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richaf

04:48AM | 05/13/05
Member Since: 05/12/05
4 lifetime posts
Bvelectrical
I just got a house that was built in ’61. Some of the outlets are 2 prong, and a couple are 3 prong.

The 2 prong outlets do not have a ground wire in the metal box (all of the outlet boxes are metal), and the couple that are grounded have only a ground wire from the outlet to a screw in the box.

Is this an adequate and safe way of grounding? My polarity/ground tester tells me that they are OK.

If this is in fact OK, I will do the rest the same way.

Note: The main panel is grounded to earth via a post in the cement in the garage.

Thanks for any help………

Rich

Tom O

04:26PM | 05/13/05
Member Since: 09/17/02
487 lifetime posts
Rich,

if these boxes are wired with Armored Cable (metal spiral wound outer jacket) then the boxes are grounded. The same applies if the boxes are wired with a metal conduit.

But, to be 100% sure, you should take a voltage measurement from the hot wire to the metal box, it should be about 120 volts. Alternatively, you could pay a 1 hour service call for an electrician to come in & verify that your metal boxes are grounded.

If, in fact the boxes are grounded, you can install a 3 prong receptacle with the jumper connectedf rom the green screw on the receptacle to a screw threaded into the metal box ( not a sheet metal or wood screw). Alternatively, there are receptacles on the market that have a spring clip on one of the mounting screws that will insure grounding continuity through the mounting screw, if you use one of these, you can skip the wire jumper to the box.

Tom

Billhart

08:01PM | 05/13/05
Member Since: 04/25/05
1918 lifetime posts
I am afraid that a voltage reading from hot to ground, specially if he uses a digital voltmeter, is less reliable than the 3 neon bulb tester.

Personally I would verify it by connecting a small load (such as a 60 w bulb) between hot and ground, but I would not recommend that to someone that does not know what they are doing.

But one test that I think would be meaningfull with a digital voltmeter would be measuring between ground and neutral. With no load on that circuit it should be zero. If you measure more than 1-2 volts then the ground is open and you are reading a phantom voltage.

Also the self grounding feature can only be dependined on surface mounted boxes.

In flush mounted boxes the wall finish can hold the receptacle off the box so the grounding fingers don't make contact.


richaf

06:45AM | 05/14/05
Member Since: 05/12/05
4 lifetime posts
Thanks for the replies.

The whole house was in fact done with metal conduit.

I will hoever do the voltage test on each plug as I chang them to be on the safe side.

Thanks again.

Tom O

01:26PM | 05/14/05
Member Since: 09/17/02
487 lifetime posts
Self-grounding receptacles actually came about because of the conditions described by Billhart. These devices ensure contact between the mounting screw & the device yoke when there is no metal to metal contact.

"250.146(B) Contact Devices or Yokes. Contact devices or yokes designed and listed as self-grounding shall be permitted in conjunction with the supporting screws to establish the grounding circuit between the device yoke and flush type boxes"

dave9123

06:56PM | 05/26/05
Member Since: 05/25/05
4 lifetime posts
Hello all,

I'm in a situation similar to richaf's. Just bought a house that was built in the early 60's, and most of the outlets are original.

My cables are wrapped in some kind of fabric and have a black white, a white wire, and a bare wire. In any box I have been in so far, the box is metal and the bare wire is attached to the bottom. This being the case, and not really wanting to pigtail a ground from the bottom of the box to the ground terminal on the new outlets, I went with self-grounding outlets.

Did two outlets in the master bedroom today. Each time I saw the metal box, but there's plaster over the edge. At the mounting holes for the screws there's a little crumbled plaster and then the screw hole.

I was expecting a clean metal edge all around.

Am I still OK using the self grounding outlets? Once installed, everything tested OK with one of those AW Sperry receptacle testers (lights were blank-yellow-yellow). From Tom O’s comment I believe I am OK, I guess I just do not understand the science of how this little clip on the screw helps. Is it making sure that the receptable grounds to the screw which then grounds to the box once screwed in?

Thanks,

Dave


househelper

05:12AM | 05/27/05
Member Since: 03/31/05
265 lifetime posts
"Is it making sure that the receptable grounds to the screw which then grounds to the box once screwed in?"

That is exactly what it is doing. By using the contact against the screw, you are not relying on metal-to-metal contact between the box and the receptacle strap to complete your ground.

dave9123

09:23AM | 05/27/05
Member Since: 05/25/05
4 lifetime posts
Thank you -- just wanted to make sure before I did too many of these!
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