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Old 02-23-2012, 06:25 PM   #1
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Potentially dangerous wiring in old house


There's one outlet in my house that is giving me problems, but not any of the others.

Here are the details:

-I have an APC surge arrester with a "Building Wiring Fault" light. The light turns on when plugged into the problem outlet, but none of the others in the house.

-I measured 118 VAC from hot-neutral and hot-ground, and 0.2 VAC from neutral-ground on a non-problem circuit. On the problem circuit (which is being used as it is the only outlet in the room), I measured 112 VAC from hot-neutral, 58 VAC from hot-ground and 54 VAC from neutral-ground.

-There is only one other circuit in the room. The house is very old, and the other outlet fixture in the room has no ground pins.

-About once a month the circuit breaker will trip. I'm not sure whether it's because of the wiring problem or if its just an overload. There are lots of appliances on the 15A breaker, for two rooms. None of the other outlets have this problem. The outlet without ground pins was not tested for voltages because it is currently inaccessible.

-Sometimes the voltage in the house will drop as low as 100 VAC if lots of appliances are running.

Any idea what the problem could be? I'm an electronics engineering student but have never worked on home wiring, nor have I studied anything about it.

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Old 02-23-2012, 06:29 PM   #2
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Potentially dangerous wiring in old house


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Originally Posted by ryanb22 View Post
There's one outlet in my house that is giving me problems, but not any of the others.

Here are the details:

-I have an APC surge arrester with a "Building Wiring Fault" light. The light turns on when plugged into the problem outlet, but none of the others in the house.

-I measured 118 VAC from hot-neutral and hot-ground, and 0.2 VAC from neutral-ground on a non-problem circuit. On the problem circuit (which is being used as it is the only outlet in the room), I measured 112 VAC from hot-neutral, 58 VAC from hot-ground and 54 VAC from neutral-ground.

-There is only one other circuit in the room. The house is very old, and the other outlet fixture in the room has no ground pins.

-About once a month the circuit breaker will trip. I'm not sure whether it's because of the wiring problem or if its just an overload. There are lots of appliances on the 15A breaker, for two rooms. None of the other outlets have this problem. The outlet without ground pins was not tested for voltages because it is currently inaccessible.

-Sometimes the voltage in the house will drop as low as 100 VAC if lots of appliances are running.

Any idea what the problem could be? I'm an electronics engineering student but have never worked on home wiring, nor have I studied anything about it.
Is the receptacle properly grounded or did someone do the old wrap awire from neutral to ground to trick a plug tester trick? I've seen this one time before and it was when someone jacked a leg of the oven to feed a receptacle in the kitchen.

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Old 02-23-2012, 06:34 PM   #3
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Potentially dangerous wiring in old house


Open ground. Check for loose or missing ground connection on the receptacle in question or the one before it on the circuit.
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Old 02-23-2012, 06:41 PM   #4
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Potentially dangerous wiring in old house


I was just confused as to why the meter would read voltages in the 50's if the ground wasnt connected to anything. Maybe its just because the multimeter is cheap/flukey?

I'll disconnect the power and check it out.
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Old 02-23-2012, 06:47 PM   #5
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Potentially dangerous wiring in old house


Pull the receptacle out and take a picture for us. It could possibly be backstabbed and be loose. A loose connection will show some voltage. There is still a chance that you have a jackwagon ground.
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Old 02-23-2012, 08:05 PM   #6
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Potentially dangerous wiring in old house






Well isnt that fun...

Last edited by ryanb22; 02-23-2012 at 08:06 PM. Reason: Resize
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Old 02-23-2012, 08:23 PM   #7
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Potentially dangerous wiring in old house


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Originally Posted by ryanb22 View Post
112 VAC from hot-neutral, 58 VAC from hot-ground and 54 VAC from neutral-ground.
It's a capacitive voltage divider causing the ungrounded ground lead to split the 112 v (58+54 = 112).
The capacitance could be Romex interconductor cable capacitance or surge arrestor capacitors.

We had an o'scope once that was giving people shocks and the scope housing read 60vac above ground. The plug ground pin was bad and the scope schematic showed an RFI filter across the AC line input.

Last edited by Yoyizit; 02-23-2012 at 08:27 PM.
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Old 02-23-2012, 08:42 PM   #8
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Potentially dangerous wiring in old house


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Originally Posted by Yoyizit View Post
It's a capacitive voltage divider causing the ungrounded ground lead to split the 112 v (58+54 = 112).
The capacitance could be Romex interconductor cable capacitance or surge arrestor capacitors.

We had an o'scope once that was giving people shocks and the scope housing read 60vac above ground. The plug ground pin was bad and the scope schematic showed an RFI filter across the AC line input.
Interesting. I'll read up about it.

Now its time to give my landlord a call. Hopefully she gets it fixed. Is the danger of ungrounded appliances more so for the user or the equipment?
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Old 02-23-2012, 09:35 PM   #9
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Potentially dangerous wiring in old house


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Old 02-23-2012, 11:11 PM   #10
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Potentially dangerous wiring in old house


Your using a digital meter. They have a tendency to read voltage that is not really there. It is called phantom voltage.
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Old 02-24-2012, 04:32 PM   #11
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Potentially dangerous wiring in old house


And since a phantom voltage cannot deliver more than about 1 mA you may not even feel a shock.
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Old 02-24-2012, 05:28 PM   #12
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And since a phantom voltage cannot deliver more than about 1 mA you may not even feel a shock.
Wtf? Under what circumstances would I feel a shock? I'm not sure what toure talking about.

Anyways, I was going to replace the receptacle with a GFCI receptacle, but I read that you shoudnt use a surge protector with them because they rely solely on ground.
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Old 02-24-2012, 05:45 PM   #13
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Potentially dangerous wiring in old house


definatly check your ground wires,
check your bonding also !
low voltage could be two issues,
1 - bad / loose / dirty connection.
2 - POCO problem.
To check which one
turn on lots of heavy loads,
check voltages at various with volt meter,
if voltage measures low at main breaker,
this would indicate a problem the main drop
or poco's system is low on steam.
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Old 02-24-2012, 05:48 PM   #14
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Potentially dangerous wiring in old house


Now its time to give my landlord a call

This is the correct answer since you do not own the house!

Ignore Yoyizit. He is more confusing than Chinese arithmetic!
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Last edited by jbfan; 02-24-2012 at 05:50 PM.
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Old 02-24-2012, 05:51 PM   #15
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Potentially dangerous wiring in old house


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poco's system is low on steam.
Maybe that's what happens when my 28.8kw tankless water heater kicks in on a cold winter day and dims the neighbors lights.

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