Meggering with a GFCI in the circuit

[wptski]

141205-1939 EST

wptski:

I had not looked at the Y axis labeling.

Now looking at it as resistance plot it does not make sense.

In the GFCI one can ignore the MOV for voltages less than the 1 mA conduction point of the MOV. Below the 1 mA voltage point (Vp) the MOV resistance is greater than Vp/0.001 . Assuming Vp is 220 V for a GFCI MOV, then R = 220/0.001 = 220,000 ohms.

The various resistive components of the GFCI at a voltage below Vp for the unit I tested were way below 220 k.

What does your megger do if you test a 100 k or 10 k resistor?

What resistance does a VOM read on your GFCI?

A GFCI is not a simple resistance, but it would seem for a megger test it would somewhat simulate a resistance.

We need to know more about how your megger works to try to understand the plots.

.

gar:

Glad you asked to try a resistor as I had a major connection mistake, my bad! My Fluke 87V reads 6.88M from L-N with polarity observed in the RESET condition.

At 250V connected to L-N again for 10 minutes with polarity observed which lights the LED during the test, the output voltage again drops. The below is just the graph. Since the reading is so low, it depends where in the circuit it is, otherwise it would have to be removed anyways for a valid test.

 

[gar]

141205-2113 EST

wptski:

If you can run the following experiment it would tell us something about the megger.

Use a 1000 ohm resistor as a shunt for current measurement. A 1 volt reading = 1 mA. This avoids using the meter current range, and prevents blowing one of the current range expensive fuses.

Do a short circuit test on the megger. That is connect the megger to the 1000 ohm resistor, and read the voltage to measure current.

Add in series with the current sense resistor a 220 k 1/8 to 1/4 W resistor. Measure voltage to measure current. If the megger is using a 250 k resistor in series with the output on the 250 V range for current limiting, then the approximate current would be 0.5 mA. You can also measure the voltage across the 220 k resistor. Should be about 117 V. A 270 k resistor would be equally suitable, or in between. Or any other value you want to use. Don't go too high in comparison to 10 M or the meter becomes a significant part of the inserted resistance.

.

 

[wptski]

141205-2113 EST

wptski:

If you can run the following experiment it would tell us something about the megger.

Use a 1000 ohm resistor as a shunt for current measurement. A 1 volt reading = 1 mA. This avoids using the meter current range, and prevents blowing one of the current range expensive fuses.

Do a short circuit test on the megger. That is connect the megger to the 1000 ohm resistor, and read the voltage to measure current.

Add in series with the current sense resistor a 220 k 1/8 to 1/4 W resistor. Measure voltage to measure current. If the megger is using a 250 k resistor in series with the output on the 250 V range for current limiting, then the approximate current would be 0.5 mA. You can also measure the voltage across the 220 k resistor. Should be about 117 V. A 270 k resistor would be equally suitable, or in between. Or any other value you want to use. Don't go too high in comparison to 10 M or the meter becomes a significant part of the inserted resistance.

.

At 250V, across the 1K it read 1.26V and the 220K it read 260V. A bit higher than 1mA but that was listed as nominal.

 

[gar]

141205-2349 EST

wptski:

With the megger just connected across the 1 k resistor, no other resistor, was the reading 1.26 mA? Or was the 220 k in series at the time of the 1.26 mA reading?

If the 1.26 mA was with just the 1 k resistor, then what was the current with the 220 k resistor?

.

 

[wptski]

141205-2349 EST

wptski:

With the megger just connected across the 1 k resistor, no other resistor, was the reading 1.26 mA? Or was the 220 k in series at the time of the 1.26 mA reading?

If the 1.26 mA was with just the 1 k resistor, then what was the current with the 220 k resistor?

.

No it wasn't for that 1.26mA reading but I did measure it across the 1K alone and it was 3.2mA. With both in series, reading across the 220K was 1.18mA.

 

[gar]

141206-2953 EST

wptski:

If I assume the megger is a constant voltage source with a high internal impedance, resistance in this case, then from your short circuit data, 1000 ohm load, the calculated internal resistance is about 250/3.2 = 78 k ohms with 1k subtracted, or 77 k.

Next make the load 1 + 220 + 77 = 298 k. Now the current should be 250/298 = 0.84 mA. But you measured 1.18 mA.

What does this mean? I don't know without further experiments. It can imply a non-linear current limiting resistor. I doubt that it implies active current control since that is more difficult.

Even raising the source voltage to 270 V does not get close to 1.18 mA.

.