Insulation testing of thermocouples

[gravitate]

Hi basicallty at work we have been asked to test some thermocouples for Insulation resistance and loop resistance. We were told to test at 100v but our meter only has 250v setting. We attach one terminal of the tester to the wires of the thermocouple and the other negative terminal to the stainless steel sheath. If we get a reading of less than 1 giga ohme we take it as needing replacement.

My question is this. How do I do the loop resistance test?

Also what does it actually mean in Lamen terms if the thermocouple has a insulation resistance of less than 1 giga ohme?

 

[IQ Electrical]

I've tested 2-wire thermocouples with a low resistance ohm meter, looking for readings of less than 1 ohm.
Thermocouples can then be tested against a thermometer and voltmeter (a voltage in millivolts will increase with temperature).

I've never seen IR testing on a thermocouple and you'll struggle to get an IR tester at 100V.

A Gigohm is 1000 million ohms

 

[UNG]

I've tested 2-wire thermocouples with a low resistance ohm meter, looking for readings of less than 1 ohm.
Thermocouples can then be tested against a thermometer and voltmeter (a voltage in millivolts will increase with temperature).

Thats how I would test them for operation, for calibration you would need an oil bath for the reference temperature

I've never seen IR testing on a thermocouple and you'll struggle to get an IR tester at 100V.

A Gigohm is 1000 million ohms

Not sure what a 100v IR test would prove on something kicking out such small voltages

I have a MFT that will do IR tests down to 50v

The struggle would be finding a meter that gives readings to more than 1 Gigohm

 

[BlueToBits]

Join both of the thermocouple leads together and test between that joint and earth.
If the thermocouple has a low IR the test voltage may destroy it. It should be tested at twice the operating voltage to a maximum of 500V. There are major differences between thermocouples and thermostats. Ensure you know what you are testing. Also check your public liability insurance because an IR test can do damage.

I assume you mean Megohm and not Gigohm.

In layman's terms it's the amount of electric current that the thermocouple could potentially cause to leak away into the earth through its insulation whilst it is in use. The regs say that the potential leakage caused by an insulation resistance test result of 1,000,000 ohms or above is satisfactory.

 

[BlueToBits]

The loop test is to ensure that the protective device will operate within the required time in the event of a phase to neutral short circuit at the furthest point of utilisation (often 0.4 seconds). There are several ways you can do this by testing, or by testing and calculation. A lot will depend on the operating voltage of the thermocouple and its protective device. It is possibly safer to use the R1 + R2 method. This can be approximated by measuring the end to end resistance of the thermocouple, halfing it and adding it to the resistance start to end of the thermocouple sheath. Add this to Zs of the device that the thermocouple connects to (assuming everything works at 230V). Check the result against the type and rating of the device protecting the thermocouple, which may be an internal fuse. You MUST check that the equipment can withstand the test currents and voltages, and that the tests you are carrying out are appropriate for the equipment and its method of installation or erection. The person asking for the testing may not appreciate this, but may blame you, as the expert, for any damage. If you are not 100% sure, you would be best advised to subcontract the testing to someone who is.

 

[Marvo]

I don't understand why you would want to test the insulation resistance of a thermocouple. The thermocouple is a very low voltage device connected to a control circuit which will in turn be supplied by a step down transformer. The voltages for the normal operation are in the millivolt range, I'm not keen on putting 100volts anywhere near the junction of a thermocouple even if the leads are connected together, at best you'll need to calibration check it when you're done, at worst you'll need to replace it if the test damages it.

If a thermocouple does develop a low insulation problem during operation a few millivolts might become available as a touch voltage, the actual fault current would be so insignificantly small it wouldn't be a safety issue. If the IR test is supposed to confirm their operation then a calibration test would be a better way to do this.

The only way I can think of to perform the test at 100volts with a standard megger tester would be to test them simultaneously in batches of five at 500v. This would in theory put 100 volts across each assuming they're all similar insulation resistance. It's a hit and miss way of doing it and even then a standard tester won't accurately indicate e fault in the gigaohm range.

I would start asking your boss some questions.