Induced earth voltage??

[cmbcmb]

Hi folks, could I have some advice please? I have 12 volts between the (presumably earthed?) metal cooker extractor hood and the earth of a wall socket. This rises to 20 volts when the fan and light are switched on. Yet when I connect a test lamp (no matter whether 3V or 12V or 240V) in parallel with the voltmeter, the voltage drops to zero, and the lamp remains completely unlit. I don't understand what's going on. Is this some sort of induced voltage on the cooker earth?
(a) Is this dangerous?
(b) What is causing it?
(c) How do I stop it?
Many thanks.

 

[MarkieSparkie]

cmbcmb, I presume you are measuring the voltage with digital voltmeter, multimeter or multifunction tester.
If so, as you have found, as soon as you load this voltage it disappears. This what electricians call a 'Phantom Voltage' and is of no concern. The input impedance of modern test instruments is very high >10Mohms and therefore it requires less than a micro-ampere to produce a voltage reading. This means in practice that voltages that are induced on all conductive materials from adjacent circuits or other electro-magnetic sources.
Back in the day when analogue instruments were the norm, electricians favoured less sensitive versions of voltmeter because they were less responsive to the phantom voltages that are encountered regularly.

 

[cmbcmb]

MarkieSparkie, many thanks for your advice; we will cook safely.
Cheers, cmbcmb.

 

[telectrix]

yeah. but yourself a proper AVO Meter, analogue.

 

[Marvo]

As stated above, it might not indicate a problem but I would certainly suggest for safety and peace of mind you test that the earth readings of the appliance and its circuit is within the specs of your local regulations. You need to do an impedance test, not a voltage test to be sure.

 

[R1CH-G]

yeah. but yourself a proper AVO Meter, analogue.

Good point!

 

[cmbcmb]

Have raided ark and found analogue multimeter, which says voltage is negligible, and when in parallel with digital meter drops the digital reading 20V to about 0.2V.
Continuity test says hood is not earthed, but label inside hood proclaims double insulation.
Thanks to telectrix, Marvo and R1CH-G for advice.