Ze/Zs of 3 phase supply

[appspark]

Hey all, going over some testing before my final assesment. i've heard a couple of times when testing a 3 phase supply using a single phase to earth test you double the reading as your Ze/Zs, so if you test L1-E=0.07 Zs=0.14. also what i have done until now is test all three Lines to earth and recorded the highest value, i heard from someone else they average the result, although i think every time ive tested they've been the same with maybe one line 0.01 higher.
Thanks.

 

[Simonslimline]

Hey all, going over some testing before my final assesment. i've heard a couple of times when testing a 3 phase supply using a single phase to earth test you double the reading as your Ze/Zs, so if you test L1-E=0.07 Zs=0.14. also what i have done until now is test all three Lines to earth and recorded the highest value, i heard from someone else they average the result, although i think every time ive tested they've been the same with maybe one line 0.01 higher.
Thanks.

You double or more accurately multiply by 1.732 to get the PFC. This is using the PEFC or PSCC depending on readings.

 

[Simonslimline]

Or just measure between the phases if your tester can do this.

 

[Leesparkykent]

As above. Guidance note 3 gives us a rule of thumb of x2 but more accurately x1.732 as Simon mentions. You conduct the 3 PSCC tests and 3 PEFC tests and either double or use the 1.732 value to the highest obtained measurement.

 

[appspark]

Thanks, that helps a lot. im sure our new meters have line to line options now, just not sure what tester il be handed for my test.
Thanks again.

 

[Simonslimline]

Hey all, going over some testing before my final assesment. i've heard a couple of times when testing a 3 phase supply using a single phase to earth test you double the reading as your Ze/Zs, so if you test L1-E=0.07 Zs=0.14. also what i have done until now is test all three Lines to earth and recorded the highest value, i heard from someone else they average the result, although i think every time ive tested they've been the same with maybe one line 0.01 higher.
Thanks.

Not sure what you mean here! Zs = Ze+(R1+R2) although measured values will normally be lower due to parallel paths etc.

 

[appspark]

i was meaning say in a factory you have main board which is actually Ze, but sub boards are actualy Zs from main board, but that board's Ze if you catch my drift.

 

[Simonslimline]

i was meaning say in a factory you have main board which is actually Ze, but sub boards are actualy Zs from main board, but that board's Ze if you catch my drift.

It will be recorded as Zdb and the test carried out as you would perform a Zs.

 

[Sintra]

Not sure what you mean here! Zs = Ze+(R1+R2) although measured values will normally be lower due to parallel paths etc.

From what I've read I think the op is doubling the impedance value as a rule of thumb at x2 hence his L-E reading of 0.07 becoming a Zs of 0.14.

To the op just to clarify you do not double the L-E impedance reading to obtain Zs it's only the PEFC OR PSCC that is doubled (rule of thumb) or multiplied by root3 to get the PFC.

 

[Risteard]

This is using the PEFC or PSCC depending on readings.

It's only for PSCC. In a three-phase installation the PSCC will always be greater than the PEFC. So double the highest reading between phase and neutral (or measure directly between phases if your meter allows).