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Old 03-21-2009, 12:06 PM   #1
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HOw to tell if a socket is propery grounded


Hi,
I just bought a network storage drive with redundancy (RAID 1) on it to protect important files like all my pictures, music etc. I live in an old house with horrible wiring, very old and even aluminum in some rooms and I want to make sure that a surge won't wipe out everything I have.

I'm trying to tell if one socket is adequately grounded. I figured if I got 120V across the live wire and the ground, then it was grounded. But I'm getting 30V. I checked the socket terminals too and get 120v.

I'm wondering if that 30V I get means the socket is ok, or if my electronics are in danger. Also, why would it be at 30V since my understanding (Circuits 1) tells me that to get 30V that ground must be at 90V.

Thanks!

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Old 03-21-2009, 12:22 PM   #2
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HOw to tell if a socket is propery grounded


If you have 120v between the ground and hot (live) wire that is not a definitive method that you have ground. In a newly built and inspected house maybe- in an old house there is always the possibility of a bootleg ground (where the ground is attached to neutral outside or beyond the main panel or subpanel).

The only way you know of a true ground is following the cable back to the electrical panel.

You could always install a GFCI outlet to give you some protection in an older questionable circuit situation but to me the best is running a brand new line directly to the source (main panel). If you care about your electronics, run a fresh line and include adequate surge protection.

Also, if you are getting 30v something is definitely wrong in the wiring. If using a meter, you should get full voltage (110~120v) or nothing. You mentioned aluminum- there could be faulty connections there too.

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Last edited by handyman78; 03-21-2009 at 12:27 PM.
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Old 03-21-2009, 12:30 PM   #3
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HOw to tell if a socket is propery grounded


Quote:
Originally Posted by Jdrewrd View Post
Hi,
I just bought a network storage drive with redundancy (RAID 1) on it to protect important files like all my pictures, music etc. I live in an old house with horrible wiring, very old and even aluminum in some rooms and I want to make sure that a surge won't wipe out everything I have.

I'm trying to tell if one socket is adequately grounded. I figured if I got 120V across the live wire and the ground, then it was grounded. But I'm getting 30V. I checked the socket terminals too and get 120v.

I'm wondering if that 30V I get means the socket is ok, or if my electronics are in danger. Also, why would it be at 30V since my understanding (Circuits 1) tells me that to get 30V that ground must be at 90V.

Thanks!
If you are using a digital meter, then it is likely that the 30 V you measure is just a phantom caused by capacitance and the outlet is not really grounded. An analog meter or small load connected between hot and ground would show whether the outlet ground was connected to anything or not.
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Old 03-21-2009, 12:49 PM   #4
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HOw to tell if a socket is propery grounded


Thanks guys. It is a digital meter. The voltage does hold after extended periods of time and varies from socket to socket as well. I did a quick inspection of the wiring in my garage which runs under the house and all the wiring very visible. I don't think the wiring even carries three wires, so that pretty much answers my question. I guess I need to look into methods for pigtailing or the task of completely rewiring. I'm sure that will take many weekends to finish. I put everything on a switch so I will just turn it off when it storms for now. Thanks again
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Old 03-21-2009, 02:12 PM   #5
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HOw to tell if a socket is propery grounded


Another problem with computers or most any electronic device on ungrounded outlets is the fact the they almost all use RC networks to earth ground to filter noise. The lack of this filtering path can lead to corrupted data. A GFCI will add saftey but will not compensate for the missing ground.

Last edited by hayewe farm; 03-21-2009 at 02:34 PM.
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Old 03-21-2009, 06:43 PM   #6
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HOw to tell if a socket is propery grounded


The problem with old systems is that you don't know what the grounding consists of. The metal sheath or conduit carrying wires may have been used as the grounding conductor with joints and clamps at every outlet box that are now oxidized and not making good contact.

The only way I know of for proving a ground is to string a long wire from the panel ground bus bar up across floors to the receptacle in question (don[t connect the end you are holding). Use an ohmmeter or multimeter to test the resistance between what you think is ground say the screw that holds the receptacle cover on, and what you know is ground which is the end of the wire you strung.

Worse yet, a ground system made up of sheaths, boxes, and conduits could be intermittent.

Modern systems have a ground wire even if metal conduit is used throughout.

You can install a new ground wire, visible on the surface if you don't want to fish it, up and over doorways and down to your panel if you wish, preferably more or less following the route of the circuit serving the receptacle in question.
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Last edited by AllanJ; 03-21-2009 at 06:54 PM.
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Old 03-21-2009, 07:57 PM   #7
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HOw to tell if a socket is propery grounded


I would just run a new line to the computer. So it can be short as possible, and have its own, unspliced ground to the panel.
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Old 03-21-2009, 08:43 PM   #8
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HOw to tell if a socket is propery grounded


I would definitely run a new dedicated circuit if I wanted to be sure of protection. That is the ideal that is recommended even for modern circuits. Newer electronics get more and more sensitive all the time.
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Old 03-21-2009, 09:04 PM   #9
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HOw to tell if a socket is propery grounded


If you are crazy about power quality, you might as well pull mc instead of romex Nah, just run romex.
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Old 03-21-2009, 09:10 PM   #10
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HOw to tell if a socket is propery grounded


Temporarily power a hair dryer from a known good 120v and your suspect ground and hook up a DVM from a known good ground to the suspect ground. Use extension cords to make the hookups.
If the suspect ground wire is #14 and goes 50' back to the panel then the voltage on this ground should increase ~130 mV or less w/respect to the known good ground, when you turn the dryer on, if it pulls 10A. For a 100' run the pass/fail value is ~260mV.

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