electrical ground problem

[Chip Orange]

My brother called me and we were talking about the new multi-meter he had to buy to effect some repairs on his boat. Something caused me to suggest he measure the potential between his neutral and safety ground in one of his rooms (as he was having some odd problems) and while mine is always 0, his is around 13 volts.

Can anyone tell me if this is a safety issue (I think it is), and suggest what might be the cause? He lives in a very old structure, that's had some remodeling done at various times, and so has some rooms with no safety ground on the electrical outlets, and a central box with those screw-in fuses, and even some of those tubular shaped ones. (he rents, doesn't own).

thanks.

Chip

[I-zheet M'drurz]

Stupid possibility that needs to be considered: when you say "new multi-meter" it could mean he's never used one before or that he might not be quite in touch with the display on this one. Could he possibly be overlooking a tiny little "mV" indicator on the display?? It happens all the time, people are actually looking at a few millivolts (which is both safe and normal) but they think they're seeing Volts.

[Joseph Meehan]

First I would suggest using an old analog meter to recheck that. Those new digital meters have caused a lot of un needed concern due to their high impedance and sensitivity. AC circuits passing close to other circuits can cause wires to pick up voltage from each other via induction. The old meters could not measure this voltage because their low internal resistance bleed it off. The new meters can measure it. It is not a safety issue or a fault.

If you in anyway doubt your ability, don't proceed with the following!

If that does not explain it, then the next step would be to make sure all the neutral and ground connections at the fuse box are clean and secure. (Any aluminum wire?) Start checking out each circuit and see what you get. You may find the circuit(s) with the odd readings have vampires (those plug in transformers) or things like touch lamps, light sensors etc.

[Speedy Jim]

Good chance someone has replaced the original 2-prong (non-grounded) with a 3-prong and the ground terminal isn't connected to anything:-(

The 13V is just stray or loosely coupled AC the sensitive meter is reading.

If you really want to test the ground, connect a small load (60W lamp) from the Hot slot to the Grounding U-slot. If, as I suspect, the Ground is open, the lamp won't light.

Jim

[SQLit]

If he is actually reading 13v, not milivolts. Then some one has tied the neutral and ground together some where. Finding it will be "the luck of the draw". If I was doing it, it would be in the last box I opened, not the first.

[John Grabowski]

I've never seen more than 5 volts difference in potential on some jobs. Given the age of the service and the structure it is not surprising that you have this problem. Under certain circumstances there is a chance of getting a shock, particularly outside, the bathroom, and the kitchen.

The first thing to do is to check the main grounding conductor. The ground clamp on the water pipe should be clean and tight. If not, remove the old clamp, clean the pipe with emery cloth or sandpaper, and then install a new ground clamp. Next check the connections in the fuse box. They should be tight and the main grounding conductor should be connected to the neutral conductor in the fuse box or the meter socket.

They wern't required at the time of this home's construction, but there should be bonding jumpers connecting the hot and cold water pipes as well as the gas pipe and heating pipes together.

After this is done, check your readings again. If you still have the problem, put an ammeter on the main grounding conductor to see if there is a current flow. There may be the possibility of some wires being crossed somewhere in the house. Not unusual in an older structure that has had multiple occupants, remodeling, and outdated wiring. Tell the landlord it really is time for a service upgrade.

John Grabowski

[Chris Lewis]

According to Chip Orange :

To summarize:

If he has a digital voltmeter, he may be reading millivolts not volts: "user error" ;-)

If he has a digital voltmeter, and it really is giving him volts, he's probably reading stray inductive pickup AND the ground isn't connected. It's acting as an antenna - problem: ground isn't really grounded. This can be quite a hazard as a hot-case short on an appliance, say, could electrify the case on _everything_ on that circuit.

Under normal circumstances, the ground-neutral voltage should be very close to zero. If nothing is being drawn on that circuit, it will be zero. If the maximum draw is being pulled on that circuit, and the ground is properly connected, the maximum voltage between ground and neutral you should see is less than 3V. If it's more than that, there's a _severe_ wiring problem, which will usually be more obvious because lightbulbs will be dim, etc.

