Voltage present on cable to light fixture even when off

[JohnnyMalaria]

Hi,

I've just witnessed an impressive but disturbing failure of a ceiling fixture (115V in US). It has two bulbs and the wire to one caught fire. I had turned it on a couple of minutes earlier. The fixture is part of a three-way lighting circuit.

Anyway, while checking the voltages with my DMM, I came across something strange. With the fixture completely removed and just the cable coming out the ceiling (hot, neutral + ground) I got the following readings:

Switched on:

Hot-Neutral: 120V
Hot-Ground: 120V
Neutral-Ground: 0V

(all well and good)

Switched off:

Hot-Neutral: 42V
Hot-Ground: 42V
Neutral-Ground: 0V

(not so good, right?)

What do I need to check?

Thanks.

[ArthurDent]

I'm assuming that the DMM you're using has a 10Mohm or some similar high impedance input. In your 3-way circuit if you have two single-pole double-throw switches with the common on one switch going to the line (hot) and the common on the switch at the other side of the room (going to the light fixture) in the off position, the wire that would be hot at the light fixture is connected to a wire that goes to the first switch where it is in the off position. This long wire is next to a wire that is carrying line voltage and the capacitive coupling is responsible for the voltage you're seeing. You can check this theory by leaving your DMM connected and connecting a small (7W) test lamp across the meter leads and see if the voltage drops to zero. If it doesn't, you have a problem.

[JohnnyMalaria]

I'm assuming that the DMM you're using has a 10Mohm or some similar high impedance input.

Yes, the manual says 10Mohm. It's an 80's vintage Fluke 8026B.

You can check this theory by leaving your DMM connected and connecting a small (7W) test lamp across the meter leads and see if the voltage drops to zero. If it doesn't, you have a problem.

I don't have that low a lamp. Is there a particular reason I couldn't use a 60W incandescent bulb?

[ArthurDent]

A 60W bulb would work as well. The reason for using a low wattage bulb is to easily see if it is just leakage caused by wire-to-wire capacitance and not something else. You could use anything that would represent a resistive load. Even a properly rated 2 watt 10K resistor should cause the 42 volts to drop quite a bit.

My reasoning is that the wire used for the 3-way switch has a bare wire (ground) and 3 insulated wires. One of those (white) neutral is at ground potential and one is connected through the first switch to 120 with the remaining wire that goes through the other switch to the fixture being disconnected at the fixture end and you are reading 42 volts. I'm guessing you could view this as 3 capacitors from the disconnected lead, one to 120, one to neutral, and one to ground. So 1/3 of the coupling comes from 120 and 2/3 comes from wires at ground potential so you see about 1/3 of 120 or 42 volts.

Or you could have a 3DFS Vector Q2 PFC device connected and it is 'saving' electricity for you.     

[Dubbie]

yep I get this too on my 3way lights

The power capacitively coupled is even enough to trip the RCD if I snip through the Phase and Earth wire (thus shorting them) as I have found out.