Stray voltage removal for mains testing input stage

[daedalux]

When testing wiring in mains devices lots of times the capacitive coupling is so high that with a 10Mohm meter you can't even distinguish a conected cable from a floating one. This leads to the need of a LoZ meter (low impedance meter) or stray voltage remover input box, that I've seen manufactured by Fluke.

I have some doubts on this topic.
- Does a typical 20Kohm/V analog meter have low enough impedance to overcome this problem up to about 500V?
- The LoZ input stage is usually quite unlinear with a PTC, and i've seen that the impedance in comercial products is as low as 2Kohm. But what is the value of the ptc and resistor power required? Has the impedance to be that low? Can a 100K permanent resistor you can put in an copable wattage up to 600V be enough for these purposes with the bonus of it being linear at least?

Some field experience in overcoming this problem would be unvaluable.

[Vtile]

What I have tried to search around (without access to full standard library) in the past there seems to be no definitive answer to anything related to stray voltages. Another thing to consider is the definition of stray voltage and especially the stray current, what is the maximum level on your application.

For my limited experience with poking around with AVO8 (20kohm/Vdc (50uA FSD) // 1kohm/Vac (1mA FSD)) analog, it indeed doesn't see stray/leakage voltages (currents) in mains systems (ie. stray voltages in floating desk lamp metal body) if it would it wouldn't be a stray voltage anymore, but a severe insulation fault obviously. You can do some rough maths also based on the insulation resistance levels given in standards. I wouldn't draw a solid conclusions from it though.

Would this be a better suited for the "Test gear" section of the forum rather than metrology?

[Krytron]

I found the same situation a long time ago, when I started working as a electrical maintenance tech.  I was looking at a 240 volt wire that was disconnected at the breaker box.  My DVM was indicating 85 volts !  My mentor told me to throw away the meter, if you cant trust it. ... !

The lession I learned is to use a meter that draws a little load or connect a load and measure across it. 

What I found reliable for AC electrical mains testing is a Amprobe RS-3 clamp on ammeter/voltmeter.  It has no impedance spec, but I tested it by plugging it into a 120 volt AC source and then measured how much current it drawed - the Amprobe meter indicated 115V on the 150 volt range and a ammeter that was put in series indicated it was drawing 0.565 milliamp.  Works out to 212K load...

[Simon]

If you want to add a 100K resistor that will certainly help compared to a 10M input but it should be insulated though. 100K is 1/100 so any stray voltages you see now will be around 1/100 what they were as you simply have a voltage divider between the meters input and the impedance of the capacitor formed by the wires at the mains frequency.

[David Hess]

- Does a typical 20Kohm/V analog meter have low enough impedance to overcome this problem up to about 500V?

20K/V * 500V is 10 megohms so no.

- The LoZ input stage is usually quite unlinear with a PTC, and i've seen that the impedance in comercial products is as low as 2Kohm. But what is the value of the ptc and resistor power required? Has the impedance to be that low? Can a 100K permanent resistor you can put in an copable wattage up to 600V be enough for these purposes with the bonus of it being linear at least?

This is why I like standard 3/4" banana jack spacing.  I have a dual banana jack plug with like a 2 watt 22k ceramic composition resistor attached so the banana jack plug goes into the meter and the test leads go into the banana jack plug.  A 5 watt metal oxide resistor might be better.