[Toller]

I have never seen the false voltage that everyone refers to, but presumably they have. Maybe my volt meter isn't "good" enough.

I expect the way to avoid that is to measure hot to neutral and hot to ground. They should be the same (assuming there are no extremely heavy load on the circuit). If they are not, there is a problem. If there is a real

13v between neutral and ground, then there should be a 13v difference between H-N and H-G also.

Since he rents, it is the landlord's problem, not his.

[Chris Lewis]

According to Toller :

It's not necessarily "good" (or not) per-se. It's a matter of how much current the voltmeter draws to make a measurement. Unless the current draw is ultra-low (high impedance meter), the voltmeter will short-out inductive pickup and you won't see anything.

Analog meters tend to have relatively low impedance (ie: 50,000 or less ohms/volt). They'll short out the signal and show zero volts.

Many consumer-grade digital voltmeters have impedances in the 100s of megaohms/volt. Those don't short out impedance effects, and the voltage you see could be surprisingly high. In the hundreds of volts in some wierd situations.

I suspect some "electricians voltmeters" (ie: high-end Fluke commercial gear) deliberately reduces the impedance to short out inductive effects.

A loose wire is just an antenna. It picks up signals. A high impedance meter can measure those signals.

Not really. The inductive pickup isn't necessarily symmetric.

It's worth diagnosing it if he wants the landlord to do anything about it.

[Toller]

Thats my point. An inductive pickup might show 13 volts where there isn't any, but it is not going to show 107v where there is really 120v. If the H-N shows the same as the H-G, there is probably no problem regardless of the N-G showing 13v.

I have a moderately price digital meter. Maybe I have just been lucky in never getting a false reading.

[Chip Orange]

Thanks for the idea, but no, he's familiar with it's operation (as he works on his boats electrical systems), and this one happens to speak its readings, so it would bring to your attention millivolts instead of volts. He also measured it from the live to the neutral side to make sure it was around 120 volts.

thanks anyway,

Chip

[Chip Orange]

I'm not sure what these are; do you mean "cheater converters" to let you plug a three-pronged device into a two-pronged outlet?

why would these cause a problem?

thanks.

Chip

[Chip Orange]

Thanks Jim,

this sounds like a good theory and a good test.

If this is the case, any problem with him just connecting the safety ground wire to a real ground source (one that is not in the fuse box). It sounds to me like this would improve safety, but I seem to recall this is not the code way of doing things.

thanks.

Chip

[Chip Orange]

I'm a little confused by this, if they were tied together (which is what I thought normally happened in the fuse box) then wouldn't that make them be at the same potential?

thanks.

Chip

[Chip Orange]

thanks so much for these steps; I'll get in touch with him about them.

The landlord is of course a real problem, refusing to fix anything, and always having "one of his buddies" do it when it's done.

thanks again,

Chip

[Chip Orange]

Thanks; I suspect the safety ground isn't connected at all on this circuit, as he doesn't see this consistantly throughout the entire house. I didn't realize even 3 volts was considered acceptible, so that's good to know.

thanks.

Chip

[Chip Orange]

Agreed. The landlord has made it clear he's not going to fix much, and if my brother insists, the landlord will just sell the property as he considers it a money-pit anyway.

This would, for other reasons, end up being hard on my brother, so we first thought we'd ask for some info here to try and figure out how much of a problem there was before taking further action.

thanks.

Chip

[Speedy Jim]

Yes, connecting to a good ground (cold water pipe) *used* to be a helpful solution, but it's not sure-fire and depends on a lot of factors. You're correct; it's no longer allowed.

Jim

[Joseph Meehan]

No, just those blocks that plug in the wall and supply low voltage to various devices like battery chargers. Those 2 to 3 prong devices are unsafe unless the grounding device (a wire or tab) provided is screwed onto a grounded part of the device. In older applications the box itself may have been grounded. In that case, replacing the outlet is in order, not a cheater.

Because they provide a small current even when the device is turned off. For example when your cell phone charger is plugged into the wall, but the cell phone is not plugged in. That small current could result in some unexpected meter readings.