Insulation Resistance Measurement for Cables


Thread Starter



Can you please answer these 4 questions for me?

1. Is it possible to perform an insulation resistance test for 30KV cables?
As this test is only mentioned by IEEE for rotating machines. If yes, how can we warm the cable up beyond dew point?

2. Does the cable's length matter during the insulation resistance measurement?

3. Do you advise the "Tan delta" test for that?

...And finally...

4. Are there other methods to make this measurement without consideration for the weather variables?

Thank you in advance and best regards.

P.S. Happy new year everyone :)

Phil Corso, PE

Responding to Voltman's 06-Jan-07 3:47 queries:

A1) Certainly

A2) Humidity greatly influences insulation resistance readings. Warm up cable by applying low-voltage supply to one end of cable, while remote end is shorted. Raise conductor temperature some 3-8 degrees C, above ambient. Current to use is proportional to rated temp-rise above ambient. Temperature of conductors can be found using resistor-rise measurement.

A3) Yes, if test equipment, and interpretive expertise is on-site. However, this test is unwarranted for cables!

A4) If, and only if, the resistance measurement shows evidence of poor insulation integrity, then hi-pot the cable. Either AC or DC can be used. However, be aware of failure ramifications*.

* Such ramifications are covered in Control List Thread # 1001589379 "Di-Electric Strength Test!"

Regards, Phil Corso, PE
Phil, you actually skipped question 2. I am currently testing #8 5kv cables we installed in series that average 25k feet in length. The test standard is greater than 500 megohms. We can get a cable up to almost 750 megohm when we test from the middle toward each end but when we plug the two parts togather, as they operate, the resistance drops below the test standard.

I wonder if it is a matter of current leakage related to capacitive leakage and absorption leakage. However it seems to me that if we get a realitivly quick stable reading for each half of the loop the two tied together should not take too much longer to stabilize.

If the two halves test satisfactorily then I think the cable should be accepted.

What do you think?

Phil Corso, PE

In response to Shawn’s 22-Jul (10:02) query: good pickup Shawn; and I apologize for having overlooked Voltman’s question about length.

A) Re: your 1st question... length vs time!
In your case, because longer test-time was associated with longer cable your conclusion wasn’t far fetched. However, the technically correct reason is that insulation volume doubled. For example, had you tested two cables of equal length but different insulation thickness, the thicker one would require a longer test-time. Following is a brief explanation.

When testing insulation resistance, R=E/I, where E is the applied DC voltage, and I the observed current. Omitting temp and humidity factors, then, what other time-related factors are there? The observed current involves leakage paths over the insulation surface, and through its volume, resulting in three current components… one transient and two steady-state:

1. Capacitance Charging Current: Contrary to popular belief, while it starts out high, it decays relatively quickly… in the order of tenths of seconds.

2. Absorption Current: It too starts high, but then decays over a period involving minutes.

3. Leakage Current: It is essentially a small but constant current both through and over insulation.

B) Re: Your 2nd question... Minimum Acceptable Value!
Assuming both cable halves had similar test results, and absent the caveat(s) in my 06-Jan-07 response to Voltman, I would accept the Rule-of-Thumb value, i.e., 1-megohm (minimum) plus 1 megohm for each 1,000 volts of operating voltage, for a total of 6.0 MegOhms! However, unless cable is new, I would be suspicious of the 500-750 MegOhm values!

Phil Corso (cepsicon)

Neil Christopher

Phil Corso wrote: "A3) Yes, if test equipment, and interpretive expertise is on-site. However, this test is unwarranted for cables!"

Just curious why you would consider Tan-Delta testing unwarranted for cables?

Phil Corso, PE

Responding to Neil Christopher’s 05-Jul-08 (11:32)... If you have the instrument, then I would certainly use it.

By “unwarranted for cable” I meant the expense to procure one just to test cable. An insulation-resistance tester, like a Megger® is certainly adequate. I will even concede that the 3000V-unit is somewhat more user-friendly than the 500V-unit.

But, then, the slot-insulation test feature of the Tan-Delta instrument, while desirable for electric machinery testing, is unnecessary for cable testing.

Regards, Phil Corso ([email protected])