Thermocouples in Class I, Division 1 Areas


Thread Starter

Bruce Thompson

I have found many schematics showing thermocouples in Class I, Division 1 areas but none showing how cables, connectors, etc. are used to acceptably provide connections to the thermocouples.

Does anyone have a resource for this information? What will be acceptable to the local Authority Having Jurisdiction (AHJ)?

I would like to have an open connector panel in the area that I can plug T/Cs into. Will the AHJ accept this?
Responding to Bruce Thompson's Sep 5, query and Bob Peterson's Sep 7, reply.... based on the info provided "should be OK" is libelous:

I doubt that the AHJ will approve the installation unless the T/C too, is Certified by an appropriate testing facility for use in either the XP or IS installation!

Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

Bruce Durdle

In the IEC world, at least (and I think in the US environment) thermocouples are treated as "simple apparatus" in IS loops and do not need certification - they are incapable of developing sufficient energy to cause ignition. (They still need to be connected to a certified transmitter.)


Kelly, Richard

As usual, Phil's response is on the money.

The best piece of advice I can give you... go talk to your AHJ and ask him (or her) what it is expected in this installation. They would rather deal with an issue like this upfront then have to fail an installation after the fact.

Note: any further comments are based on the Canadian Electrical Code, while there are vast similarities between the rules of the CEC and the US NEC, you will need to check the validity of any advice given for your particular area.

What this system needs in order to be passed by the AHJ depends upon one question, is this system going to be built intrinsically safe or not?

If it is built intrinsically safe, meaning that all equipment to be used is approved as being intrinsically safe, the normal rules pertaining to electrical systems in hazardous locations (CEC rules 18-100 through 18-380) do not apply. You will still need to provide separate raceways (or mechanical barriers between sections in one raceway) so that intrinsically safe circuits and wiring are separated from non intrinsically safe circuits and wiring. You will also need to provide vapor seals in the raceway to prevent explosive gases from migrating to other locations.

If the system is not designed to be intrinsically safe, then every aspect of the system must meet the standard rules for electrical equipment in the particular hazardous location.

In any case... I would not accept the "open connector panel" in the hazardous location as the only reason for having it located in the hazardous location is convenience, which is not a valid reason IMHO.

Best Regards...

Rick Kelly

Senior Plans Review Officer and
Qikiqtaaluk Region Electrical Inspector

Safety Services C&GS Protection Services Government of Nunavut

Phone (867) 975-5419
Fax (867) 975-5453
Responding to Bruce Durdle's Sep 15 comment... I disagree. Following are my reasons:

1) I answered the question as it related to Class I, Div 1 Areas, which comes under the jurisdiction of the USA NEC.

2) Bruce T. brought up the question of connections To T/Cs. XP enclosures address the problem.

3) I agree, that a T/C is considered a device that by itself can't cause ignition (technically, however, it is not impossible.) But, it must be
connected to an amplifier. And, they can't be used in classified area unless suitably altered, example- IS.

3) The IEC definition of "simple apparatus" has been used in the US for years. But, the NEC does not have a general definition except that used in connection with nonincendive devices, i.e., while suitable for use in Div 2 areas, they are not permitted in Div 1 areas.

4) All of the European projects I was involved with required vendor document "certifying" that the T/Cs selected were suitable for use with their recommended IS barrier.

Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

Bruce Durdle

Hi Phil,
Just to clarify things for me ...
I am looking at a copy of the NEC as annotated by Crouse-Hinds.

Section 504.4 states that "All intrinsically safe apparatus and associated apparatus shall be listed. Exception - simple apparatus, as described on the control drawing, shall not be required to be listed.

"Simple Apparatus" is defined in 504.2 as "An electrical component or combination of components of simple construction with well-defined electrical parameters that does not generate more than 1.5 volts, 100 milliamps, and 25 milliwatts, or a passive component that does not dissipate more than 1.3 watts and is compatible with the intrinsic safety of the circuit in which it is used.

FPN: The following apparatus are examples of simple apparatus:
(a) Passive components; for example, switches, junction boxes, resistance temperature devices, and simple semiconductor devices such as LEDs
(b) Sources of generated energy, for example, thermocouples and photocells, which do not generate more than 1.5 V, 100 mA, and 25 mW."

I would interpret this as indicating that thermocouples etc do not need certifying if used as part of an IS circuit - they would with any other method of protection, and the transmitter or amplifier must be suitably certified. Where am I going wrong?



Michael Griffin

In reply to Phil Corso - I did not see anything in Bob Peterson's reply which could be construed in any way as "libelous" (see the last word in your first sentence quoted below). His opinion on the use of thermocouples may or may not be correct, but I do not see how it can be considered libel.

