how to use a voltage tester

[smartlady]

hi

i have three red wires and three black wires on my light fixture on the celling, there are two neutral and one live black wire how do i use a two probe tester to find the live one ,as i have never used this type of tester before and i want to use it safely.

[verne]

not just smartlady then but a determined one too! most people with an interest usually start mucking about with low voltage stuff - batteries and bulbs and stuff at school and fixing torches and pushbike lights and work up to mains voltages as they become more familiar with the techniques and equipment.

one of the best bits of kit you can have to start with is a mains tester screwdriver... it will indicate the live wire with a single connection... just touch the blade of the screwdriver to each wire in turn while keeping your thumb over the other end.

for the job you want to do, you will probably not need the voltage tester function of your new bit of kit. most of these sort of things; whether an old fashioned multi-meter, a digital meter, or an led bar display will have a resistance or continuity function (or both) operated by its own internal battery and it is this that you will be using. some time with the instructions is recommended and some time practicing...

you first need to forget the individual wires and identify the pairs or numbers of cables you have... switch the power off... pull the wires / cables down through the hole in the ceiling. if you can see where the wires were before the fitting fell off you can put them back - generally all the red wires will be connected together; usually three; one live in, one live to the next light and one to the switch for this light.

generally all the blacks will be connected together (except one); usually two; one neutral in, one neutral to the next light and it is to these that the blue wire of your light fitting will be connected to.

that should leave one black wire which will be live only when the lightswitch is on and it is this that connects to the brown wire of your light fitting.

if you are not absolutely certain where the wires went you need to seperate them and start from scratch and identify the job that each pair (cable) of wires does. using your new meter / tester on its resistance range find the pair that goes to the light switch... almost no resistance with lightswitch on and open circuit with lightswitch off is what you are looking for. when you have found this cable i suggest you mark it with tape.

you can proceed with further tests and identify the cable(s) going to the next light by repeating the test on the remaining cables and running around the house until you find a switch that causes some sort of resistance reading on your gadget.

the remaining pair of wires must (by elimination) be the supply live and neutral. this fact can be verified if required by making safe and switching the power on and then testing with your new gadget (on the correct range or you will break it) or simply with the mains tester screwdriver.

connecting up and testing the light should now be fairly straightforward.

next a word about safety. if there are other people in the house make sure they know what you are doing and why the power is off. draw the fuse if you can or tape over the main switch. always have an assistant watching you while you are testing with the power on and make sure he or she knows where the phone and the main switch is and that they can RUN!

identifying the switch pair is the important task here. once that is done the remainder is fairly intuitive... happy wiring!