Class I Div II - Thermostat

[Tommyr]

We have an explosion proof box that we are putting in a class I div II location. The question is, would it be acceptable to place a standard A419 thermostat in the XP enclosure and run the cable through the enclosure to have the temp sensor in the class I div II room?

My interpretation of the code is that this is okay as long as there is a boundary seal and a process seal (501.17).

Another thing that I am curious about is how to determine if a cable is 'capable of transmitting gasses or vapors'. In the case that they are, the code states that they are to be sealed after the jacket and other coverings have been removed (501.15(E)(1)).

 

[rbalex]

I have started to answer this about four times and finally came to the conclusion that I had no idea what you wanted to do.

Let's start by asking what cable(s) are you talking about and where do you plan to mount the sensor?

 

[petersonra]

I'm not sure what you're trying to do either but if it's really just a thermostat it would probably be simpler just to buy an explosion-proof thermostat and mount it in the process piping. I don't think in the long run it would cost you any more to do it this way rather than doing it in the convoluted way I think you were trying to do to save a few bucks on the switch itself.

 

[don_resqcapt19]

I also don't know exactly what is being asked. Most of the A419 thermostats that show on a search have remote capillary tube temperature sensors. My guess is that he wants to install the thermostat itself in the explosion proof enclosure and extend the sensor tube out into the classified area.

I think he could do that as long as the tube was extended through a seal fitting.

 

[Tommyr]

Don is correct. We want to install the thermostat itself within the XP enclosure and extend the sensor through the enclosure wall into the classified area. Sorry for the confusion.

 

[rbalex]

Don is correct. We want to install the thermostat itself within the XP enclosure and extend the sensor through the enclosure wall into the classified area. Sorry for the confusion.

Is the sensor in direct contact with the process fluid?

 

[Smart $]

For those unfamiliar, this is an A419 t-stat...


The sensor is electrical, i.e. it connects to the controller with a cable. Spec' sheet lists it as a Type PTC Sensor (thermistor).

http://cgproducts.johnsoncontrols.com/met_pdf/125188.pdf

 

[petersonra]

For those unfamiliar, this is an A419 t-stat...

View attachment 15612
The sensor is electrical, i.e. it connects to the controller with a cable. Spec' sheet lists it as a Type PTC Sensor (thermistor).

http://cgproducts.johnsoncontrols.com/met_pdf/125188.pdf

this is not a capillary tube as don suggested.

I can't imagine how you would get the sensor into direct contact with a process fluid. it would need some kind of thermowell I would think.

if the whole thing is in a classified area I think you could just run conduit from the XP thermowell jbox to the xp box the thermostat is in.

I have never been real clear about what is meant by being able to transmit gas through the cable either.

 

[rbalex]

If the sensor isn't in direct contact with the process fluids Section 501.17 doesn't apply. This is what started my confusion in the first place. The OP also appears to have a misunderstanding about enclosure versus process seals. OP's understanding of boundary seals is also questionable; i.e., where (or what) is the boundary?

As as far as transmitting gases through the cable, unless it is MI cable, you may as well assume cables will transmit gases through the core, especially in Division 1. Even seals themselves leak. See Section 501.15 IN No.1. For extra scary bonus points see IN No.2.

While the scenario as proposed can be done, proper sealing will be a nightmare. Petersonra's first suggested installation is probably best - get a thermostat that is already suitable.

 

[Tommyr]

I appreciate all of the feedback guys.

The issue is that we have a customer who may have already installed the thermostats and was looking for clarification on whether or not this is an issue. Bob, you are right in questioning my understanding. It seems that every time I re-read a section I change my mind on what I think it means.

The entire XP enclosure will be within the class I div II area. The thermostat lead will then run through conduit into the classified area.

With that being said, would it just need the conduit seal as mentioned in 501.15(A)(1)?

Also, the reason I mentioned the process seal is because the fluids are confined but may be present "only in case of accidental rupture or breakdown of such containers".

 

[rbalex]

Ok, this may take a bit of back and forth questions.

Are the thermostat, conduit and sensor in the same classified location? If so, there is no "boundary" but there still needs to be an enclosure seal. The enclosure seal will need to be installed per Section 501.15(E)(1) and (3) and Section 501.15(B)(1). Since the enclosure is required to be explosionproof, these will ultimately lead back to Section 501.15(A) also.

Note: Thermostat power and signal wiring must also comply. These may or may not have boundaries.


​

Is the sensor mounted in a thermowell? If yes, Section 501.17 doesn't apply since the sensor is not in direct contact with the process fluid; if no, you may be in a world of hurt. We will have to get a detailed description of how the sensor is mounted.