Three phase testing

[RMS]

Hi all,

when testing Ze on a three phase system is it okay to measure between earth and any phase as i'm only proving the impeadance of the earth?

also when carring out PFC tests is this okay on any phase and to earth and when measuring phase-neutral fault currents do you connect the earth lead onto the neutral bar with the neurtal lead?

cheers guys.

RMS

 

[Spark123]

when testing Ze on a three phase system is it okay to measure between earth and any phase as i'm only proving the impeadance of the earth?

Measure all three, record the highest of the three ohms values. You are not only checking the impedance of the earth, you are also checking the impedance of the supply phase conductors from the transformer.

also when carring out PFC tests is this okay on any phase and to earth and when measuring phase-neutral fault currents do you connect the earth lead onto the neutral bar with the neurtal lead?

It depends on your tester, it should tell you in the instruction book. Some testers require both the black and earth test probe on the same terminal. Some testers are OK on 415v, others are 240v max. For the PFC you are measuring the maximum fault current in your system, hence you need to take phases-phases, phases-neutral and phases-earth currents and record the maximum. If your tester does not measure 415v, complete the other tests at 240v, i.e. phases-neutral and phases-earth tests, take the highest value and double it.

 

[RMS]

Thanks Spark 123,

i take it when you double the reading for PFC on single phase calculations then you are accounting for the maximum three phase to earth/neutral faults?

RMS

 

[Adam_151]

i take it when you double the reading for PFC on single phase calculations then you are accounting for the maximum three phase to earth/neutral faults?

RMS

Thats to take account of the maximum possible fault that could occur, one involving all three phases, for example a spanner dropped across a busbar, which is why its double and not route 3 of the highest single phase fault that could occur (out of the six that are possible).

Maybe we should factor Rsp in as well (resistance of spanner )