Visual inspection
The process of pat testing is quite straightforward.
Firstly there is the visual inspection. Basically if it looks broken or worn out, it is! The vast majority of items that fail do so during their visual inspection.
Applied voltage tests
Then votages are applied to test the equipment and so there are values to compare to standards to see if the item is satisfactory or not.
An analogy
To explain the applied voltage tests I use the analogy of central heating. Most people, me included, find it hard to understand electrons flowing through copper but we are familiar with water flowing through pipes. The Boiler is analogous to the power station. Radiators are electrical appliances and the pipes are the cables. The water circulates taking energy from the boiler to the radiators in a similar way to cables moving power from the power station to your television.
Now, you wouldn’t want water leaking from your central heating and you don’t want electricity leaking from your appliances.To test that you would put high pressure through the pipes to force water out of any tiny holes or to burst out of weak spots. In electrical terms Voltage is pressure. So high voltage is high pressure. The usual voltage is 230v. So the test is with 500v. There should be no leaks and a reading over one megohm (One Million Ohms). However should there be a leak in your central heating you need a drain large enough to allow all the water to flow away without you getting too wet.In electrical terms the drain is the Earth. Voltage is applied and a current flows through the earth to give a reading. There should be a low resistance and a reading below 0.1 ohm. (The resistance of the earth is also critical for the automatic disconection of the supply which is covered in the theory section of this site.)
IT equipment needs special treatment as they contain a lot of semi-conductors. They are all a bit “Leaky” so putting high voltages through them would damage them (They would “Burst” for sure). So they are measured at normal voltages and the actual leakage current flowing through the earth conductor is measured.