Terminal Block in Non-Hazardous Enclosure Located in Class 1, Div 2 Location

[msimms]

I was recently asked to finalize an electrical design at a power generation facility. The client wishes to have remote/emergency control of a natural gas slam shut valve, in the event pressure readings are deemed to be too high at certain points in the pipeline. To do this, we were tasked to wire control cables from the existing control room out to a solenoid block which electrically controls the valve, and to a set of limit switches that verifies valve position. Given the potential existence of natural gas (potential leak sources at valves and pressure transmitters) the area was deemed to be Class 1, Division 2. It is an outdoor station, so the determination was made to deem anything within 15' of the potential leak locations to be in a hazardous location.

The design was to route the new cable from the control room in existing cable tray out to the skid, drop it into RGS pipe, and then route to an enclosure next to the valve containing a terminal block. The reason for the TB, was due to the use of pigtails coming from the solenoid control block, with the connections already hermetically sealed on the valve side. The pigtails don't stretch more than 2', so this required a closely mounted box, and a terminal block seemed like the best way to tie these field cables to the control room cables. The question was whether this electrical termination at the block, was deemed to require a hazardous rated enclosure. I've been told that it is acceptable to install a ?standard? (NEMA 12, 3R, 4, 4X) enclosure in a classified area, if this enclosure has only hermetically sealed devices or non-arcing devices (like terminal blocks). I understand that a hazardous enclosure for electrical devices is required for situations where a spark can be produced by switching action or some other ignition source.

The concern of course is the potential for the connection at the terminals to become loose and cause an air gap and in the unlikely, but possible scenario that the solenoid valve signal is sent (which is 120V), a spark is produced.

So my question, through all of this design background, is whether those in the industry in anyone's opinion consider a terminal block a non-arcing device, and whether those who work within these types of environments, would be okay with installing standard enclosures if TBs are the only items installed within?

Thanks and sorry for the elaborate question setup.

 

[rbalex]

Terminal blocks are not considered to be arcing, sparking or heat producing. This is indirectly discovered in Section 501.10(B)(4) and their omission in 501.105(B)(1), 501.115(B)(1), and 501.150(B)(1).


Technically, even in Class I, Division 1, they are usually ignored when they are the only thing under consideration. See Section 501.15(A)(1)(2).

 

[BPoindexter]

Question for you Bob- previous places I have worked have always considered diodes, resistors, and such installed on terminals to be heat producing devices and thus they required explosionproof boxes. What is your opinion on that?

 

[rbalex]

The basic concepts for applications of heat producing items in Division 2 are found in 501.105(B)(2) Exception:

Exception: General-purpose-type enclosures shall be permitted if such equipment is without make-and-break or sliding contacts [other than as provided in 501.105(B)(1)] and if the maximum operating temperature of any exposed surface will not exceed 80 percent of the ignition temperature in degrees Celsius of the gas or vapor involved or has been tested and found incapable of igniting the gas or vapor.

This exception shall not apply to thermionic tubes.

This concept is found throughout various other Division 2 applications; i.e., the surface temperature doesn’t “… exceed 80 percent of the ignition temperature in degrees Celsius of the gas or vapor involved or has been tested and found incapable of igniting the gas or vapor.”

Most diodes, resistors, etc. haven’t been specifically tested; however, that means they would need to exceed the assumed ambient (40C) ambient by 28C to be in trouble with the lowest “T-Code” [.8 x 85C - 40C]. The “running” temperature of most properly applied resistors and diodes is below that. It should be checked, of course, but with the next “T-Code” higher you can boil water and there are more personnel safety issues than explosionproof enclosures.

 

[msimms]

Thanks, Bob. Good to know for future work.