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@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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