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This question has some implications on home related work with mains. Never take what you read on the internet as the truth when lives might be in the line. That is how I'll be treating potential answers also.


In an old household on some sockets the testing screwdriver lights up very a little when the ground is tested. The light is significantly weaker than on the hot wire.

The ground voltage can't be felt to the touch, the only way it's detectable is with the neon lamp screwdriver.

Other sockets don't have that leaking voltage, the screwdriver doesn't light on at all.

The house does have an RCD switch which works.

Assuming anything can be expected from the house wiring what could be the cause of the voltage on just some grounds?

Update

There was a good comment if I want an engineering answer or how to fix this practically. I would like an engineering answer in the form of a summary of plausible theories in the EE jargon that would fit this behavior.

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    \$\begingroup\$ This is on the ground connection, and not neutral? That seems to indicate a miswired socket to me. \$\endgroup\$ – Hearth Jan 20 '20 at 14:47
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    \$\begingroup\$ If you're looking for an engineering answer from a bunch of guys who don't actually wire houses for a living, then this is the right place. If you want to know what it means in terms of the safety of you, your loved ones, and your pets, and what to do about it that's code-compliant and safe, then this question should be migrated to the DIY forum -- there's some accomplished electricians that hang out there and share their expertise. \$\endgroup\$ – TimWescott Jan 20 '20 at 14:59
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    \$\begingroup\$ I suspect that the ground wire is firmly attached at the socket, but isn't grounded at the other end of the cable. So the ground wire is capacitively coupled over a great length to both power and neutral, and is weakly held in the middle. When you touch it, that shorts out the capacitance -- but your neon test light can still get enough voltage to fire, followed by enough current to light dimly. \$\endgroup\$ – TimWescott Jan 20 '20 at 15:01
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    \$\begingroup\$ You're assuming that in a "normal" situation you're grounded and the ground connection is grounded as well. That would mean no voltage should exist. Now what if the ground connection is grounded properly but you're not. If your body is well isolated from the floor (if have rubber soles on your shoes) and you somehow (capacitively) couple to the mains wires in the room and/or walls, that could make the neon tester light up. \$\endgroup\$ – Bimpelrekkie Jan 20 '20 at 15:02
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    \$\begingroup\$ If you use a neon screwdriver on a properly earthed socket, it will glow. You act as a giant capacitively coupled power supply for the screwdriver picking up mains hum. This is perfectly normal. Neon screwdrivers are not designed to be earth continuity detectors. \$\endgroup\$ – Tom Carpenter Jan 20 '20 at 15:20
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your ground wire is broken, your tester is showing you capacitive pick-up between the live and the ground.

to fix, find the break and repair it.

The ground will follow the same path as the live wire so check the first outlet on the bad circuit and whatever is before it

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