i'm in the uk, and I've taken the wall lights off and the earth wire and the neutral are both black, how do I tell which is which. thank you john
Most domestic fixed wiring in the UK is done with what is known as "twin and earth" cable. Normal twin and earth cable has two insulated conductors colored brown and blue* on cable produced today and colored red and black on earlier cables. In addition it has an uninsulated earth conductor.
Unlike the americans, we brits place sleeving over bare earth conductors inside boxes. This sleeving would normally be green/yellow striped, but on very old installations it may be plain green**.
Some properties are also wired with single wires in conduit, in the case of single wires in conduit there may not be a dedicated earth wire as the conduit can be used as the earth path.
If the light is fed with a cable, then what has most likely happened is that someone ran out of the proper earth sleeving, so they used a bit of black insulation stripped off another wire to sleeve the earth conductor. If this has happened it should be visible on a careful visual inspection, it's almost impossible to push tightly fitting insulation back onto a wire, so if this has been done then the insulation will likely have been taken from a large size of wire and will be a loose fit on the conductor.
If the wiring is in conduit then things get much more complicated. It's possible that someone used the wrong color wire, but it's equally possible that neither of the wires is in-fact a ground. For example one could be an incoming neutral and the other could be a neutral going out to the next light.
To test earths, I would suggest the following. First turn off power at the main switch, do not use the MCB for this as MCBs normally only provide single pole isolation***.
Then attach a long lead to the earthing system. I would suggest wiring it to the earth terminal of a plug and plugging it in is the easiest way.
You can then use your multimeter on continuity mode to find stuff that is connected to the earthing system.
P.S. It is always good practice to look carefully and label stuff before disconnection to avoid confution later.
* Normally live and neutral, but twin and earth cable is also used for switch drops with live and switched live. In this case the blue/black conductor should be marked with red/brown sleeving, but that does not always actually happen.
** In UK fixed wiring green/yellow replaced plain green for Earth long before brown/blue replaced red/black for live and neutral.
*** This advice on how to isolate both live and neutral applies specifically to the UK, n some countries even the main switch does not disconnect neutral and the only way to disconnect it would be to remove wires.
Make a test lamp ( a simple bulb connected to two wires). Connect one wire of the test lamp to the live wire and then using the other wire touch the black wires one by one. The bulb will turn on normally and brightly when joined to the neutral wire. It will be dim when you touch the ground wire. Or there are chances that the bulb will not turn on in case the ground has been disconnected in your house.