DIY Doctor UK DIY Forums

Hi there
I isolated the supply to my electric shower but somehow there was still 150 volts.
Basically I switch off the MCU at the CCU, and switched off the double isolation switch (big red switch).
But somehow when I tested it it was giving me a reading.
I tested it on another meter and weirdly that was giving zero reading.
Im baffled!
Pics -
https://www.dropbox.com/scl/fo/gogg53mo ... mwqrx&dl=0

Now you know why you shouldn't use a multimeter to check for mains A.C. voltage. You should use a proper mains indicator Click here I would say the 1st meter you are using is picking up an induced voltage, the 2nd meter you are using is a £4.99 special from ebay, and should never be used near mains as they can be dangerous.

The difference between the two meters is that the first is high impedance and the second one (yellow) is low impedance.
High impedance means that the meter requires hardly any current (micro-amps) to infer a voltage. Low impedance means that the meter requires a bit more current (milli-amps) to make a reading.
So although low impedance meters give more "truthful" readings, as implied above, you don't want to rely on them too much.
The reason why the high impedance meter is giving a reading is because, as said above, it's picking up mains voltages induced through the air, close by. But if the house is detached and you've switched off the mains supply for the whole house I wouldn't expect it to pick up any mains voltages.

[attachment=0]Voltage tester and proving unit.jpg[/attachment]As said a voltage testing unit should not rely on batteries, and should include a proving unit.

However I don't there are another item to carry around so I only carried one when it was a sacking offence not to.

It is likely induced voltage, but could also be radio interference, I remember an expensive AVO showing I was using 60 amps on transmit, and the wires would have melted had that been the case.

Wow didnt expect that, thank God I was wearing insulation gloves.
Many thanks all.