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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.

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    \$\begingroup\$ "voltage across the box" - exactly what does this mean? "However when i plug the ground wire from the mains onto the box in order to ground. The box reads a voltage of 240v~" - so your 'ground' wire has 240VAC on it? \$\endgroup\$ – Bruce Abbott Feb 14 at 5:02
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    \$\begingroup\$ So with one probe to ground and the other to the box, it reads 240VAC? And that is with the 'ground' wire connected to the metal box? Is the part your probe is touching bonded to the part the ground wire is attached to? What voltage do you measure at the ground wire connection on the box? \$\endgroup\$ – Bruce Abbott Feb 14 at 5:09
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    \$\begingroup\$ @Dantecks, your multimeter has two probes. You put one on the box, but where did you put the other one? Please edit your question to make this clear. \$\endgroup\$ – The Photon Feb 14 at 5:36
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    \$\begingroup\$ "when i put both probes on the box that i get a reading" - if all parts of the box are electrically connected together you should get no reading, and then it is safe. The meter has high impedance so a 'floating' part may show voltage due to EMI. But that means it is not grounded, and therefore possibly (depending on whether that part could come into contact with high voltage) not safe! \$\endgroup\$ – Bruce Abbott Feb 14 at 5:43
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    \$\begingroup\$ Can you add a photo of the test being performed? \$\endgroup\$ – HandyHowie Feb 14 at 7:59
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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).

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