Need help understanding test values for insulation resistance (NETA Table 100.1)

[Sanjayo]

What test voltage would you use on a maintenance test for an older switchboard with a nameplate voltage rating of 208/120? The reason I'm asking is because NETA table 100.1 shows a minimum test voltage of 500V for equipment with a 250V nominal rating. Would a 208V switchboard fall under that class, or is it still considered 600V class insulation? We usually test these boards at 1kV but I am wondering if we should do it at 500V.

80

[patelnipul5]

Generally switchboard with name plate rating of 208/120 volts has max. voltage rating of 240V if it is not specified as 600V on name plate. So, it is preferable to apply test voltage 500V as per NETA table 100.1 and not 1kv by simply assuming 600 volts class insulation.

Here is the link which is very helpful.

http://electrical-engineering-portal.com/measurement-of-insulation-resistance-1

[Sanjayo]

Link did not work for me but you answered my question, thanks for that. Now I have another question: NETA table 100.1 heading reads "Minimum Test Voltage," does that mean 1kV is still acceptable for a 240V max. board?

[patelnipul5]

I understand that it does not mean 1kV is still acceptable for a 240V max. board. If it is so, then all test voltage >500V is good for 240V board. But that is not the case.

I agree that the heading 'Minimum Test Voltage ' is quite confusing. But I here interpret as 'Recommended Test Voltage' or 'Accepted Test Voltage".

[Sanjayo]

Thank you for explaining this. The minimum voltage heading is very confusing and I do wonder why there is no max listed because you obviously wouldn't want to put 5kV on a 240V board, yet the table by itself doesn't imply that. Is there a rule of thumb for max test voltage (such as 1.5X nameplate voltage rating)?


I understand that it does not mean 1kV is still acceptable for a 240V max. board. If it is so, then all test voltage >500V is good for 240V board. But that is not the case.

I agree that the heading 'Minimum Test Voltage ' is quite confusing. But I here interpret as 'Recommended Test Voltage' or 'Accepted Test Voltage".

[patelnipul5]

If you try to open above link, it has very interesting information and also includes the thumb rule for meggering.

However,The Rule of Thumb for maximum test voltage for meggering is as follow.

When AC Voltage is used, The Rule of Thumb is: Test Voltage (A.C) = (2X Name Plate Voltage) +1000.

When DC Voltage is used (Most used in All Megger):Test Voltage (D.C) = (2X Name Plate Voltage).

[Sanjayo]

Link seems to be working now; good article. Thanks again!