student need help please re: pfc

[N6rul]

Some test instruments display a prospective fault current reading when carrying out an earth fault loop impedance test. Please can someone tell me why the pfc test must be carried out as a desperate test from an external earth fault loop impedance test?

My exams are near by I will be much obliged if someone can explain this to me. Thanks.

 

[Edd]

The Earth Fault Loop Is as it says The Impedance (resistance) of the Earth system.
But the Prospective fault current PFC is the current that will run in a dead short situation, so this should be tested between L and E and L and N as a dead short between L and N could give a greater value.
And on 3 phase systems a short between L and L would be even greater. A rule of thumb on 3 phase is to do L-E and L-N of all 3 phases take the higest reading and double it.
Good luck with the Exam

 

[Deleted member 9648]

^^^......plus the reason for testing is because the PFC should be less than the breaking capacity of the protective device. If you look at a typical MCB or fuse it will be marked with it's breaking capacity,usually something like M6. This means a breaking capacity of 6ka....or 6000 amps. The measured PFC at the origin of the circuit must therefore be less than 6ka or the MCB will not be capable of breaking a dead short fault current.

 

[N6rul]

thank you

Thank you, for your time and a very good explanation.

 

[SirKit Breaker]

Some test instruments display a prospective fault current reading when carrying out an earth fault loop impedance test. Please can someone tell me why the pfc test must be carried out as a desperate test from an external earth fault loop impedance test?

My exams are near by I will be much obliged if someone can explain this to me. Thanks.

The PFC/PSSC can be carried out at the same time as the Ze. Most instruments do this anyway. I don't know why you have been told otherwise. There is nothing to be gained by doing it separately, and in any case ohms law never lies, so you could get away with not doing it at all in reality. In all the testing course i have run, i have never made anyone do this separate to Ze, but i do make them calculate after to ensure that they can substantiate their results.

Cheers.............Howard

 

[N6rul]

thanks

Thank you.
It was an exam question On a previous paper, hmmmm! Trick questions! Cheers

 

[Richard Burns]

Ze is carried out Line to the earthing conductor whilst the earthing conductor is disconnected from the installation.
PEFC is carried out with the main earthing conductor connected to the installation and with the parallel paths that may be created by bonding, etc.

 

[telectrix]

just to muddy the water. why do we measure PFC with bonding connected? answer.. to give the worst ( highest ) result. then from the 2 readings, PSCC and PEFC, we record the highest, which is always PSCC excepting TNC-S , where they should be the same. now, the PSCC (L/N) will not be affected by the bonding, so what's the point? i throw this out for discussion, if anyone's bothered.

 

[lauriehurman]

N6rul
From the point of view of your exam;
PFC is at the origin of the installation for the reasons others have given - can the fuse and the earth conductor cope with that much current.
EFLI is at the furthest point of a circuit to give a worst case earth fault current and hence tripping time.

Think about a TT installation; The PEFC may only be a couple of amps but the dead short - line to neutral, would be hundreds of amps.

Hope that's helpful
Laurie

 

[N6rul]

thank you guys

Great to have mentors online.

 

[N6rul]

How would i write it in exam

Sorry guys being a bit cheeky. During the exam you get only only 2 mins to answer the questions, what would be the best way to answer this question. I understand the earth fault loop test is to determine how much current can go through the protective conductor in the event of a fault. And the Pefc is to check if the protective device ie Rcd is sufficient if a fault occurs.
Is this correct.

 

[Richard Burns]

Re: How would i write it in exam

Some test instruments display a prospective fault current reading when carrying out an earth fault loop impedance test. Please can someone tell me why the pfc test must be carried out as a desperate test from an external earth fault loop impedance test?

My exams are near by I will be much obliged if someone can explain this to me. Thanks.

Sorry guys being a bit cheeky. During the exam you get only only 2 mins to answer the questions, what would be the best way to answer this question. I understand the earth fault loop test is to determine how much current can go through the protective conductor in the event of a fault. And the Pefc is to check if the protective device ie Rcd is sufficient if a fault occurs.
Is this correct.

If your question is as above "Some test instruments display a prospective fault current reading when carrying out an earth fault loop impedance test. Please can someone tell me why the pfc test must be carried out as a separate test from an external earth fault loop impedance test?"

Then the answer would be:
Each is measuring the worst case scenario: external EFLI measured using only the main earthing conductor gives the highest resistance in relation to the maximum external EFLI allowed.
PEFC measured with main equipotential bonding in place as well as the main earthing conductor gives the maximum fault current in relation to the short circuit capacity of the protective device(s).

 

[lauriehurman]

Re: How would i write it in exam

Nail on the head.
Thanks Richard




[QUOTE=Richard Burns;
Each is measuring the worst case scenario

 

[N6rul]

thanks

I'm just loving it, this is the first forum that I have received x amount of help. Invaluable to a student like myself and others in my shoes..Thanks