I can certainly agree that the Exe "increased safety" method of explosion protection is a very useful technique, and in Europe, it is very commonly used for what would previously have been purely flameproof boxes. In the US, it is an allowable technique where plants are designed and built to IEC/AEx standards, but these are still few and far between.

To return specifically to fieldbus then, we (MooreHawke) are certainly seeing a lot of users in both offshore and onshore petro-chem applications using Exme device couplers in Zone1 with GRP enclosures. We managed to get ours dual certified (Exn for Zone 2 and Exme for Zone1) so that commonality of product across these Zones strengthens the argument for their use in Zone 1.

Overall, Exe fieldbus JB's are cheaper, lighter, and easier to use than the Exd equivalent.