PFC

Deleted member 138473 · Feb 19, 2021

Hello

I am wanting to know if any one can help me with a question I have in regards to PFC.

When testing PSCC and PEFC in order to find out the PFC, so that this value can be recorded and then compared against the breaking capacity of the protective device, on what part of the system is actually tested.

The test takes place at the DB/CU on the incoming Line and Neutral conductors with the main switch off.
So this will measure the current from external side from the DB/CU to the star transformer ?

My query is that if my understanding above is correct then this is only testing the potential fault current externally and not the internal installation.

I would have assumed the potential fault current that occurs inside the installation on the outgoing side of the DB/CU is what is needed to be tested, as this where the fault would take place and that potential current would need to be compared against the breaking capacity to ensure the protective device had a greater ka than the Ipf.

I might have gone completely off track with this would appreciate some clarity.

Thanks.

James · Feb 19, 2021

The difference will be negligible.
Normal practice is the origin of supply (main incomer or isolator)

pc1966 · Feb 19, 2021

Usually you measure the PFC (L-E fault) and the PSCC (L-N) and record the highest for single-phase setups, as the goal is to determine that the CU is able to interrupt the worst-case value (fault close to CU).

In practice, the incoming fuse on most domestic systems will peak-current limit on big faults so you won't really see much more than 10kA no matter what.

The end of circuit current will always be less, and then the goal is to establish it is big enough to disconnect fast enough (i.e. the Zs values measures should be less than the tabulated Zs values needed for the types of fuse/MCB in the CU).

Deleted member 138473 · Feb 19, 2021

James said:
The difference will be negligible.
Normal practice is the origin of supply (main incomer or isolator)

Thanks for the reply.

The current of the internal installation will be negligible ?

Deleted member 138473 · Feb 19, 2021

pc1966 said:
Usually you measure the PFC (L-E fault) and the PSCC (L-N) and record the highest for single-phase setups, as the goal is to determine that the CU is able to interrupt the worst-case value (fault close to CU).

In practice, the incoming fuse on most domestic systems will peak-current limit on big faults so you won't really see much more than 10kA no matter what.

The end of circuit current will always be less, and then the goal is to establish it is big enough to disconnect fast enough (i.e. the Zs values measures should be less than the tabulated Zs values needed for the types of fuse/MCB in the CU).

Thank you for the reply

1)During a big fault the current wont be more than 10ka.
Does that mean you would check the breaking capacity of the main fuse rather than the individual MCBs at the DB

2)The end circuit current will always be less.
Is this because there will be extra resistance, through more cabling causing the current to go up.

I will research your answers more so I understand and get back.

Thank you

pc1966 · Feb 19, 2021

MRELECTRICAL said:
Thank you for the reply

1)During a big fault the current wont be more than 10ka.
Does that mean you would check the breaking capacity of the main fuse rather than the individual MCBs at the DB

You are supposed to check this (fuse type and interrupting capacity) though the reality is the likes of the BS88 fuses have breaking capacities well above what the domestic supply can deliver.

One aspect that fuses do well (and MCB/MCCB don't) is limiting the fault energy. Yes, for medium faults in the 0.5-1kA range MCBs typically let through much less than fuses, but as the PFC increases fuses start to limit while breakers don't do nearly as much. Here is a typical fuse let-though plot:

What is shows is that by time you reach a PFC of about 2kA (symmetric, RMS) the 100A fuse starts to limit the peak current and even if your supply has 120kA PFC (an astonishing level by most standards, as the output of a 500kVA substation transformer is typically in the 15kA range) the peak fault current is about the same as if it were 7kA RMS and no fuse.

MRELECTRICAL said:
2)The end circuit current will always be less.
Is this because there will be extra resistance, through more cabling causing the current to go up.

Basically yes, the cable impedance limits the fault current.

However, a non-obvious aspect of that is the fault energy can be greater, as the fuse (or MCB) disconnection time may be far longer, so I2t is bigger. So while you will never have problems of the MCB failing on an end-of-circuit fault, you might have issues of cables overheating if the fuse/MCB does not act fast enough because it is not seeing much of an over-current.