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Current Rating of Thermocouple

05/07/2014 7:58 AM

I am using a T-type thermocouple to measure temperature. I have a current carrying bare conductor on the surface of which, I am going to place thermocouples. Obviously the current will flow through the thermcouples. I want to know where will I get the current rating of the thermoucouple i.e. the maximum temperature that which it can carry. On the basis of that I will decide how much current I must pass through the conductor.

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#1

Re: Current rating of thermocouple

05/07/2014 8:19 AM

If the thermocouple is in electrical contact with the thermocouple, then the conductor voltage and some of the current will appear at the terminals of the thermocouple's transmitter, which may or may not affect the reading, after all, thermocouple voltages are tiny in comparison with those needed to push current through power conductors!

  • Non-contact measurements such as laser are applicable here.
  • The current-carrying capacity of any conductor can be evaluated using British Standard 7671. If its capacity is exceeded, one would expect a circuit protective device to operate, disconnecting it.
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#2
In reply to #1

Re: Current rating of thermocouple

05/07/2014 8:28 AM

Actually, I am mounting my thermocouple bead on the bare conductor. The conductor is of rectangular cross-section. So I think, the current will flow through the thermoucouple wire. If I use too high current, then the thermoucouple might melt. Hence, I want to know whether any such current rating is there, for the thermocouple

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#12
In reply to #2

Re: Current rating of thermocouple

05/07/2014 10:43 AM

Your idea is silly and has no technical merit. PWSlack has already given you very sound advice and told you why what you want to do is a waste of time and effort.

If you insist on pursuing this foolish trick: Images for temperature indicating tape

You should find a more productive waste of your time.

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#15
In reply to #2

Re: Current rating of thermocouple

05/07/2014 1:56 PM

ude this from a safe distance, I promise it wont melt

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#3

Re: Current rating of thermocouple

05/07/2014 8:51 AM

The junction of the two dissimilar metals is insulated inside. Usually with some ceramic material. Which would be best to ask the manufacture of the thermocouple about the ceramics insulating properties. As the issue I see will be leakage current through the ceramic to the junction. Which will make the readings from the thermocouple unreliable. Which may mean using a thermocouple a bit larger to more insulation between the case and junction. As far as the case handling the current shouldn't be issue.

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#4

Re: Current rating of thermocouple

05/07/2014 8:59 AM

before I echo Slacks comments........whats the goal here, what is your application?

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#9
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Re: Current rating of thermocouple

05/07/2014 9:11 AM

My goal is to find temperature of the conductor. Due to current, the resistive heating will lead to increase in temperature. I want to know that value of temperature. Hence i am mounting the thermocouples on the surface of the conductor.

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#11
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Re: Current rating of thermocouple

05/07/2014 10:10 AM

#8
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#5

Re: Current rating of thermocouple

05/07/2014 9:01 AM

Is this a Mechanical Engineer's work-around so as to avoid carrying out proper Electrical design, then?

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#6
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Re: Current rating of thermocouple

05/07/2014 9:04 AM

Well no. Actually, the current carrying rating for the conductor is fixed. I more interested in the temperature distribution the conductors. the configuration of conductors will affect the temperature distribution around and on the conductors. My current value may go as high as 800A. I already have T-type thermocouple. Hence, I want to know whether I can use them. I can try to reduce current or I can go for a different thermocouple.

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#8
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Re: Current rating of thermocouple

05/07/2014 9:07 AM

Either laser temperature measurement or thermal imaging techniques, as used routinely in condition-based maintenance operations, would prove workable, and would not expose the operative to bare conductors at unknown voltages with their attendant risks to safety and health.

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#10
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Re: Current rating of thermocouple

05/07/2014 9:33 AM

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...or fit an appropriately-sized fuse in series with the conductor.

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#18
In reply to #6

Re: Current rating of thermocouple

05/07/2014 3:57 PM

Well since you aren't listening to people offering you the technical on why you don't need to worry, how about me offering you the practical take on the situation.