I've included his original reply (from a separate message) below yours for reference. My reply to you may be considered as "nit picking", but this particular nit is important for a public mailing list.

On Thu 14 September 2006 22:47, Phil Corso wrote:
> Responding to Bruce Thompson's Sep 5, query and Bob Peterson's Sep 7,
> reply.... based on the info provided "should be OK" is libelous:
> I doubt that the AHJ will approve the installation unless the T/C too,
> is Certified by an appropriate testing facility for use in either the XP
> or IS installation!

On Thu 7 September 2006 02:07, Bob Peterson wrote:
> From: Bob Peterson
> To: [email protected]
> Subject: Re: ENGR: Thermocouples in Class I, Division 1 Areas
> As long as you use correct IS barriers, you should be OK.
> Keep the control documents.
Following is my response to Bruce Durdle's and Michael Griffin's Sep 16 comments. Please bear with me, but it's a 3-parter consisting of a) the Preface; b) the "libelous" comment: and c) continuation of the Technical argument.

The Preface.
I have always participated in the discussion openly and honestly. My responses were based, in part on my experience(s) and in part on my desire to pass on to others want I have learned (a rarity amongst Engineers, just ask their wives!) I have tried to keep the discussions professional, sometimes humorous, but foremost professional. One of the Professional Engineering Canons I applied is, "Issue public statements objectively and truthfully!"

Digressing for a moment... I don't know the origin of the following but it speaks volumes, "free advice is often taken as having zero value!" It's an attitude of many List participants as indicated by the lack of the simple, but courteous phrase, "thank you!"

The Libelous Statement
I don't know Bob Peterson. My statement was certainly not meant accusingly, nor to condemn or ridicule him. Instead it was meant as an "alarm!" When I see "shoulda", "coulda", or "woulda" in a public statement, the one making the statement is a "target!" How? You ask! In this litigious society any one can sue any one else for anything! You all have a favorite frivolous law suits (mine's abut the office suicide whose estate sued the building's design engineer!)

Consider this scenario.... Bruce T's plant explodes.... someone recalls what Bob said. Immediately, Bob is put on the "person of interest list!" "Not possible!" you say.

The above situation wasn't my story, but close. I have been involved in 3 litigation... 2 as a defendant; one as a defense consultant. One, a gulf state refinery explosion with loss of life, required that I pay $40,000 a legal fee to prove "my equipment was not in the unit that exploded!"

The other was a boiler explosion in a large mid-state utility. The utility (Plaintiff) filed an $800 million law suit, naming everyone, including me, as sub-contractor to the BMS (Burner Management System) vendor. The outcome... the jurors assessed damage at about $500 million, held the utility responsible for 70% and the PLC vendor for 30%. The plaintiff's lawyer said, "We're gratified the jury recognized we should receive compensation." If you want to know the rest of the story(ies), contact me!

The Technical Discussion.
Nowhere, I repeat nowhere, does the NEC state that a "simple apparatus" is permitted in a Class I, Div 1 area. Imagine buying a $30,000 X-proof rated motor having RTD or Thermocouples temperature monitors without the appropriate Classification enclosures.

Regards, Phil Corso, PE {Boca Raton, FL, USA} [[email protected]] ([email protected])

Bruce Thompson

In respect to Phil's Technical Discussion...

Per NEC (2005 edition), Article 500.7(E) permits intrinsic safety as a protection technique in Class I, Division 1 or Division 2 areas.

Article 504.4 allows for simple apparatus (thermocouples, RTD's, etc.) to be installed intrinsically safe, without being listed (with the provision that they are installed using the proper "Associated Apparatus").

My concern rests with how to install these thermocouples using flexible cabling, connectors, etc. where we have applications that can benefit from these devices being replaced, moved, or reallocated.

Can anyone give me some examples on how these devices have been installed in this manner?

Bruce D. Thompson, PE
Hi Bruce,

That was my reading of NEC, and is in line with IEC standards.

The restrictions on wiring techniques that apply with other explosion protection methods don't affect IS circuits - Articles 501 through 503, and 510 through 516 do not apply. Theoretically, any connection can be used with an IS-protected circuit once you are downstream of the barrier or isolator providing energy limitation. (The original development of IS was driven by explosions in coal mines when guys whacked bare wires with their shovels to signal to the pit head they had a full load to be pulled up - adding IS allowed them to retain the bare wires but avoid the explosions.)

So you should be able to use any good wiring practice consistent with retaining the general circuit integrity. There are no particular issues with removing and re-connecting components on-line, and testing is also possible if Intrinsically Safe test equipment is used.

However, it is always a good idea to get alongside the guy who is going to sign it off and make sue he has no particular bees in his bonnet - preferably at a very early stage in the project.