Ken wrote in to ask:
I have a water pump with three wires – green/yellow(ground), the other two are white and green. Which is live and which is neutral?
Ken, I’m guessing that you must be writing from outside the EU – as far as I know, none of the EU member states has used that kind of wiring. In fact, I’m not sure that any of the major international standards uses it!
One of the simplest ways of finding out which wire is live is to obtain a mains testing screwdriver (also known as an electrician’s screwdriver) – it shouldn’t cost more than a couple of pounds. This looks like a normal screwdriver except that there’s a small bulb in the handle (which has a small metal patch at the top), and the shaft is usually insulated, with only the blade exposed. The screwdriver should have some kind of marking to let you know for what voltage range it may safely be used.
WARNING: DO NOT use a mains testing screwdriver for a mains circuit with a voltage higher than the maximum voltage indicated!
To test for a live current:
First of all, make sure that the appliance you’re testing is switched off. (If it’s switched on, then current is likely to be flowing through both wires, so you’ll be none the wiser.)
You’ll also need to have access to the bare wires or – better – to the terminal screws securing them to the appliance.
Then hold the screwdriver so that the handle’s between your thumb and middle finger and your index finger is pressing on the metal patch, and touch it to ONE of the two wires or screws.
WARNING: DO NOT let the blade touch more than one wire or screw at once, or form a bridge between the wire and the earth/ground connection – at the very least, you’ll melt the blade of the screwdriver, and possibly far worse.
When the blade of the screwdriver contacts the live wire or terminal, you should see the bulb inside the handle glow (usually reddish-orange). But DON’T assume that if the first wire glows, the second must be neutral. There may be a fault in your wiring. So always check both wires – or all three, if there’s an earth wire too.
Likewise, if neither wire glows, then there may be a fault with your tester – so ALWAYS test your tester before and after use, for instance on a socket that you know works.
Another way of checking is to trace the wiring back to the consumer unit (“fuse box”) and checking where it’s wired up to at that point. But make sure you do it thoroughly – crossed wires at any point (for example, an incorrectly wired socket or junction box) could mislead you into thinking that a wire is neutral when it is, in fact, live. So keep that mains tester handy!
And to avoid confusion in future (for instance, if you sell the house on) it would be as well to have the wiring replaced, by a competent electrician, with wiring that complies with the local standards – or, if there is no local standard, at least an internationally known one like the EU or US standard.