[LAU] ground loop hell

jim jim at well.com
Sun Feb 8 20:43:16 UTC 2015

(again, hoping to help someone or another)

     Two prong receptacles in old wiring connect one prong
to the "hot" lug at the service and depend on the bonding
between the neutral lug and the ground lug at the service
entrance so that the neutral wire is also the path to earth
     The receptacle has a cover that is attached to the
receptacle itself by a small (#6-32) screw. This screw is
electrically connected to the neutral and can serve as a
"separate" ground lug.
     To power a device with a three prong plugs to a two
prong receptacle, get an adapter (about a dollar at any
hardware store). Note the plug for your device fits in at
one end of the adapter and the other end of the adapter
has two prongs and a small (usually green) tab.
     Some adapters' prongs are "modern" such that the
neutral prong is wider than the hot prong. Some two-
prong receptacles accommodate such adapters, and
some do not--they present connections for two,
similarly sized prongs, both small. It's probably good
to buy an adapter with two small prongs (not only will
it fit all receptacles, you can turn it "upside down" to
try to reduce noise or shock hazard.
     The (green) tab attaches to the plate cover screw.
Before you plug the adapter in, remove the screw,
then plug in the adapter, then screw back the plate
cover screw through the (green) tab. This not only
gives your device a pseudo-separate ground path, it
fastens the adaptor to the socket, providing pretty
good strain relief.

On 02/08/2015 12:14 PM, Hermann Meyer wrote:
> Old buildings didn't use grounds, they just have 2 cords and use the 
> second as ground and backline. In German this is called Nullung.
> Potential issues are clear with this technique.
> -- 
> Ralf Mardorf <ralf.mardorf at alice-dsl.net> schrieb:
>     On Sun, 8 Feb 2015 09:06:40 -0800 (PST), Len Ovens wrote:
>     >It appears we have power coming from two power entrances with two
>     >different earth grounds to the panels.
>     I experienced potential difference for the ground in old buildings
>     between two main sockets, less than 3 meters difference between those
>     sockets, both fed by the same power, coming from the same fuse and
>     connected to the same concrete-footing ground electrode.
>     Even if the power should come from two different sources, one building
>     should have one concrete-footing ground electrode for ground, so in
>     theory there shouldn't be a potential difference.
>     Buildings sometimes don't care about school books.
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