General Usage Thermocouples in Class 1 Division 2 Hazardous Location

[mcollings]

I have been trying to locate direct language in the NEC that allows for the use of unclassified wiring techniques for thermocouples in a Class 1 Division 2 location. The only thing that I can see as an option is to consider the circuit as non-incendive to qualify.

Background:
1) Our thermocouples use simple quick-disconnect plastic heads
2) We use ordinary thermocouple extension wire out of the wire trays to connect to these thermocouples
3) Our data acquisition system is rated Class 1 Division 2

Code:
Using NEC 2014

This type of thermocouple (non-amplified, no transmitter) I believe falls under a 'simple apparatus' definition that does not need to be listed, and allows them to be connected in a non-incendive field wiring circuit.

An exemption to Article 725.121 (A) allows the thermocouple to not be listed as a Class 2 power source.

I believe Class 1 Division 2 wiring techniques only consider 'Normal' operating conditions (open, short or grounding of wiring), without the added protection from equipment faults (abnormal conditions) from our data acquisition system. The fact that the thermocouple wiring is connected to a PLC should not require the added fault protection of an IS barrier.(?)

Question:
Is our wiring technique acceptable based on NEC?
Could the absence of a formal 'control drawing' for this common scenario be the only reason for non-compliance?
Can documentation or certifications be referenced from third parties or consultants that makes this practice defensible?

 

[petersonra]

I have been trying to locate direct language in the NEC that allows for the use of unclassified wiring techniques for thermocouples in a Class 1 Division 2 location.

What is an unclassified wiring technique?

The only thing that I can see as an option is to consider the circuit as non-incendive to qualify.

You do not get to make that choice.

Background:
1) Our thermocouples use simple quick-disconnect plastic heads
2) We use ordinary thermocouple extension wire out of the wire trays to connect to these thermocouples
3) Our data acquisition system is rated Class 1 Division 2

None of this matters.

Code:
Using NEC 2014

This type of thermocouple (non-amplified, no transmitter) I believe falls under a 'simple apparatus' definition that does not need to be listed, and allows them to be connected in a non-incendive field wiring circuit.

An exemption to Article 725.121 (A) allows the thermocouple to not be listed as a Class 2 power source.

Does not matter in your case, while being true.

I believe Class 1 Division 2 wiring techniques only consider 'Normal' operating conditions (open, short or grounding of wiring), without the added protection from equipment faults (abnormal conditions) from our data acquisition system. The fact that the thermocouple wiring is connected to a PLC should not require the added fault protection of an IS barrier.(?)

What "added protection" does your DAS provide?

Question:
Is our wiring technique acceptable based on NEC?

I am not sure you can use the quick connects if there is no control drawing that says you can. It is not a chapter 3 wiring method.

Could the absence of a formal 'control drawing' for this common scenario be the only reason for non-compliance?

A control drawing is not just a formality. It is a requirement.

Can documentation or certifications be referenced from third parties or consultants that makes this practice defensible?

Consultants probably not. An NRTL of some sort, maybe. It is hard to say since i am not sure what it is you want this third party to do for you.

A simpler solution would seem to be to get a plc that has a control drawing associated with it for use on T/C. one would think that if your DAS is listed as being suitable for C1D2 such a drawing would likely be available.

 

[mcollings]

General Usage Thermocouples in Class 1 Division 2 Hazardous Areas

General Usage Thermocouples in Class 1 Division 2 Hazardous Areas

By unclassified wiring I'm referring to methods not covered directly under the scope of NEC 500-505...probably covered by NEC 300.

Not trying to be argumentative, but the decision to qualify as non-incendive is similar to making the decision to use the method of an intrinsically safe system if the equipment is available (rather than explosion-proof, e.g.)...that's all. The problem I have is trying to prove, what at face value seems obvious, that thermocouple extension wire is passive under 'normal' conditions.

The DAS says Class 1, Zone 2 AEx nA IIC T4. It also says "connected signals must be within the following limit:
Capacitance....0.2uF max." There is a picture of a wiring diagram to properly connect to the module. Would this be a valid control drawing?

If an IS system is our only alternative, would it allow the wiring practice described in the original posting, quick disconnects and extension wire out of the tray, or is that still not allowed under NEC 300?

 

[rbalex]

By unclassified wiring I'm referring to methods not covered directly under the scope of NEC 500-505...probably covered by NEC 300.

Not trying to be argumentative, but the decision to qualify as non-incendive is similar to making the decision to use the method of an intrinsically safe system if the equipment is available (rather than explosion-proof, e.g.)...that's all. The problem I have is trying to prove, what at face value seems obvious, that thermocouple extension wire is passive under 'normal' conditions.

The DAS says Class 1, Zone 2 AEx nA IIC T4. It also says "connected signals must be within the following limit:
Capacitance....0.2uF max." There is a picture of a wiring diagram to properly connect to the module. Would this be a valid control drawing?

If an IS system is our only alternative, would it allow the wiring practice described in the original posting, quick disconnects and extension wire out of the tray, or is that still not allowed under NEC 300?

Article 501-505 wiring methods are already recognized in Chapter 3. You may have too great a fixation on finding a non-explosionproof system. Class I, Division 2 is fairly permissive about wiring methods for non-arcing, sparking or high temperature (ASH) equipment.[See Section 501.10(B)]. The wiring you described in the OP is fine in Division 2 and the wiring itself isn't required to be either intrinsically safe or nonincendive.

Although they have their place, I'm not a big fan of either intrinsically safe or nonincendive systems. Beyond their requirement for a control drawing, they get no relief from Class I bonding/grounding requirements.

This will be an interesting mixing/matching of NEC Class I, Divisions and Zones concepts. That "AEx nA" mark on the DAS is very important. The "nA" marking indicates the DAS is non-sparking which would be permitted under 501.105(B). Section 501.5 permits certain Zone marked equipment in Division classified locations. Protection Technique "nA" is acceptable in Division 2. If it is "AEx" marked it is listed.