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Joined: Apr 2001
Posts: 84
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donles Offline OP
Member
Usually in old houses, I find that the metal device boxes are grounded and that it's relatively easy to upgrade to grounding receptacles. HOWEVER, today I came across ungrounded steel boxes. The old romex either did not have a ground because I cannot see one and the bx is not grounded either. What struck me funny was that I measured approx. 60 VAC between the ungrounded conductor and the steel device box. With the supply breaker open, I measure infinite resistance between the device box and the grounded neutral conductor. I can't make sense of the 60 volt reading. Can anyone explain?
The box in question is above a basement with an exposed copper water pipe so I plan to run a grounding conductor between it and the box.
I guess I had a pretty good run with old houses until now. This one also likes to ruin my holesaws and sawblades with it's concrete walls.

Don

Joined: May 2001
Posts: 552
T
Member
Hi Don. If you use the water pipe to tie your ground to for the receptacle,it has to be within 5ft of the point of entrance of the water pipe per 250.50 and 250.130(C)but you probably already knew that.As for the 60 volts [Linked Image]You mentioned bx and type NM,what type wire is used?If it is bx are the connectors corroded?


Donnie
Joined: Jan 2002
Posts: 163
D
Member
Don - this resistance -

(someone step in here and shove my feet in my mouth if I'm misguided)

RE: Old metal boxes being 'grounded'....I've found that most of these old metal boxes are not truly 'grounded'...they are bonded. There is this thin, small gauge copper wire running from box-to-box and connecting here and there to metal pipes...thus showing a 'ground' when tested...but this is not the equipment grounding conductor called for when required to ground devices...and this situation would give you a pretty high resistance reading also....i.e. this 'bond' is not going to clear a fault very effectively....OK, go ahead and shoot.

Joined: Oct 2001
Posts: 597
E
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I've had "phantom voltage" readings occasionally in situations like you're describing.

A house that was originally wired with Knob and Tube, that had very few outlets at the beginning, will have new outlets extended off the K&T. As the years pass and outlets and load are added, the service gets an increase in size and specific loads are broken out and connected to individual branch circuits. Most of the outlets extended off the K&T remain as originally added.

One common extension method around here has been fishing in BX (pre-drain wire AC). The source end would be made up where the hot and neutral were available off the K&T, most of the time, in an accessible joist or stud cavity. A junction box was rarely used. The metal armor, stripped back a foot or so, should end at a "birds eye" fitting, but commonly a couple of wraps of friction tape was all that was done. The black and white from the BX were then soldered to the K&T and reinsulated. That's it.

Now, if the BX got fished in along or over another grounded surface, say old iron water, sewer or gas lines, the casual contact will fool test instruments into showing a ground.

I've had the runs that aren't in any contact with any ground show a capacitve coupled voltage of 50~70 V. The higher the input impedance of my tester, the higher the voltage it reads on the sheath. Bonding the sheath or the outlet box bleeds of the capacitive charge, and everything is fine.

One can get the same effect with BX or AC extensions off of pre-ground wire NM.

Al


Al Hildenbrand
Joined: Apr 2001
Posts: 84
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donles Offline OP
Member
Thanks for the replies. I ran a single stranded #12 to the panelboard.

txsparky: >As for the 60 volts You mentioned bx and type NM,what type wire is used?If it is bx are the connectors corroded?<
Cloth covered with 2 or 3 rubber insulated conductors - no bare copper gnd.

dana1028: >RE: Old metal boxes being 'grounded'....I've found that most of these old metal boxes are not truly 'grounded'...they are bonded. There is this thin, small gauge copper wire running from box-to-box and connecting here and there to metal pipes...<
I don't think that I have seen that.

Don

Joined: Aug 2001
Posts: 7,520
P
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Don,

The capacitive coupling can really lead to some puzzling readings if you are using a digital meter with a high input impedance.

The typical DMM has a 10 meg impedance, and it takes only a very small value of capacitance to give a reading of half the supply voltage.

Joined: Sep 2002
Posts: 131
T
Member
What you are seeing is the effect of magnetism. Anytime a current in induced into a wire a magnetic field gets built up around the wire The magnetic feild couples itself to the metal box. Once the box is grounded the magnetic field is then shunted to ground and the 60volts goes away.

[This message has been edited by tsolanto (edited 09-04-2002).]

Joined: Apr 2001
Posts: 84
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donles Offline OP
Member
Thanks to all for the capacitive coupling explanation. I will reread those posts and file them away in my memory (hopefully) for the next time.
Don


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