# Digital insulation meter measurement limits

A Meggar digital insulation meter user manual warns:

"When using 500 V measurement to measure resistance less than 1.5 MΩ or 1000 V to measure resistance less than 5 MΩ, measurement time must not exceed 10 seconds."

The problem is that i need to make measurement beyond the 10 seconds - a measurement of 60 seconds and more.

I assume that when the measured resistance is less than 1.5 MΩ for 500 V and less than 5 MΩ for 1000 V, I might be able to exceed the 10 seconds, but how many seconds/minutes can I make the measurement according to the measured resistance? For instance, if i measured 10 MΩ with 500 V, can I make the measurement for 60 seconds, 3 minutes etc.? Is there a rule of thumb for this situation?

• Look at your requirements carefully. You probably need to verify that the insulation resistance is greater than a certain value for 60 seconds. If the insulation resistance is so low that any of the limits you state kick in, then you already flunked the test and there is no need to continue. – Olin Lathrop Aug 9 '17 at 18:31
• If you want to measure for 60 seconds and more, you need a different insulation meter suitable for the measurement time you need. Using the meter you have for more than 10 seconds could be dangerous, may damage the meter or may deliver inprecise results. – Uwe Aug 10 '17 at 10:29

Remember Ohm's law:

500V across less than 1.5M gives greater than 0.33mA and over 0.17 watts

1000V across less than 5M gives greater than 0.2mA and over 0.2 watts

The watts are about the same in both cases, so their 10sec limit may be to prevent internal heating of the meter's own HV supply ...but more likely it's for fire hazard. If the >0.2 watts is being drawn by a corona or an arcing point-fault in your cable insulation, then 10sec of a >fifth-watt heating may cause permanent damage, even flames.

If the cable leakage draws less than 0.167 watt from the meter's supply, then you're within their published V-I limits, and the 10sec doesn't apply.

It's a good idea to perform a one-second test, then if you find >1.5M or >5M of insulation leakage, go and find the break in the insulation.

• Thanks,wbeaty.But how many seconds/minutes can i maximum make the measurement when,as you said,i am in their published V-I limites(<0.2W)? – xchcui Aug 11 '17 at 6:02
• @xchcui Their 10sec time limit only applies for high power (above 0.2W.) For low power, time is unlimited. – wbeaty Aug 11 '17 at 6:59

10 sec limitations are for inducing high voltage for measurement of small current to ensure high resistance. If more time required for testing then ur instrument design to be changed so that high voltage inducing and insulation to be withstand.