That is a clear indication that your ground wire is not properly connected. If the breaker is on you should see 110V between ground and the black wire, and 0 volts between ground and the white wire. Trace that ground wire back and see if you still get the voltage at upstream boxes.
According to Justin West :
It's fairly common. At least until recently, all Canadian kitchen counter outlets HAD to be done with "multiwire branch circuits" (CEC terminology for multiple hots and one neutral).
The main techniques of importance with multiwire branches is that you ALWAYS pigtail neutrals.
If your ground and neutral are properly connected back to the panel, the most you're going to see (in terms of a neutral voltage relative to ground) is about 2-3V (circuit delivering max load). The ground should be unaffected.
If you _ever_ see a neutral-ground voltage of more than 2 or 3 volts, then something is definately wrong. "stray" (aka "induced" aka "phantom") voltages are indeed common when you use high impedance voltmeters - but these will _never_ occur between conductors that are currently "in circuit".
In a correctly operating circuit, the only time you'll see a phantom voltage is on a switched off hot. You can't conclude anything from such a measurement unless you can load the hot enough to short out the stray component.
In contrast, ground and neutral are supposed to be interconnected (back at the main), and neither are EVER supposed to be switched. Thus, if you see more than a couple volts between a ground and a neutral, then one of them is broken. The voltage _may_ be "stray impedance". Or it may be something with real punch behind it. Either way, there _is_ a problem.
One way to distinguish whether the problem is with the ground or the neutral is to measure the voltage between the offending ground, and something that's grounded on another circuit (or often a copper pipe). If you still see voltage, the ground is broken. If not, it's the neutral. If it's the neutral, the circuit won't be working at all. If it's the ground, certain devices on it (eg: metal-cased 3-wire devices) may have live cases.
Further, anything that gives you a tingle ain't stray impedance. It's been my experience that "tingle" is almost always a sign of "[near] full line current available, the resistance of your shoes are saving you from being thrown across the room".
According to mm :
Yes. But as I mentioned elsewhere, phantom voltage or not, measuring voltage between ground and neutral means there's a very real problem.
Grabowski mentions the possibility of a loose neutral in a shared neutral circuit. That indeed is possible. However, I think it more likely that somewhere upstream of that outlet there's a poor ground connection. Many not-quite electricians are quite sloppy at connecting grounds together.
A loose neutral in a shared neutral circuit has other side effects. Like having lights brighten when you turn something else on. The circuit will seem "sick" in other ways. Besides, shared neutral circuits are usually only a tiny fraction of the circuits in a home.
Disappear? Not necessarily. On a 150 volt scale a 20,000 ohm per volt meter has a resistance of 3,000,000 ohms. A hot-ground 'leak' of 12 meg ohms would give reading of 28 volts from 'ground' to ground!.
bud--
12 volts can give you an even stronger feeling if applied directly to the tongue. 12V can hurt, but this is a common way of testing 9V batteries. A good one tastes like lemon juice without the lemon.
OK, you did say that.
OK. I'll abandon that idea. Thanks
Remove NOPSAM to email me. Please let me know if you have posted also.
Very good response. At least for me, as I was actually able to understand it.
However, I do have a followup question. I recently replaced a fire alarm system in a condo tower which had the amplifier stack, fire alarm system, and security panel on 3 seperate hots but used one neutral.
I opened the disconnect for the ampifier stack, and fire alarm panel, and measured from hot to neutral (0V), Hot to ground (0 volts), neutral to ground (0v). When my assistant opened the neutral he noticed some arcing as the security system was still live. Thus I learned about the multi-wire branched circuits.
How would i go about knowing if a neutral is live or not? As all my measurements above, and (gasp, I know it's not a good method) my magic wand fluke voltage checker all showed 0V.
Thanks, JW
According to Justin West :
You can't, really. The best you could do is use some sort of current detector (eg: clamp-on ammeter) to look for current flow in the neutral before you disconnect it. Alarm systems may be carrying quite low current, and a clampon ammeter might not register that low.
"Shared neutral" is usually used to refer to a 4 wire 240V circuit, where you share the neutral between the two hots. Or a 5 wire 3 phase circuit, where you share the neutral between the three hots. In these circuits, the neutral is only carrying the _difference_ between the hot currents.
When you share neutral between two hots on the same leg of the panel, it's possible to overload the neutral, because the neutral current is the sum of both hots, not the difference.
The two paragraphs above are because I _suspect_ that you're not really seeing a shared neutral - especially because of the reference to three hots - unless the alarm system is hanging off a 3 phase circuit.
What I more suspect that it's actually a _single_ 120V circuit, you disconnected an intermediate junction, and the "disconnect" only disconnected _part_ of the circuit, whereas the neutral fed thru to more equipment that got its feed _before_ the disconnect. Eg: the downstream equipment "feeds through" where you disconnected, but the downstream hot didn't.
Mind you, I've not worked with large multi-unit buildings, so maybe they are wired differently.
Shared neutral circuits are supposed to be wired such that you can remove a device in the middle of the circuit without interrupting the neutral or ground. Eg: pigtails.
If you're working on a suspected shared neutral circuit, when you have the slightest doubt, you should take the time necessary to track down where all the hots are, and kill them all at the panel. I personally will not wire shared neutral circuits without a tied breaker, despite the fact that code doesn't always require it.
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