500.6(A)(2) Ex#1

[don_resqcapt19]

Does this exception mean that I can install a valve position limit switch that is reated for Class 1, Division 1, Groups C and D in a Group B, butadiene location as long as I install a seal off at the limit switch enclosure? It appears to me that is what the words say, but this limit switch package requires a seal even in a Group C or D location.

 

[rbalex]

While this particular application required a seal anyway, not all do. Both Exceptions 1 & 2 permit using something other than a Group B rated enclosure for certain specific materials.

 

[don_resqcapt19]

Bob,
Thanks...that is how I read it too, but the enclosure has a screw on cover and the threads are not as fine as I would expect to see on a Group B rated product. How does the seal have anything to do with the hot gas escape path through the threads? I thought that was one of the big issues with Class B products.

 

[rbalex]

I just looked at butadiene's "stats" in NFPA 497 and it's something of a "maverick." In classic Zones it is only Group IIB rather than IIC.

Since 1999, Group B has been defined as "...having either a maximum experimental safe gap (MESG) value less than or equal to 0.45 mm or a minimum igniting current ratio (MIC ratio) less than or equal to 0.40."

Butadiene has an MESG of 0.79mm, but a MIC ratio of only 0.13 which still considerably higher than hydrogen's (0.019). I'd say the relatively large MESG permits the use of Group D equipment where suitable seals are installed.

 

[don_resqcapt19]

Thanks Bob