pfc

[m1ck_b87]

Hi all
Ive just done a ze and pfc and the pfc measure 8.2 which is greater than the breaking capacity of the Mcb's.
What should I do

 

[telectrix]

nothing. it's the capacity of the suppliers fuse that's relevant.

 

[m1ck_b87]

Yes your right thanks telectrix

 

[bartkusal]

That would be really low Ze, in region of 0.03 ohms, is the premises right next to distributor’s transformer? And is it domestic property?

 

[bartkusal]

Sorry Telletrix, Can you explain that. So where I should take the PFC readings to make sure that consumer installations equipment complies?

 

[m1ck_b87]

Domestic property and could you please explain abit more telectrix what happen when the faults on the socket circuit and the pfc is greater than the breaking capacity surely this is unsafe

 

[jamesBoater]

Have you taken Zs readings from the sockets?

 

[telectrix]

as above, your PFC on the circuits after the MCBs will be a lot less than at the source, due to the impedance of the circuit.

 

[Richard Burns]

If you have a domestic consumer unit and standard DNO fusing then the British standard requires that the consumer unit as a whole can withstand a fault current up to 16kA.

When measuring PFC this is done at the origin as any further measurements will be lower and so you have the worst case scenario.

 

[bartkusal]

as above, your PFC on the circuits after the MCBs will be a lot less than at the source, due to the impedance of the circuit.

That’s right and I knew that.
It’s just I thought if the circuit is really short, let’s say, a radial socket in next door cupboard and to add R1+R2+Ze (0.03) the PFC from that could be over 6 kA, as most MSB’s are this size in domestics, but then I imagined the R1+R2 as low as 0.10 ohm and calculated probable PFC it came to 1.77kA, so nothing to worry about, just didn’t thought that 0.10 ohms will make all the difference to comply, thanks.

 

[bartkusal]

If you have a domestic consumer unit and standard DNO fusing then the British standard requires that the consumer unit as a whole can withstand a fault current up to 16kA.

When measuring PFC this is done at the origin as any further measurements will be lower and so you have the worst case scenario.

When you say the whole consumer unit, do you mean with all PD in it, if that’s the case non complies as far as I understand, because all PD are 6 kA maybe sometimes 10 kA, but definitely not 16 kA. And then what code would you give on EICR, C3 or C2, I would see it as a danger under fault conditions.

 

[dave274]

in the electricians guide to the building regs it says that pfc should preferably be obtained by enquiry and would be given as a worst case of 16ka except in London.

 

[markythesparky]

as above, your PFC on the circuits after the MCBs will be a lot less than at the source, due to the impedance of the circuit.

what if the fault is right at the consumer unit on the lighting circuit or sockets? the PFC would be very large and the MCB wouldn't trip.

 

[Risteard]

There is a 16kA conditional rating which applies only in the UK. So your devices are fine in this instance.

 

[tony mc]

Have a read of this as your meter is not as accurate once the values get low ! Higher-accuracy low Ze test procedure