We measure bare conductors operating at extra-low voltage and 3000A or even higher (>10,000A) with standard thermocouples on a regular basis. No issue, no special thermocouples needed, no special isolation methods employed.

If your really concerned then check to ensure that your thermocouples are electrically isolated from your temperature measuring electronics.

If however you are not performing standard conductor heat cycle testing at only a few volts (which I hope you are) and are in fact trying to measure live conductors and high voltage then don't use thermocouples at all or the voltage will likely kill you.

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#24
In reply to #18

Re: Current rating of thermocouple

05/07/2014 11:34 PM

I think JoAT meant to say,

If you're really concerned, then check to ensure that your thermocouple and temperature measuring electronics are electrically isolated from the heater elements.

The issue is having high voltage from the heater element at the measuring electronics which can

a) be a safety shock hazard

b) destroy the electronics

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#23
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Re: Current rating of thermocouple

05/07/2014 11:04 PM

Go ahead and use the thermo couples. As proposed it will likely make you eligible for the annual Darwin award.

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#7

Re: Current rating of thermocouple

05/07/2014 9:05 AM

No. You need to understand how a thermocouple works to properly use it in this application.

A thermocouple produces a very small voltage difference from the effect that heat has on dissimilar metal. To accurately preserve this tiny voltage the sensing circuitry should be a very high input impedance. This way the voltage drop from a long wire run is virtually zero. The input electronics is designed to have a high common mode rejection and a low pass filter to remove power line signal noise. The thermocouple will not care if it is attached to any voltage, the electronics measuring the small voltage will care greatly if the thermocouple is attached to a voltage.

What you wish to do can be done but some very careful design work must be considered.

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#13

Re: Current Rating of Thermocouple

05/07/2014 12:29 PM

You have confused the method of thermocouple temperature measurement....please read the following article at highlighted link....

http://www.analog.com/library/analogdialogue/archives/44-10/thermocouple.html

The heat generates millivolts which are then used to est temp....they cannot be in direct contact with a live conductor, and used to measure temperature at the same time...

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#14

Re: Current Rating of Thermocouple

05/07/2014 12:37 PM

Perhaps a better solution would be to maintain the isolation of the thermocouple by using an insulator and thermal grease. The grease between the TC and the conductor will allow accurate capture of the temperature, and the insulator ( mica or other ) will provide the electrical isolation. Then you can measure in safety. Other thermal transfer media are available, such as silicon pads if you do not wish to have grease.

I mention safety because most technical people working on a thermocouple circuit will not be expecting high voltages or large currents, so use insulation and operate the TC as it was designed to be used. ( My advice ).

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#16

Re: Current Rating of Thermocouple

05/07/2014 2:04 PM

Stop right now! 600 Amps at some unspecified voltage and a non-isolated thermocouple is a formula for disaster or death.

Rent a thermographic camera and you'll get much better data without ever going near the conductor. If you can't afford that then get a non-contact IR thermometer and take readings where you were going to put the thermocouple. Anything involving contact with that conductor can have disastrous consequence which obviously you are incapable of foreseeing.

Better yet use some simulation software available on the web, finding the temperature distribution on a conductor is quite common and has been done many times before in FEM software.

No one with any common sense should be helping you to continue down your current path.

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#17
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Re: Current Rating of Thermocouple

05/07/2014 3:00 PM

Remember, the gene pool may benefit.:>) My smileys are on strike today.

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#19
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Re: Current Rating of Thermocouple

05/07/2014 4:15 PM

Aw. I was looking forward to a new Darwin award.

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#20
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Re: Current Rating of Thermocouple

05/07/2014 4:54 PM

This gets even more ridiculous when read through the second time!

The fact the hapless OP is even suggesting such a stunt almost assures him a spot on the next Darwin Awards list. If he survives this one, that is.

