Acceptable Insulation Resistance Values for 15 kV Switchgear

[GRAEE]

We have newly acquired 15 kV Switchgear and conducted insulation resistance test in all 6 poles (1,3,5,2,4,6). However the result varied significantly when the 2-4-6 poles are tested with respect to ground. Using the 5 kV, Vanguard-Insulation Resistance Tester, the largest insulation resistance recorded for 2-4-6 poles to ground is only 600-Mega ohms. While the 1-3-5 poles to grounds are having 100 Giga ohms and above insulation resistance. Looking at the manual, no shunt resistor or PTs wer connected. Verifying it to the NETA, the insulation resistance for circuit breakers/switchgear should be at least 5 Giga-ohms at 20 deg Celsius and 5 kV dc test voltage. Based from manufacturer's manual, using 2.5 kV insulation resistance tester, the insulation resistance should be at least 50 Mega-ohms.

Are there any established standards/acceptable insulation resistance values that support the manufacturer's criteria? It seems that the criteria they are using is too small and lead us not to energize the switchgear until we get clear explanation from the manufacturer of the factors that contribute to the low resistance values.

 

[Zogzog]

Difficult position that I have been in myself. I don't know where the OEM's get the values they put in instruction manuals but I am guessing it comes from some in house testing to determine the minumum safe IR. Is 600M safe? Sure it is, but there is obviously something wrong if some are 100G and others are 600M, so it warants further investigation or chances are you could have a premature insulation failure sooner than expected.

Have you conducted any overpotential testing such as VLF or Tan Delta?

 

[dpmac]

Can you provide additional information on switchgear type (Metal enclosed, metal clad, vacuum bottles, arc chutes, etc)

 

[oldfieldguy]

How much equipment is involved in each test? For example, on certain vacuum circuit breakers, poles, 1,3,5 are short pieces of buss connected to the tops of vacuum bottles with one stand-off insulator. The other poles, 2,4,6 are connected tot eh bottoms of the vacuum bottles and have several links and standoffs. Typical readings show a big difference between the two conditions.

old field guy

 

[DTR2011]

Are you grounding all adjacent poles during testing? Have you considered the placement (or condition) of the test leads?

You list this as switchgear, but are refering to poles. Is this switchgear or breakers?

How about AC HyPot tests? Have you conducted?

 

[GRAEE]

Zogzog, dpmac,oldfieldguy & smallgreek,

Thanks for the response.

This is actually a metalclad switchgear and 8 vacuum circuit breakers were drawn out to be tested. Only insulation resistance test was conducted. Taking all each insulation resistance per pole such as pole 1-to-ground, pole 2-to-ground, pole 3-to-ground, pole 4-to-ground, pole 5-to-ground, and pole 6-to-ground. The test leads were connected to the pole and the other one to the body ground.