Class I Division 2 Group D Nonincendive circuits

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HazLoc

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If I use a listed NEC Class 2 power supply with rated output below the ignition curves, does this make my circuits nonincendive and therefore able to feed NRTL approved nonincendive and intrinsically safe field devices; and use non-hazardous location wiring methods, quick disconnects and simple devices, such as switches, without intrinsic safety barriers?
 

rbalex

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You might sneak this by under Section 500.8(A)(3) but I wouldn?t count on it since it would still need to be installed under a control drawing required by Section 501.10(B)(3).
 

steve66

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I'm not sure, but I thought intrinsically safe equipment had to remain safe even with a component failure.

I'm also not sure how that would apply to a class 2 transformer. Would you have to consider the possibility of a primary to secondary short on the transformer??
 
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I think I'm confused about something. When you say rated output below the ignition curve, are we talking about the temperature rise of the device, or the power output? If it's temperature rise, I don't understand what bearing this will have on the circuit design.

Is the power supply located inside the C1D2 area?
 
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Ok, I looked up the ClassII PS, so disregard my last post.

From my reading, it seems like you need to verify with the PS manufacturer that the PS is UL rated for nonincendive circuits. This would be fine for your Div 2 area, but not for a Div 1 area. See the comments following 501.10 B 3 (2008)

Hopefully someone with more experience can verify this.
 

rbalex

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Within the general means of defining Groups there are two terms that are commonly used: Maximum Experimental Safe Gap (MESG) and Minimum Igniting Current (MIC) Ratio. A third term, Minimum Ignition Energy (MIE), while basically valid, is usually too difficult to determine and so is avoided. Note that none of them are directly related to temperature.

For the purposes of the OP, MESG is pretty much irrelevant. However, theoretically a power supply with a maximum output below a specific gases? MIC or MIE might fly under Section 500.8(A)(3) as nonincendive without specifically being listed for Division 2. That said, the manufacturer would still have to create a control drawing indicating the proper use of the unit. I doubt it would be worth the effort without going ahead and having it listed as well.
 
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I would like to start at the original question.

If I use a listed NEC Class 2 power supply with rated output below the ignition curves, does this make my circuits nonincendive and therefore able to feed NRTL approved nonincendive and intrinsically safe field devices; and use non-hazardous location wiring methods, quick disconnects and simple devices, such as switches, without intrinsic safety barriers?

The relatively easy answer is NO. Product defined as associated apparatus or used within a hazardous location (field devices) must be certified by an OHSA approved NRTL. A Class 2 power supply has been evaluated against fire or shock hazard. As part of this evaluation, there is no consideration as to its ability to cause/prevent ignition of a hazardous location. It seems simple enough to say that the Class 2 power supply must be safe as long as the voltage/current output is below the ignition curve. It?s true - physics state that a point below the ignition curve will not ignite the specified hazardous ?classified? area. However, there is more to consider.

Let?s deal with intrinsic safety first. A device is only considered intrinsically safe (associated apparatus or a field device) after multiple fault conditions are introduced at random places within the product and after additional safety factors of at least 50% (sometimes even more) are applied to the output as well as to the rating of components within the circuit. An intrinsically safe device is only safe after it meets a wide range of electrical and mechanical requirements. OSHA has granted the NRTL?s as the only agencies able to claim a product meets the requirements specified in the appropriate standards for safety. A Class 2 power supply is not evaluated to the appropriate ANSI standard(s) for intrinsic safety (or nonincendive/Division 2). Next, let?s consider nonincendive, this type of device does not normally have faults considered as part of an evaluation for safety; however, the output parameters are still just a small part to the overall certification of a product that is allowed to be connected to an NRTL approved field instrument or simple apparatus.

Simple apparatus, such as switches and quick disconnects, are not required to be certified by an NRTL but they still require connection to a certified intrinsic safety barrier or nonincendive barrier to operate within a hazardous location.
 
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