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#21
In reply to #20

Re: Current Rating of Thermocouple

05/07/2014 6:21 PM

Given he is likely still doing his post-grad thesis (based on his past recent posts on busbars) I hope he is staying away from live real-world low or high voltage power conductors, otherwise the real world is going to bite him in the arse before he has a chance to learn.

:(

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#22
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Re: Current Rating of Thermocouple

05/07/2014 7:11 PM

Post-grad? Sadly, he'll probably end up as an "engineer". Oh well, at least he'll have us to fall back on for the hard stuff, like conductor sizing.

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#25

Re: Current Rating of Thermocouple

05/07/2014 11:36 PM

Friends,

There are limits to all methods of temperature measurement. IR sensors measure the thermal energy emitted by the surface, which is very significantly affected by the emissivity of the surface. Polished copper bus bar and tarnished copper bus bar will have very different emissivities and may appear in IR to be at very different temperatures. Thermocouples in direct contact with the surface can give a reasonably accurate voltage output to an appropriate meter. I have done this, as a means to verify or correct the IR camera data. HOWEVER, when doing so, I have been fully suited for any potential arc flash energy and wearing voltage-rated gloves. My meter was a Fluke 87V with the standard K thermocouple that came with the meter. Because the meter was not connected to any external equipment, the actual voltage on the conductor was not going to falsify the reading. I asked Fluke about the insulation of the thermocouple wires and was told that they are not rated at any voltage level (in contrast to the standard test leads that have voltage ratings).

If you are going to use a thermocouple on bus bars, please be VERY careful. Make sure that you are properly clothed for any potential arc flash incident (this is actually a requirement just to open the equipment), and use proper voltage-rated gloves. You can sleeve the thermocouple leads in appropriate insulating sleeving if they will be close to conductive materials ad a different voltage than the one being measured.

So, before you do this,

  1. Make sure that you have valid calculations for the available fault current and arc flash energy.
  2. Get the necessary training and equipment to do it safely,
  3. Keep non-protected people away,
  4. Anticipate that a valid temperature measurement can take a number of minutes before you are close to equilibrium, and is dependent on the quality of the connection to the live bus bars you are measuring.

Darwin awards? Unlikely if proper precautions are taken. IR? not always a reliable/accurate method.

--John M.

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#26

Re: Current Rating of Thermocouple

05/08/2014 1:15 PM
Re: Current Rating of Thermocouple
After a careful reading of the question, it is not clear if the OP has no understanding of what a thermocouple is and basic electrical theory,

or If he is unable to express his question using the correct terms in the English language.

Is he asking for the usable temperature range of a type T thermocouple? That information is readily available from any thermocouple manufacturer such as Omega.

Or is he asking if the current in the conductor that he is attaching the thermocouple to will effect his thermocouple circuit? The answer to that is no,unless there is a potential difference between the conductor and the other end of his thermocouple circuit. In that case the results could be deadly depending on the difference

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#27
In reply to #26

Re: Current Rating of Thermocouple

05/08/2014 3:37 PM

With 800A of current in the wire the voltage gradient across the thermocouple may overwhelm the thermocouple millivolt signal. As I said earlier, this can be done but not easily.

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#28

Re: Current Rating of Thermocouple

05/08/2014 7:12 PM

For what it is worth. I would make very sure that the current carrying conductor was in the ground side of the circuit & that the thermocouple is placed as close to the ground point as thermally reasonable. This will reduce the common mode impressed voltage & minimize danger in the measurement circuit. Be very VERY carefull.

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#29
In reply to #28

Re: Current Rating of Thermocouple

05/08/2014 10:16 PM

I've got to give you a GA for that. You present the danger, the uncertainty of any obtained result and a method to obtain valid results in very few words.

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#30

Re: Current Rating of Thermocouple

01/18/2019 2:46 PM

Yes , you can mount thermocouple on low voltage <12V , high current carrying conductor's to measure temperature, but the data acquisition system must be galvanically isolated from the electrical network of the conductor under test, load and its current source, even the earth connection of the measurement system can create current loop through thermocouple,

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