Thermocouples in Class 1 Div II Equipment

[fifty60]

504.30 (B) states that conductors in an intrinsically safe circuit shall be separated from each other by one of the following means : 1) the conductors are within a grounded shiel. 2) the conductors of each circuit have insulation with a minimum thickness of .25mm.

Does this apply to thermocouples? I have equipment that has Class 1 Div II interior. The only circuits entering the Class 1 Div II location are thermocouples. The thermocouples are ran from the Class 1 Div II interior by through a sealed fitting and then into an IS barrier located on the sub panel. Generally, all of the thermocouples use the same conduit.

Does 504.30 (B) apply to thermocouples? It specifically says "conductors" and gives the exception "Unless otherwise identified". The intent of the thermocouples are not to be a conductor. If I call this out on the control drawing am I able to run them in the same conduit?

 

[Smart $]

I think we need to retrace your steps...

To start with, see 504.2 definition for Simple Apparatus. I have never seen a situation where a thermocouple is anything "more" than simple apparatus.

 

[fifty60]

I read that to mean the T/C itself does not need to be rated Intrinsically Safe, but that it still has to through a barrier to make the circuit IS. We normally run the t/c straight from a controller into the inside of our equipment. When IS is required, we run the T/C through an IS barrier so that we get a 4-20mA signal at the output of the barrier, and the 4-20mA signal is then input into our controller located in the safe zone.

 

[fifty60]

I see the definition, but in the explanatory note above the definition it states (paraphrase) "...thermocouples....are not necessarily intrinsically safe because of the devices they are connected to. Should a fault occur inside the device it could be transferred to the thermocouple"

I know this is the explanatory note, but it seems to be the route we have taken in the past. The definition of simple apparatus does seem to contradict this however....

 

[Smart $]

I understand you've evaluated your control loops to require IS barriers. As such, if you are not using shielded or metal jacketed T/C wire, then 504.30(B)(2) applies.

 

[GoldDigger]

Something that takes a TC signal in and produces a 4-20ma signal out is more than just an IS barrier it is a transducer/transmitter and its output is no longer thermocouple wire.
Is it common to identify a transducer that incorporates an IS barrier as just a barrier?

Tapatalk!

 

[fifty60]

Some of the T/C's are shielded and some of them are not. I generally do not connect the shielding to ground. I am a little extra weary of this now since the Barriers ground reference the output of the my 24VDC power supply (which powers the barriers, but not actual controllers operate at 120V, also ground referenced from the secondary of a transformer located on the equipment).

Would the T/C shields have to be grounded to meet this exception? Would the noise cancellation benefits outweigh the possible ground loops?

 

[Smart $]

Some of the T/C's are shielded and some of them are not. I generally do not connect the shielding to ground. I am a little extra weary of this now since the Barriers ground reference the output of the my 24VDC power supply (which powers the barriers, but not actual controllers operate at 120V, also ground referenced from the secondary of a transformer located on the equipment).

Would the T/C shields have to be grounded to meet this exception? Would the noise cancellation benefits outweigh the possible ground loops?

Exception?

Shield has to be grounded to avoid insulation thickness alternative. There'd be no ground loops if only one end of the shield is grounded If the shield is part of the T/C lead wire, it may be bonded at the process end and grounded by contact. In this case, you'd isolate the controller end of the shield.

You're still required to meet the insulation thickness requirement for the unshielded T/C wires.

 

[don_resqcapt19]

If you are already using conduit, why are you looking at IS? There is no advantage or requirement to using IS in a Class I, Division 2 area if you are installing the conductors in a metallic raceway.

 

[Smart $]

If you are already using conduit, why are you looking at IS? There is no advantage or requirement to using IS in a Class I, Division 2 area if you are installing the conductors in a metallic raceway.

That's what prompted my first reply... but upon his evaluation confirmation, I assumed that perhaps the individual T/C leads break out
of the conduit within the HL.

 

[fifty60]

I will not have any ground loop problems with the barrier grounded, and the thermocouple leading to the barrier grounded at the barrier end? I know not to ground both ends, and I am comfortable grounding one end when the power supply is floating. The ground referenced barriers force my power supply to be ground referenced as well. This is why I am worried about the ground loop.

 

[Smart $]

I will not have any ground loop problems with the barrier grounded, and the thermocouple leading to the barrier grounded at the barrier end? I know not to ground both ends, and I am comfortable grounding one end when the power supply is floating. The ground referenced barriers force my power supply to be ground referenced as well. This is why I am worried about the ground loop.

As I said, depends on whether the shield is grounded by contact at the T/C end. If it is, isolate at the barrier end.

 

[rbalex]

If you are already using conduit, why are you looking at IS? There is no advantage or requirement to using IS in a Class I, Division 2 area if you are installing the conductors in a metallic raceway.

