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End of line fault
Nick Parker
32 Posts
Question
Why calculate end of line fault current? 
Is it for verifying that thermal stress could be withstood by the cable before the breaker gets tripped?
Is it to verify that breaker to be tripped in a certain time (1s or 5s) for the fault current magnitude at the end of line?

I have a colleague who says, only phase to phase faults to be calculated at the end of line.  I ask why not the phase to earth or Phase to neutral for which his reply is, Generic MCCB or MCBs would not work for earth faults.

The Generic MCCBs would work for earth fault provided the fault magnitude is greater than the magnetic trip threshold (Im). Now what if the Phase to earth fault's magnitude is lesser than the Magnetic Pick up. Should RCD be provided?

I think it is the job of the designer to find the minimum fault current at the end of the line (whether it is P-P or P-N or P-E) and select the breaker which guarantees trip the minimum fault current
5 Replies
mapj1
3509 Posts
Yes your colleague is mistaken, in the general case there may be no RCD, so the MCB/fuse is handling all cases, L-N L-E and L1-L2. in such a case the far end Zs is important, as it sets the breaking time  - in an extreme case if the cable is long and thin enough, nothing may trip at all, you just get some very hot cable....

If, and only if, there is an RCD or other earth fault detection, then you may argue that the MCB is only handling the faults of the L-N and L1-L2 kind. It is not the general case, and always with one protective mechanism or another, all credible cases should be considered.
Mike.
broadgage
647 Posts
Yes, both of the "worst cases" need to be considered.
Close to the OCPD the fault current must be determined and shown to be within the breaking capacity of the MCCB or other protective device. This should be whichever is HIGHEST out of phase/earth, phase/neutral, or phase/phase.
It would in most cases be prudent to allow for changes to the supply arrangements that may increase the fault current. For example load growth that requires a larger transformer, or multiple transformers. Or the replacement of local generation by a grid connection.

At the distant end of the circuit, the fault current must be determined and shown to be sufficient to operate the MCCB or other protective device in the desired time. This should be the LOWEST of phase/earth, phase/neutral, or phase/phase. It may be possible to ignore the phase/earth scenario if reliance is placed is placed upon an RCD or upon earth fault protection incorporated in the MCCB for such events.
AJJewsbury
2574 Posts
Generic MCCB or MCBs would not work for earth faults
That's generally only true for TT systems - in TN it's usual for overcurrent devices (MCBs, MCCBs, fuses) to provide earth fault protection too.
   - Andy.
Nick Parker
32 Posts
AJJewsbury:
That's generally only true for TT systems - in TN it's usual for overcurrent devices (MCBs, MCCBs, fuses) to provide earth fault protection too.
   - Andy.

Do you meant to say that in TN systems, the earth fault current is sufficient to operate the generic breakers?

Zoomup
2897 Posts
Nick Parker:
AJJewsbury:
That's generally only true for TT systems - in TN it's usual for overcurrent devices (MCBs, MCCBs, fuses) to provide earth fault protection too.
   - Andy.

Do you meant to say that in TN systems, the earth fault current is sufficient to operate the generic breakers?

It certainly can be, especially in TN-C-S situations. Just draw the circuit and see the earth fault loop path. In B.S. 7671 the earth fault current is defined as resulting from a fault of negligible impedance. The cables to and from the earth fault point will in practice have an impedance of course.


411.4 onwards.
Z.

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