Energy let-through for Type 2 MCB's ?

In the past with 2.5mm2 T&E cables with a 1.0mm2 C.P.C. protected by a 30 Amp bit of fuse wire for a ring, did we really have lots of melted cables or fires caused by that arrangement?

Quote: 

"OK do the calculation for a 2kA or 1.5kA  fault current then if you prefer - it's still likely to be more than would reasonably be expected."

The nice thing about fuses is that they continue to get faster as fault currents increase - so the energy let-through doesn't keep increasing - unlike MCBs where the speed of moving parts puts an upper limit on how fast they can open - so energy let-through tends to increase as fault currents increase. A 30A/32A cartridge fuse is probably perfectly adequate for a 1.0mm² c.p.c - unlike an MCB. BS 3036s will be a little slower so probably fail today's limits, but I suspect not by much.

If so the breakers and cables will be happy with a lower fault current. Or a large one, but not for long eh?

Pretty much yes - too low a fault current and the MCB won't open instantaneously, too high and the energy-let through increases beyond what the cable can withstand - it's only the limited area in between that's useful. Fuses are a lot simpler in that respect, not having an upper limit as it were (at least until you get to their breaking capacity).

   - Andy.

In the past with 2.5mm2 T&E cables with a 1.0mm2 C.P.C. protected by a 30 Amp bit of fuse wire for a ring, did we really have lots of melted cables or fires caused by that arrangement?

Quote: 

"OK do the calculation for a 2kA or 1.5kA  fault current then if you prefer - it's still likely to be more than would reasonably be expected."

The nice thing about fuses is that they continue to get faster as fault currents increase - so the energy let-through doesn't keep increasing - unlike MCBs where the speed of moving parts puts an upper limit on how fast they can open - so energy let-through tends to increase as fault currents increase. A 30A/32A cartridge fuse is probably perfectly adequate for a 1.0mm² c.p.c - unlike an MCB. BS 3036s will be a little slower so probably fail today's limits, but I suspect not by much.

If so the breakers and cables will be happy with a lower fault current. Or a large one, but not for long eh?

Pretty much yes - too low a fault current and the MCB won't open instantaneously, too high and the energy-let through increases beyond what the cable can withstand - it's only the limited area in between that's useful. Fuses are a lot simpler in that respect, not having an upper limit as it were (at least until you get to their breaking capacity).

   - Andy.

Thanks Andy. There must be millions of '70s houses that still have 2.5/1.0mm2 ring final circuit cables installed. Perhaps a percentage of them still have hot wire fuses. I do not think that there is evidence that that arrangement was a real problem. After all it complied with the regs. and British Standards at the time of installation. Also, the M.K. range of M.C.B.s was of very high quality. Further investigation revealed to me that the range was upgraded to an M6 rating.

The thing with the adiabatic equation is that it assumes that all heat is retained in the cable during a fault, and is not lost by conduction, radiation or convection, which may not be the case at all in reality.  So the equation is idealistic. This situation also makes it  err on the safe side.

"I" in the equation refers to the R.M.S. fault current flowing into a fault of negligible impedance. Does such a thing exist in reality? I think that all earth faults will have some resistance/impedance that will limit fault currents.  Also, the disconnection time must not exceed 5 seconds.

I have a gut feeling that the earth fault current with its arcing spitting and popping, will not be a steady fault current, but a varying fault current, rising and dipping with time. So it may not be accurately described as a "bolted fault current."

www.youtube.com/watch

Z.