Megohm testing

[brian john]

When any testing raises questions, Stand back and think what am I missing, isolate and re-test.

 

[gar]

090704-1853 EST

wptski:

You were testing with a source voltage of 50 V from your earlier post. This is well below the the normal applied voltage to the breaker circuitry (120 V RMS, +/-170 peak). If your megger source resistance was high enough you might not damage the GFCI with a higher voltage.

.

 

[wptski]

When any testing raises questions, Stand back and think what am I missing, isolate and re-test.

I always start with 50V and when I got the low reading, I stopped.

 

[brian john]

When meggering feeders or branch circuits that might have other equipment connected, I also start low, then work my way up.

 

[wptski]

090704-1853 EST

wptski:

You were testing with a source voltage of 50 V from your earlier post. This is well below the the normal applied voltage to the breaker circuitry (120 V RMS, +/-170 peak). If your megger source resistance was high enough you might not damage the GFCI with a higher voltage.

.

gar:

Yes, there seems to be no damage. Just wonder why the 50V RESET the tripped Shock-Buster? It makes the same big snapping sound just like when it's manually RESET.

 

[mdshunk]

I always start with 50V and when I got the low reading, I stopped.

I see. The reason you start with a low voltage is to double-check for missed connected loads. When you get a bad reading right off the bat like that, you've got something still connected. In resi, it's stuff like GFCI's, dimmers, lamps, furnaces, chime transformers, attic fans, etc.

 

[wptski]

I see. The reason you start with a low voltage is to double-check for missed connected loads. When you get a bad reading right off the bat like that, you've got something still connected. In resi, it's stuff like GFCI's, dimmers, lamps, furnaces, chime transformers, attic fans, etc.

I forgot to mention that I always start by using the resistance function of the Fluke 1507 but a max reading will show as >22.00K and then proceed to insulation at 50V. A reading with a DMM showed 9.5M with the GFCI on the circuit.

A Fluke T+ Pro wouldn't catch that either with a max of 10K but will catch a wall transformer on the circuit.

 

[gar]

080704-2218 EST

wptski:

If you noticed the National sample circuit had an MOV and a bridge rectifier at the input across line to neutral. A typical DMM will only use a low source voltage for measuring resistance. Hardly enough to forward bias a diode or two and also there is probably a substantial internal current limiting reasistance in the meter.

At the 50 V level you are going to activate more of the electronics. I have never tried proding a GFCI to see how it responds, and so I have no idea about the reset.

.

 

[ramsy]

I use a tone & probe to check for attached utilization equipment, lamps, or shorts, but never considered the output.

The specs on my GB unit show 8-v AC square wave output tone, series current limited by 4000 ohms. Is this crudely inverted 8 volt AC more powerful than most DVM's or volt meters?

 

[wptski]

080704-2218 EST

wptski:

If you noticed the National sample circuit had an MOV and a bridge rectifier at the input across line to neutral. A typical DMM will only use a low source voltage for measuring resistance. Hardly enough to forward bias a diode or two and also there is probably a substantial internal current limiting reasistance in the meter.

At the 50 V level you are going to activate more of the electronics. I have never tried proding a GFCI to see how it responds, and so I have no idea about the reset.

.

The Shock-Buster isn't a typical GFCI. I also failed to mention that I piggybacked the it into the Leviton and it didn't function as before.

There's a new low ohm function in the Fluke 289 for testing motors that uses 20V. I wonder how that would work?

 

[gar]

080705-1332 EST

ramsy:

My Simpson 260, circa 1948, on the Rx1 and Rx100 has an open circuit voltage of 1.6 V, and on the Rx10,000 range the open circuit is 7.4 V. The short circuit currents are 110 MA, 1.23 MA, and 60 microA.

My Fluke 27 has an open circuit voltage probably less than 1 V, and short circuit current of 0.5 MA.


wptski:

Any GFCI is going to be some sort non-linear load across the hot line to neutral. If the device was powered from some other source, then you would only be passing the current carrying conductors thru the GFCI and there would be no-load between line and neutral.

.

 

[76nemo]

The Shock-Buster isn't a typical GFCI. I also failed to mention that I piggybacked the it into the Leviton and it didn't function as before.

There's a new low ohm function in the Fluke 289 for testing motors that uses 20V. I wonder how that would work?

That's the same low-ohm function that is on the 1507, is it not???

 

[wptski]

That's the same low-ohm function that is on the 1507, is it not???

Nope, the 1507 is >4.0V <8.0V.