Hipot vs Megger

[maxamize]

I test terminal connectors for 480v 3ph motors with 2200v AC Hipot. Some persons contend that a 500vDC megger with a minimum allowable resistance of 100Mohm will find cracks or shorts in a glass encased terminal better.

Please tell me what will be a sure test.

 

[UKpete]

I think DC testing is ok, it stresses the insulation (it has the advantage that you don't get a small leakage current due to capacitance that you get with AC).

However, 500V isn't high enough. Even without surges and spikes, the 480V refers to the RMS, the peak line voltage will be 1.41 times this. Even a 1000V megger is a bit low, especially if it's an inverter fed motor on a long lead.

There should be a specification for the motor insulation test (eg 2 x Vline + 1000V, line to earth), and the terminal test should be at least as good as this.

In the absence of a spec, why not stick to your original test?

 

[electricpete]

One argument for dc is the test equipment is cheaper/lighter. Since you've already got the ac set that doesn't apply.

One argument for ac is that it recreates a similar stress to what is seen in-service. I have personally witnessed that the 1.7 conversion factor assumed between ac and dc is not accurate. 13.2kv motor passed ac test at 18kv, then failed dc test at 24kv which is <1.7 times as much.

One advantage of dc is that you have a leakage current value, rather than just a go-no/go. That leakage current will be a sensitive indicator of potential problems. (Not so for ac where capacitive current will drown out resistive leakage current right up to the point of failure). By that logic, it seems that dc can be made more sensitive. One other thing that is possible if you see a suspect dc test... you can repeat the test in a &quot;step&quot; manner. slowly increase voltage in steps and measure current at each step. A non-linear curve tells you that you are approaching breakdown.

 

[cbarn24050]

Hi, do both but 500v is nowhere near enough as has been stated allready, but there is no need to spend a lot of money, an old microwave oven has everything you need to make 1.

 

[busbar]

“I test terminal connectors for 480v…” If the devices under test are mass-produced items, automated insulation testing–-go/no-go AC HiPOT “weeding out” is a very practical method. Gut level, 2200VAC is a more 'conservative' test, even compared to 1kVDC.