Ok so simple question though it might seem stupid. I'm wiring up a simple metal enclosure with a control unit inside. AC current from the wall, 240v~ to the unit.
I'm containing it in a metal case specifically so I can ground it (and the frame it is connected to).
With everything wired up, (and not grounded) its all good. voltage across the box are where they should be, not shorts. if I place my multimeter on any point of the case no voltage, as you would expect.
However when I plug the ground wire from the mains onto the box in order to ground. The box reads a voltage of 240v~. (when I put the probes from the multimeter on the box it reads voltage where there wasn't before.)
TO CARIFY. If I put the probe on the metal casing anywhere on it, back front inside outside both probes on the grounded box. Not the components I read a voltage. This occurs when the ground is attached.
Am I having a brain fart here or should the box have no voltage so long as there are no shorts?
{EDIT}
Thank you for the information, I will try a different outlet. and probing the case with a resistor when I get a chance. will update when I do.
[Final Edit]
So after testing it seems to have been stray currents. By shifting to a different power point and checking with a resistor it shows no voltage. I also double checked with a pen to be safe, all seems good.
Also tested all the circuit breakers and safety circuits in the house because the electrician who did it was a bit of a bodge, job safer to double check.
Best Answer
The reason why you measure an AC voltage on the box when you have the box/housing grounded might be stray currents.
Your multimeter has a very high input impedance and thus it displays such voltages from stray currents. If you put an oscilloscope probe on your desk and do not connect it, you might also see some 50Hz AC voltage on the oscilloscope display.
You can test this theory: Put a 100k Ohms resistor between your multimeter terminals and measure again.
If you still see a 240V AC display, then there's definitely something really bad. If you don't see any display anymore, then you know that this came from stray currents and it is totally harmless (because stray currents have extremely high output impedances and thus no driving strength).