Evaluating insulation resistance values for heat trace cables

[edienberg]

Was wondering if anybody had some thoughts on this?

What is a recommended method for evaluating insulation resistance (IR) test results of Heat Trace cables? The system in question was put into service 10 years ago, and to the best of my knowledge IR tests have not been done routinely so there is not any histortic data to compare to.

Below how many M-Ohms would the cable be considered bad and below how many M-Ohms would it be considered to have reduced life?

From IEEE 515-1997:

4.1.2 Insulation resistance test
The resistance of the insulation shall be measured between conductors and the metallic outer covering, or a specially applied conductive metal ground plane, by means of dc voltage of 1000 Vdc for mineral insulated heaters and 2500 Vdc for polymer insulated heaters. The measured value shall not be less than 50 M-Ohms.

Erik

 

[VEBill]

It's not my field, but 1000+ volts might be too high. I've seen many electrical devices and cables marked as being qual. tested to 600 volts. You should double-check Vmax before hipot testing.

 

[Zogzog]

See attached table from ANSI/NETA specs, the measured values should be tempature corrected to 20 degrees C.

 

[electricpete]

Some limits take into account the length of the cable.

=====================================
(2B)+(2B)' ?

 

[electricpete]

Good point by VE1BLL, I'm not used to seeing megger voltages above 500 vdc except in medium voltage systems.

=====================================
(2B)+(2B)' ?

 

[Zogzog]

Good point Pete, forgot to mention that. But under no case should <2M be acceptable.

 

[resqcapt19]

Tyco says "9.2 Insulation Resistance Criteria.
A clean, dry, properly installed circuit should measure thousands of megohms, regardless of the heating cable length or measuring voltage (0–2500 Vdc)."

 

[7anoter4]

From Raychem Installation Instructions for Gardian W51 120 V preassembled self-regulating heating cables:
Cable testing and maintenance
"Using a 2500-Vdc megohmmeter, check the insulation resistance between both of the rectangular (power) prongs on the plug and the round (ground) prong after installing the heating cable. Minimum reading should be 1000 megohms."

http://tycothermal.com/assets/Ameri...nuals/1274/H54499_Gardian_W51_II_0511_web.pdf