Prospecctive Fault Current purpose....

[whazza22]

Hi

Can someone explain the reason for testing the PFC and what it confirms?

And what is the reson the value is doubled for 3-phase?

Thanks

 

[vernam616]

Prospective fault current, meaning the maximum current that will flow through the system in the event of a fault


Prospective Earth Fault Current - is the L/E reading with the protective bonding in place.

Prospective Short Circuit Current - is the L/N reading.

PFC is the higher of PEFC and PSCC (almost always the PSCC figure).

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[trypod]

The current which is likely to flow in a circuit if line and neutral cables are short circuited is called the prospective short circuit current (PSC). It is the largest current which can flow in the system. and protective devices must be capable of breaking it safely. The breaking capacity of a fuse or of a circuit breaker is one of the factors which need to be considered in its selection. Consumer units to BS EN 60439-4 and BS 88 (HBC) fuses are capable of breaking any probable prospective short-circuit current, but before using other equipment the installer must make sure that their breaking capacity exceeds the PSC at the point at which they are to he installed.
The effective breaking capacity of overcurrent devices varies widely with their construction. Semi-enclosed fuses are capable of breaking currents of 1 kA to 4 kA depending on their type. whilst cartridge fuses to BS 1361 will safely break at 16.5 kA for type 1 or 33 kA for type II. BS 88 fuses are capable of breaking any possible short-circuit current. Miniature circuit breakers to BS EN60898 have their rated breaking capacity marked on their cases in amperes (not kA) although above 10000 A the MCB may be damaged and lower breaking currents (75% for 10000 A and 50% above that level) must be used for design purposes.

 

[JUD]

Hi

Can someone explain the reason for testing the PFC and what it confirms?

PFC is measured to confirm that the fault current that could occur does not exceed the breaking capacity of the overcurrent protective device i.e the main cut-out fuse.

And what is the reson the value is doubled for 3-phase?

The reason it is doubled for 3-phase is because the fault current between phases will be higher than between a single phase and neutral. It's actually 1.73 times higher but it is doubled to err on the side of caution.

 

[ringer]

To ensure that your mcbs/rcds can withstand the pfc. If not then they may melt and fuse closed circuit, unable to trip. Typical single phase rating is 6kA. Typical 3ph is 10kA. 3ph potential is higher than 1ph.

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