A Cable Tester like many other devices does exactly what it says on the Tin. Tests Cables.
That's a bit vague! What exactly does it do?
There are many different types of cable tester which will work in different ways and of course have different features. The basic cable tester will be a single device usually powered by an internal battery. This device will have various sockets on it to allow connection of various Cable Types. Simply plug both ends of the cable into the device, follow the units instructions (testing may require a button push or a switch operation).
What do they test?
Every cable tester is different but most have a similar set of basic features. Each of the particular cores to a cable are tested, this is PIN 1:1 2:2 and 3:3 for an XLR etc. The cable is also tested for Short Circuit between 2 pins, open circuit on one pin. or any other misconfiguration or fault with the wiring.
It may be necessary to move the cable within the socket to see if any difference occurs. Sometimes a faulty socket can read OK on a cable tester until you move it. Technical Term : Wiggle.
Do they do anything else?
Cable testers sometimes have an in built signal generator. One end of the cable is plugged in to the unit and another to an amplifier or mixing console.
What if i've run the cable already?
Some cable testers come as 1 complete unit, you will probably have to connect both ends of the cable to the unit. tricky when you've already run out a cable, particularly a multicore.
Other testers come in 2 parts. a Send and Receive part. These can be split up and placed either end of a cable. These will often have 1 connector type only (often XLR).
there are other small cable testers which plug into a XLR socket. These require Phantom Power sent from a mixing console to work. You can find a fault and trace it back to the desk if necessary.
What if i dont have a cable tester?
You can use a Multimeter to test cables. this can be a bit tricky and as with the single unit style testers you need both ends of the cable with you. check resistance between pins 1:1 2:2 3:3 4:4 5:5 etc or T:T R:R S:S etc. This should be VERY low. Then check the same between alternate pins. For an XLR his would be 1:2 1:3 2:3. Its difficult to move the cable connector about whilst holding 2 multimeter probes. Simply plugging a cable in to test it may not be the best idea. A signal cable may not cause any issues if it is faulty (other than a few crackles). A speaker cable with a short circuit could do serious damage. and DO NOT even THINK about testing a mains cable by USING it.