Another Megger Reading Question

[Pitt123]

We were performing an insulation check on a 3 Phase Delta connected resistive element heater and found that is had an extremely low megger reading to ground of 0Meg. We performed measurement at 500V and 1000V and found 0Megs or a short both times.

Is there something we are missing in regards to measuring a resistive heating circuit that is causing it to look like there is a L-G short somewhere? Or is what we are seeing what we are seeing.

When the L-G reading is checked with a regular meter it reads about 85kohm. So even if power was applied the L-G current would be very minimal is this correct? 277V / 85kohm = 3ma ?

So why would the digital meter read 85kohm while the megger reads 0? Is this just a function of the scale ranges on the two?

 

[zog]

We were performing an insulation check on a 3 Phase Delta connected resistive element heater and found that is had an extremely low megger reading to ground of 0Meg. We performed measurement at 500V and 1000V and found 0Megs or a short both times.

Is there something we are missing in regards to measuring a resistive heating circuit that is causing it to look like there is a L-G short somewhere? Or is what we are seeing what we are seeing.

When the L-G reading is checked with a regular meter it reads about 85kohm. So even if power was applied the L-G current would be very minimal is this correct? 277V / 85kohm = 3ma ?

So why would the digital meter read 85kohm while the megger reads 0? Is this just a function of the scale ranges on the two?

It is a matter of using the right tool for the job, most multimeters only have about a 4V output.

 

[Pitt123]

It is a matter of using the right tool for the job, most multimeters only have about a 4V output.

So would the fact that a megger has a 500 or 1000V output stress the insulation more and thus bring forth any faulted areas. In other words, the lower voltage may not stress the insulation enough or produce enough voltage to drive a current though the faulted resistance?

 

[zog]

So would the fact that a megger has a 500 or 1000V output stress the insulation more and thus bring forth any faulted areas. In other words, the lower voltage may not stress the insulation enough or produce enough voltage to drive a current though the faulted resistance?

Right, in a heater element you most likely have some corrosion there at the fault. Just as ohmeters don't work accurately at higher resistances, meggers are not very accurate at lower resistances.

 

[Pitt123]

Right, in a heater element you most likely have some corrosion there at the fault. Just as ohmeters don't work accurately at higher resistances, meggers are not very accurate at lower resistances.

So does the output voltage of the two different instruemnts make a difference in the circuit resistance to ground. In other words does the higher votlage cause more of a breakdown to ground and thus the impedance to ground to change?

Or does the impedance to ground always stay the same and its just the instrument measuring range that appears to make the resistance to look different. So in my case the circuit resistance to ground is 88kohm no matter what, but it just appears different on the two meters because of measuring range?

I would think this may hold true in a circuit where there is maybe no insulation, but wouldn't the insulation break down more depending on voltage exposed to it? So with a 4V stress on insulation it might not break down and therefore will have more of an impedance to ground than if you exposed it to 1000V due to the fact that 1000V would break down insulation more and lessen the impedance?