PEFC - Design Vs testing

[DonClif]

Hi guys,

I might be having a brain fart on this one, but can anyone enlighten me on this one:

(Stop me if I'm talking bum at any point)

When designing a circuit you calculate the R1+R2, add it to the Ze, end up with Zs.
You then do Uo x Cmin ÷ Zs to get your PEFC.
So you have a PEFC based off of the entire circuit from the socket to the transformer.
BUT when you are testing an installation you complete the PEFC test on the 'feed' of the main switch, with the switch open.
So than means you get a PEFC reading from the DB to the transformer.

If I'm not completely away with the fairies tonight, and I have this correct, then why do we effectively get the same thing from different places and using different parts of the circuit?

Is this just one of those: "Because that's how it is" situations, or should I know the answer?

Cheers

 

[Fleeting]

I think you need to elaborate. PEFC theoretically should decrease the further you calculate from the origin all things being equal. What do you mean by the same thing?.

 

[binky]

As I've always understood it, PEFC is to check incoming supply parameters rather than whole circuit, with a view to checking main switch and fuse can cope with a dead short on the supply.

 

[Fleeting]

You do both PEFC and PSCC then record the highest.

 

[SPECIAL LOCATION]

Hi guys,

I might be having a brain fart on this one, but can anyone enlighten me on this one:

(Stop me if I'm talking bum at any point)

When designing a circuit you calculate the R1+R2, add it to the Ze, end up with Zs.
You then do Uo x Cmin ÷ Zs to get your PEFC.
So you have a PEFC based off of the entire circuit from the socket to the transformer.
BUT when you are testing an installation you complete the PEFC test on the 'feed' of the main switch, with the switch open.
So than means you get a PEFC reading from the DB to the transformer.

If I'm not completely away with the fairies tonight, and I have this correct, then why do we effectively get the same thing from different places and using different parts of the circuit?

Is this just one of those: "Because that's how it is" situations, or should I know the answer?

Cheers

I think you my be confusing... (1) Ensuring the fault current at the furthest point of you circuit will be sufficient to operate the protective device fast enough.. (2) And verifying that all switchgear / protective devices are capable of withstanding a worst case short circuit fault at the origin.

With point (1) at the design stage you need to make sure your R1+R2+Ze, (obviously without an actual measured Ze have to use the nominal values), will not exceed the max permissible Zs, otherwise the fault current will not be sufficient to meet disconnection times.

e.g. for a 32A 60808 B max Zs is given as 1.37.
Uo (230) x Cmin (0.95) / 1.37 = 159.4A
or the 160A listed in the quick ref table on the time current graph for 60898's on page 417.
So you need to design you circuit to ensure you get an earth fault current >=160A to minimise any danger until the supply has been disconnected.

We are not actually recording any fault currents at the far end of a circuit..
But we are recording Zs or (R1+R2+Ze) to verify Zs is low enough and thus fault current will be large enough to perform ADS.

Whereas, with point (2) you measure PEFC (Line to Earth) and PSSC (Line to Neutral) and note the highest as the PFC.

Two different considerations that need to be taken account both relating to fault currents.
But NOT the same thing at each end of a circuit.
{there are no boxes on an electrical certificate for any fault currents for individual circuits)

 

[DonClif]

I think you my be confusing... (1) Ensuring the fault current at the furthest point of you circuit will be sufficient to operate the protective device fast enough.. (2) And verifying that all switchgear / protective devices are capable of withstanding a worst case short circuit fault at the origin.

With point (1) at the design stage you need to make sure your R1+R2+Ze, (obviously without an actual measured Ze have to use the nominal values), will not exceed the max permissible Zs, otherwise the fault current will not be sufficient to meet disconnection times.

e.g. for a 32A 60808 B max Zs is given as 1.37.
Uo (230) x Cmin (0.95) / 1.37 = 159.4A
or the 160A listed in the quick ref table on the time current graph for 60898's on page 417.
So you need to design you circuit to ensure you get an earth fault current >=160A to minimise any danger until the supply has been disconnected.

We are not actually recording any fault currents at the far end of a circuit..
But we are recording Zs or (R1+R2+Ze) to verify Zs is low enough and thus fault current will be large enough to perform ADS.

Whereas, with point (2) you measure PEFC (Line to Earth) and PSSC (Line to Neutral) and note the highest as the PFC.

Two different considerations that need to be taken account both relating to fault currents.
But NOT the same thing at each end of a circuit.
{there are no boxes on an electrical certificate for any fault currents for individual circuits)

This perfectly sorts my confusion.
Thank you.

I suppose i was wondering why we don't physically record fault currents at the end of a circuit, like with the PEFC/PSCC tests at the origin, but as you say the Zs covers that.

Also, am I understanding correctly that the Ia is the fault current required to operate the device in any of the times specified in the 'curve graphs' in Appendix 3? Or is it specifically the times in the tables to the right of the same graphs?

I've got a reasonably good understanding of how to test, and what each test is for, and I'm currently flying through my level 4 design, but connecting the dots between theory and practical sometimes has me in a bit of a mix.

I'll get there...

 

[SPECIAL LOCATION]

This perfectly sorts my confusion.
Thank you.

Its always easier to answer questions when someone gives a clear description of what they currently think they understand, and what bit(s) they don't understand.

 

[DonClif]

Its always easier to answer questions when someone gives a clear description of what they currently think they understand, and what bit(s) they don't understand.

I'm guessing I did a poor job of that, I'll be clearer in the future.
Thanks again

 

[NozSpark]

The only reason to work out the PEFC at the end of the circuit would be to stick the figure into the adiabatic equation to work out if the CPC is adequately sized,,,, however that only gets you the size at the end of the circuit and misses the beginning totally

 

[binky]

I'm guessing I did a poor job of that, I'll be clearer in the future.
Thanks again

It's hard to be clear when confused . Do feel free to ask questions, you are learning, and need as much help as us old pharts can give you, albeit being an old phart I've forgotten a lot

 

[SPECIAL LOCATION]

I'm guessing I did a poor job of that, I'll be clearer in the future.
Thanks again

Actually I think you did a pretty god job of describing what your current understanding was..
Which is why I made my assumptions about where you were getting confused?

Please don't be put off about asking any question you need clarification on..
And 'ALWAYS'.. give as much background info / current understanding as you can to put your question into context...

Your question details, and the thread title were good, if not better than some of the questions we have had in the past...

So I don't feel you did a poor job!!!