Low power DC devices - intrinsically safe or not

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megloff11x

Senior Member
There is a complicated (and expensive) process for verifying MESG and other criteria used in the listing of devices for hazardous areas. It may seem silly at first glance, but any non current limited Voltage could make a spark big enough to ignite the more flammable vapors and dusts.

Are the standard communication means (RS232 at 12V, Ethernet, RS485, etc.) considered intrinsically safe and is there a citation or supporting document for this?

You can always plug in within a purged & pressurized cabinet, but sometimes it's easier to have the connection outside.

I would ask the same for various DC low powered sensors. Obviously a non-hazardous listed, unsealed, relay switching something is out, but what of other devices?

The demand for hazardous listed sensors and computer communication interface connectors and cables is small enough that I haven't heard of anyone going to the expense of listing.

We say that most Squirrel cage type motors can go in Class I Division 2 because no brushes = no sparks. Is there a similar rule of thumb for DC devices and communication interfaces?

Or must you submit the system or facility to the listing agency and have them test those parts too, on your nickel?

I'd like to know which ones are intrinsically safe and which ones must be located in a purged and pressurized area.

Matt
 

rbalex

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Location
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Professional Electrical Engineer
There is no rule of thumb. Intrinsically safe / non-incendive circuits, components, apparatus, systems, etc. are defined per UL and ISA standards. See the FPNs in 500.2 and 504.2 that reference the specific documents.

The reason the content of the FPNs are not code themselves is that the NFPA Manual of Style does not permit direct citing of other Standards - even other NFPA Standards.

In addition, all intrinsically safe / non-incendive installations must be per a Control Drawing.
 
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