UK Three phase testing PFC

[CokeCanBrad]

am i correct in thinking for determining the PFC, you can do a PEFC test aand PSCC test and which ever one is highest you can double and choose to be your PFC also is it true for 3 phase that PSCC Is usually always higher?

Ive seen some people croc on to the earth and croc onto the neutral and test between each phase too determine the PFC take the highest reading and double.

Then other people say just croc on to the neutral and test between each phase and take thhe highest reading and double, that will be your pfc

Who is right and who is wrong?

 

[Bellendian]

If your tester is suitably rated, then no problem to take pscc line to line and record the highest value.

 

[CokeCanBrad]

If your tester is suitably rated, then no problem to take pscc line to line and record the highest value.

sorry i forgot to mention assume that i dont have a tester that is capable of testing between phases

 

[pc1966]

If your system has the same neutral conductor as the phase, then the "3 phase" fault current is double the single phase current, as under a balanced load ("bolted fault" I think is the USA expression) you see only the phase impedance limiting the current, where as a single phase test has both phase and neutral, so half the current.

Of course, there are deeper assumptions about the upstream transformer source impedance and linearity, etc, but I think it is good enough. Any one able to comment?

 

[Bellendian]

Just re read. You only double the line neutral figure as a rule of thumb if tester can't handle line line voltage. So line line direct reading would normally be the figure recorded.

 

[davesparks]

First you don't test PFC, you measure the loop impedance and your tester calculates the PFC from this.

On a three phase supply you measure the L - L loop impedance and the tester will give you a value for the PSCC, do this for all 3 possible combinations of phases. The highest value of PSCC is recorded.

If you tester is not capable of measuring the L-L loop impedance then you can measure the L-N loop impedance for all three phases and calculate the probable PSCC from the highest value the tester gives you. To do this take the value given by the tester and multiply by the square root of 3.
(Some people just double the figure, not sure why, maybe they're incapable of operating a calculator)

But if you are working with three phase supplies you should really have test equipment suitable for the type of work you are doing.
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If your system has the same neutral conductor as the phase, then the "3 phase" fault current is double the single phase current, as under a balanced load ("bolted fault" I think is the USA expression) you see only the phase impedance limiting the current, where as a single phase test has both phase and neutral, so half the current.

Of course, there are deeper assumptions about the upstream transformer source impedance and linearity, etc, but I think it is good enough. Any one able to comment?

It shoukd be multiplied by the square root of 3, not doubled, as far as I know.

 

[CokeCanBrad]

First you don't test PFC, you measure the loop impedance and your tester calculates the PFC from this.

On a three phase supply you measure the L - L loop impedance and the tester will give you a value for the PSCV, do this for all 3 possible combinations of phases. The highest value of PSCC is recorded.

If you tester is not capable of measuring the L-L loop impedance then you can measure the L-N loop impedance for all three phases and calculate the probable PSCC from the highest value the tester gives you. To do this take the value given by the tester and multiply by the square root of 3.
(Some people just double the figure, not sure why, maybe they're incapable of operating a calculator)

But if you are working with three phase supplies you should really have test equipment suitable for the type of work you are doing.

your answer makes the most sense too me. In the regs it does say its simpler to double it as its lower on the side of caution anyway. i dont have a cat 4 meter so i have no option.

After ive done my Zs test i recconect the main earth croc onto the neutral test l1, l2, l3 select highest reading and double it.

and ill get that ampage reading i assume on the Loop L-N setting on the megger meter correct?

 

[davesparks]

your answer makes the most sense too me. In the regs it does say its simpler to double it as its lower on the side of caution anyway. i dont have a cat 4 meter so i have no option.

After ive done my Zs test i recconect the main earth croc onto the neutral test l1, l2, l3 select highest reading and double it.

and ill get that ampage reading i assume on the Loop L-N setting on the megger meter correct?

What is a 'main earth croc'?

I'm surprised that it can't measure L-L if it's a megger tester.

 

[pc1966]

If you model the supply impedance as a star-connected source (see page 76 of the IET's "Electrical Installation Design Guide") then what you measure as phase-phase has sqrt(3) more voltage and double the impedance, so it is 0.866 of the worst-case three-phase "bolted fault" current, but 1.73 more than the single phase current.

For most simple faults the phase-phase case is worst, as a full three phase fault is not normally the start.

But under arc-flash conditions the resulting plasma is very conductive and mobile so what starts as, say, a phase-earth fault (or phase-phase) can rapidly become a full three-phase fault where all 3 conductors are carrying the fault current and the neutral is not really needed, so its impedance is secondary to the situation.

 

[CokeCanBrad]

What is a 'main earth croc'?

I'm surprised that it can't measure L-L if it's a megger tester.

sorry thats my grammar it should of said after ive done my zs test i recconect the main earth, croc on to the neutral (crodile clip

 

[Risteard]

Doubling the phase to neutral may actually be a more realistic worst case scenario than multiplying by root 3 as a bolted fault between all three phases will be higher than between any two.

 

[Ian1981]

First you don't test PFC, you measure the loop impedance and your tester calculates the PFC from this.

On a three phase supply you measure the L - L loop impedance and the tester will give you a value for the PSCC, do this for all 3 possible combinations of phases. The highest value of PSCC is recorded.

If you tester is not capable of measuring the L-L loop impedance then you can measure the L-N loop impedance for all three phases and calculate the probable PSCC from the highest value the tester gives you. To do this take the value given by the tester and multiply by the square root of 3.
(Some people just double the figure, not sure why, maybe they're incapable of operating a calculator)

But if you are working with three phase supplies you should really have test equipment suitable for the type of work you are doing.
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It shoukd be multiplied by the square root of 3, not doubled, as far as I know.

To calculate it then The rule of thumb is to double the phase to neutral PSSC, this is what is in any literature I’ve read (GN3) and what I was taught on the inspection and testing course anyway, but heard if the square root method to be honest.
May as well test between phases of course

 

[pc1966]

I think the most accurate test is to measure PSSC between two phases and multiply by 2/sqrt(3) = 1.155 as that excludes the neutral's impedance (which may not match the line value) to get the worst-case "bolted fault" current. Also that allows for the generator/transformer impedance being significant w.r.t. line impedance.

But as many MFT can only check 230V then taking max PSSC of all three L-N measurements and doubling it is probably good enough.