2391 Practical re sit, Need a little bit of help...

[YoungBart]

Hi all,
On friday I have the re sit for my 2391 practical, I'm confident on all the dead tests, Zs, RCD's and functional checks etc. And I'm fairly confident with the PEFC and PSCC, However on our test rig we have 2 boards (A main board, and a 'Garage' board) So do I need to do the PEFC and PSCC at both boards?
Thanks in advance.

 

[Midwest]

I've just done my 2392. My understanding (and I'm sure I will be corrected if I have this wrong!), is that BS7671 requires the PFC (PEFC+PSCC) to be determined at the origin and at other relevant points. For simple installations, where all circuits eminate from the CU, that would be there. In the 'your' installation, the PFC should also be determined at the garage CU. My notes read that PFC should always be determined at the point of the installation where the electrical energy is deliverd. Only the same as for determining the ZS of those circuits.

 

[timbobelfast]

IIRC it was one SOTR for 3 ph DB and one SOTR for single ph CU. Therefor PFC measured on both boards.

 

[SirKit Breaker]

Your main board should be supplying your garage board.

The main board is where the supply comes in to the installation. So you need to take Ze and PSSC/PEFC at that point.
You then have a submain from your main board supplying your garage board, this is a distribution circuit.
You have a choice, either measure Ze PSSC/PEFC at the garage board, and record the result as Zdb, or calculate them, whichever floats your boat.

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

 

[YoungBart]

Thanks all, I think finding the PEFC at the garage board should be alright, just do it as I would measuring the Zs but just press the PFC button. I assume the PSSC will be the same as for the main board. I just need to make sure I have the 'garage' breaker ON at the main board.....I think....

 

[trev]

You measure at the 3p board and calculate the single phase one. At least that's how I was told to do it

 

[Midwest]

Thanks all, I think finding the PEFC at the garage board should be alright, just do it as I would measuring the Zs but just press the PFC button. I assume the PSSC will be the same as for the main board. I just need to make sure I have the 'garage' breaker ON at the main board.....I think....

Yep it's one of the live tests, before energising the garage circuits, test for PFC & PSC (record the worst figure, ie. the highest ka). The garage cu isolator will be in the off position, you are testing the distribution circuit from the garage CU back the house CU back to the supply transformer. Record as Zdb as Sirkit said (additional record of schedules & test reports to your EIC/EICR). You should carry out your garage CU ZS test before PFC tests.

 

[ringer]

Have a think about WHY you would do a PFC at the sub board and what you would use that result for (whether measured or calculated). It is not good enough to just record a result because the form has a box that needs to be filled in. There is a reason for this particular test.

 

[ackbarthestar]

Agreed....
Don't forget, when testing for pfc/psc on the distribution board, to make sure all earthing and bonding connections are in place.

 

[pencilpusher]

The PSCC and PEFC should be at the highest at your main CU. With ohms law taking effect, the run to the garage will add more resistance to the circuit and so your PSCC/PEFC at the DB in the garage should be lower.

 

[partypest]

Like the other chaps just said, what is the relevance of the PFC an PSC, when would you get those currents? What would currents of that magnitude do? Would a low or high result be desirable? Answer those questions and you'll understand why you're doing it.

 

[YoungBart]

As far as I understand the reason we need to do the test is, to make sure that equipment (breakers) can cope with any potential fault currents. But thank you all for your help. I passed it today, and was very relieved to do so. AM2 in a few months time, so hopefully before long all the 4 years hard work will have an end product.

 

[adslev]

If it's 3 phase then what do you do to the pfc value ........?

 

[YoungBart]

Oohh I know this.....double it

 

[ackbarthestar]

Well done - YoungBart !

Look up how to wire DOL starters - hint