That's what prompted my first reply... but upon his evaluation confirmation, I assumed that perhaps the individual T/C leads break out
of the conduit within the HL.

With so many applicable wiring methods available for Division 2 [Section 501.10(B)] and the fact that thermocouples aren't arcing, sparking or high temperature themselves [Article 501,Part III], there's virtually no reason to use intrinsically safe OR nonincendive systems for them.

 

[Smart $]

With so many applicable wiring methods available for Division 2 [Section 501.10(B)] and the fact that thermocouples aren't arcing, sparking or high temperature themselves [Article 501,Part III], there's virtually no reason to use intrinsically safe OR nonincendive systems for them.

I agree... but all we can go on is posts #3 and 4.

 

[rbalex]

I agree... but all we can go on is posts #3 and 4.

I don't believe the misunderstandings displayed in #3 and 4 warrant requiring the system to be IS, especially since there also appears to be a misunderstanding of how a control drawing is used in the OP; i.e., he can't create his own - it can only be created by a manufacturer. Your response in #5 was correct though: if you're going to identify a circuit as IS you have to go the whole 9 yards whether it needed to be or not.

 

[fifty60]

What manufacturer is responsible for the control drawing? The barrier manufacturer? I am the OEM of the end equipment, and am installing the barriers at a customer request. There are sheath heaters in the hazardous area, and thermocouples recording air temperature and component temperatures inside of the hazardous area. The customer has made the further request to make it Class I Division II interior. There was not a request to make circuits Intrinsically Safe, or to label them as such, but there was a request to use intrinsic barriers and to label the equipment Class I Division II Interior. No concern about the sub panel or the inner wiring of the equipment.

I agree with your feeling on the T/C's. Isn't there danger, however, of a malfunction from the controller sending a dangerous amount of energy through the thermocouples in an instance of a fault?

The sheath heaters are connected on the wall, so the heaters themselves are in the hazardous area but the connections are in the unclassified area.
The thermocouples are ran from the unclassified area and enter into the hazardous area. They are ran from temperature controllers on a sub panel located in the unclassified area of the equipment. It is my understanding that the thermocouples will have to enter the hazardous area through sealed fittings.

Some of the thermocouples are shielded, but the others are not. When you say "isolate the thermocouples at the barrier" are you saying to not ground them if the T/C's are grounded at the input terminals of the barrier?

From an NEC standpoint, am I concerned with any openings into the hazardous area that do not have electrical wiring running through them? For example, a window or port in the side of the equipment? Where would I find procedure for making sure windows and ports meet Class I Div II requirements?

 

[Smart $]

Let's get back to my first reply, first sentence only...

Just exactly what material(s) and processing takes place within the enclosed space that makes it a hazardous location?

 

[fifty60]

The only information I receive is the AIT so that I can set the controller temperature limits accordingly, the request to use the intrinsic barriers where applicable, and the request to make the interior class I div II. This is not a dwellings or a residence, it is a piece of equipment similar to an oven. There are windows, a port to access inside the oven, connection points for the sheath heaters, and entry points for the thermocouples. Would I have to remove the ports completely to make the equipment class I div II? If my T/C's are grounded at the controller terminals, should I not ground the shield on the T/C cable? Where does the control drawing come from?

 

[Smart $]

... Would I have to remove the ports completely to make the equipment class I div II?

Not my area of expertise.

If my T/C's are grounded at the controller terminals, should I not ground the shield on the T/C cable?

Whether the T/C circuit is grounded has no bearing on whether or not you ground the shields. If you don't ground the shields, then you have to meet the insulation thickness option if you want to run the wires of multiple T/C's in the same conduit.

Where does the control drawing come from?

The manufacturer(s) of the associated equipment.

 

[fifty60]

I am starting to lean more towards the T/C's being a simple apparatus. I know there is actually no debate here, they are a simple apparatus by definition. The only aspect of the circuit that would be of any concern is the PID controller, limit controller, and data logger that the thermocouples are connected to. All of these devices are locate in the safe zone. The T/C travels through the safe zone and enters the hazardous area and then branches out. There is no conduit inside of the hazardous area, only t/c wire.

Given the fact that the T/C are connect to PID controllers, data loggers, and limit controllers, would this change the simple apparatus classification? Do I have to take possible faults in these equipment into consideration?

The problem is I will have 11 t/c's in all entering the classified area. We normally would not separate them, so to separate them will take a lot of effort on our side. I do not want to separate them if I do not have to.

Can I use the barriers without necessarily following all of the steps to make the circuit IS? If I am not labeling it as IS, but only using the barriers for added protection, can I then run the thermocouples without the required .25 in separation?