Unfused spur.

davezawadi (David Stone):

No Andy, it is not that you must assume the energy let through is the maximum, it cannot be unless the fault current is very high. The B32 for instance characteristics in the Regs shows a vertical line on the graph at 600A fault current, which according to the graph is between 10ms and 5 seconds. This is the instantaneous region, and the worst case is your energy let through. You are assuming the worst case can occur anywhere in the characteristic, but this is not the case. You will see the note on the graph, which says the worst case is at the high end of fault current. You do not and cannot have this, so the let-through must be less. Instantaneous operation is usually considered 10ms, not you see an unreasonably quick figure. Therefore the energy let through is I2t Watts, so the current is the important factor.

I read the graph at 160A down to 100ms - but that's an aside. I agree the energy let-though will be less than 18,000A²s, but my point is that in order to show compliance we need to actually quantify how much less - specifically that it'll be no higher than 13,225 A²s to suit 1mm² Cu in a PVC cable. I still think we don't have the MCB data to do that.


If from Z's latest posting we can deduce that the fault current will be below 575A (but above 250mA) then we can use the 40ms disconnection time of the 30mA RCD and that should be fine.


   - Andy.

davezawadi (David Stone):

No Andy, it is not that you must assume the energy let through is the maximum, it cannot be unless the fault current is very high. The B32 for instance characteristics in the Regs shows a vertical line on the graph at 600A fault current, which according to the graph is between 10ms and 5 seconds. This is the instantaneous region, and the worst case is your energy let through. You are assuming the worst case can occur anywhere in the characteristic, but this is not the case. You will see the note on the graph, which says the worst case is at the high end of fault current. You do not and cannot have this, so the let-through must be less. Instantaneous operation is usually considered 10ms, not you see an unreasonably quick figure. Therefore the energy let through is I2t Watts, so the current is the important factor.

I read the graph at 160A down to 100ms - but that's an aside. I agree the energy let-though will be less than 18,000A²s, but my point is that in order to show compliance we need to actually quantify how much less - specifically that it'll be no higher than 13,225 A²s to suit 1mm² Cu in a PVC cable. I still think we don't have the MCB data to do that.


If from Z's latest posting we can deduce that the fault current will be below 575A (but above 250mA) then we can use the 40ms disconnection time of the 30mA RCD and that should be fine.


   - Andy.