Standard DNO Fault Current - incompatible MCB rating

Neil Barrett said:

in lieu of an existing EICR

Not quite sure what that means. A PEFC of 16 kA implies a Ze of < 0.02 Ω. How close is the transformer?

Are the final circuits 3-phase or single-phase?

This will be the line fault and not the earth fault and I would assume that the worst case line fault current that the DNO would state would be adjacent to the Transformer, so I could estimate that the cut-out is about 40 metres from the transformer and can model this in Amtech to see what fault current I might get.  The majority are single phase but there would still be a risk of a fault between final circuits of different phase which would present this issue if occurring close to the board. I just think this must be a common occurrence.... 

This seems to be a bit muddled, but if your service cable is 25 mm² copper equivalent (plenty for 100 A/phase), that is about 0.06 Ω for a start.

If there are 3-phase final circuits, then your PFC is approximately twice your PEFC for line/line faults. If the supply is PME, the L/E and L/N faults at the DB will be more or less the same.

From the sound of it, 10 kA MCBs will be fine.

This seems to be a bit muddled, but if your service cable is 25 mm² copper equivalent (plenty for 100 A/phase), that is about 0.06 Ω for a start.

If there are 3-phase final circuits, then your PFC is approximately twice your PEFC for line/line faults. If the supply is PME, the L/E and L/N faults at the DB will be more or less the same.

From the sound of it, 10 kA MCBs will be fine.

Thanks Chris, understood. 

I was applying the DNO stated worst case characteristics

SSE

"Nominal Voltage U/Uo = 400/230v
Nominal frequency f = 50 Hz
Prospective fault current Ipt: 25kA (3 phase), 16kA (1 phase)
External loop impedance Ze:
0.35 ohms typical maximum for TNCS system
0.8 ohms typical maximum for TNS system
The Ze values may be higher on older networks and should be verified by measurement."