50 volts across terminals

[Tom Desmond]

An entire 15amp circuit is shutting off at my house without blowing its fuse and I'm going nuts trying to find the cause. My house is old, and the ungrounded wiring, (consisting of just a hot and an neutral conductor without a separate grounding conductor), is deteriorated to the point that the insulation on the conductors will break and fall off if not handled very carefully. The problem circuit turns off, and turns on for no apparent reason. I noticed that when the circuit is on, a 50 volt potential exists between the neutral conductor at a certain light fixture and its metal bx conduit, measured with a multi meter. There's also 50 volts between the metal conduit and the neutral wire from another, properly working circuit, and no volts between the two neutrals, so it seems that the bx conduit is what is charged and not the neutral. So it seems that voltage is "leaking" from the hot lead into the conduit. But why just 50 volts and not the entire line voltage of 120 volts? Is there any other explanation?

Another strange thing is that there is about 50 volts between the two leads that bring power to a different overhead light fixture when the switch that controls the light IS TURNED OFF. When the switch is on, the voltage between the leads is 120 volts, just like you would expect, but it reverts to 50 volts when the switch is off.

Any advice anyone can give me will be appreciated.

Tom Durham, North Carolina

[RBM]

[Jeff Wisnia]

This may not answer all your questions.

But, if you are using a solid state digital voltmeter....

Hopefully, you understand why that kind of meter's extremely high input impedance combined with capacitive coupling of ac signals between powered and unpowered conductors can let the meter display voltage readings which are accurate but exist only because there's just microamps of current flow.

If you don't understand those principles, learn about them or you'll be following false trails which will lead you to incorrect conclusions.

HTH,

Jeff

[toller]

Either of the replies you got could be right. Or it could be something else entirely.

You say there is no voltage between neutrals. Neutrals on different circuits, or the same circuit?

What is the voltage between the hot and the conduit? How about voltage between the hot, the neutral, and the conduit to a grounded water pipe?

You must have a bad connection or a short or both. It is probably an easy fix, but you have to find it first. If it was me, and I wasn't sure I knew what I was doing, I would leave the breaker open until an electrician had a look at it.

[volts500]

Tom Desmond wrote (in part):

Turn the circuit OFF and call a qualified electrician. Your problem is not DIY! Old BX cable is known to rust and cause fire hazards because the rusting causes the impedance of the armour to be so high that the breakers or fuses won't trip. The symptoms that you describe may be an indication of such. The problem may be something as simple as a bad connection, but, IMHO, you need an electrician. I have a master electrician's license in two states, both of which would condemn your home if code enforcement caught wind of it. Not trying to be negative, just safe.

[Wes Stewart]

Took the words right outta my mouth.

Use an analog meter, a cheapie, or better a Simpson 260 for this stuff and not be taken in by vapor volts.

[Chris Lewis]

According to volts500 :

I agree with volts500. Your system has deteriorated to the point where you shouldn't be bothering to diagnose and pinpoint faults. Instead, you should be replacing whole circuits. If you understand wiring well enough to do it yourself, fine, but otherwise, you need professional assistance.

Unless that 50V is a phantom high impedance mismeasurement, the whole circuit is likely unsafe.

[CL (dnoyeB) Gilbert]

I agree with Chris Lewis. This looks like a dangerous situation. In the automotive world we call it a "resistive" short. The problem is resistance creates heat. You could be at 50V because the other 70V is burning off in the form of heat somewhere. Thats one way to find the problem actually. But I'm sure you agree if this is a resistive short its quite dangerous.

Also the random on/off points to a short. If the conduit is charged, and if current is bleading off, then you definitely have a resistive short.

I just moved from a home with same old wires that deteriorated in my hand. Not recommended but if you want too look I would open up each box on the circuit. Often for instance, they will wire to 1 electrical outlet, and double connect on the terminals, and run wire from that to another box. Those double connections can develop resistance. They could do this on a switch, or an outlet. Its rare on lamps and fans where they tend to use twist caps as opposed to doubling up on the screw.