and how do you do the psc, by calculation? I would do this as a live test, then refer tabular values and perform polarity whilst isolated and again whilst live. I would also be doing the Zs whist live, as you would need to test the Ze anyway which would again be a live test.Theses test that I'm performing are standard tests that have to be preformed on a gas boiler before you service that boiler. The four tests to be completed are earth continuity, short circuit, resistance to earth and polarity. Three tests are done with the boiler isolated and obviously polarity is a live test. Theses tests are testing the boilers internal components and wiring looms within the boiler itself and not the source. Tests are being completed with a auto range multimeter.