Residual voltage question

tiernsee

My OH is driving me mad. He has a mains voltage tester and has been going around the house sticking it on metal objects like a radiator, the gas fire, a metal light fitting some of them are showing a voltage of 12V when he touches them and he now thinks that there is something wrong with the earth.

Any electric experts can give an explanation, please.....

thescouselander

Exactly what sort of tester is it and how is he connecting it?

tiernsee

It is a contact tester, you touch the end to a wire (or what you want to test) and press a button and it either displays nothing, a flash symbol or a voltage reading.

tiernsee

It looks like this one:

http://www.diy.com/departments/bq-pen-type-voltage-tester/177924_BQ.prd

brightontraveller

Technical bit
Non-contact voltage testers do not actually measure voltage. It measures the movement of electrons through the conductor, Reading it is an electricfield or electromagnetic radiation, the “voltage” is reported as a secondary reading based on movement of electrons and their associated wave functions.

Reality
Those type of tester are often as accurate as a blind dart player ?
They often give false readings for a multitude of reasons...

forgotmyname

Similar issues with a stud finder. It will detect a power cable even when there is no power in that area.

WobblyDog

Making measurements near mains-voltage power circuits is more complicated than it first appears, even when using quite high-specification test equipment.

The voltages and currents in live circuits are alternating backwards and forwards 50 times a second, which means they can induce voltages and currents in disconnected bits of metal nearby, which can confuse basic test equipment.

It's an interesting subject to learn about, but the results from a tester like the one you linked to can sometimes be misleading:
http://en.wikipedia.org/wiki/Stray_voltage#Stray.2Fcontact_voltage_detection