Overloaded fused spur, opinion please

I have read the rest of this thread and we seem to be back to this ridiculous idea that currents of any duration are exactly the same as far as effects on parts of the circuit are concerned. This is not, and never has been the case. Taking the limiting case, a short circuit trips the CPD, at 50 x In. Has anything been "overloaded"? Is anything damaged by this large current, which exists for a short period? I would expect not, even if the fault was made by means of a switch. People here and probably elsewhere often worry about cable ratings, and exact compliance with the tables in the "Book" to the last amp. This is particulary common when ring circuits are discussed, for example the aparent 21A rating of 2.5mm cable above. In some way these tables would be better re-written with current ratings in Ampere-minutes, or something actually connected to the temperature rise caused by loading. This would make understanding of diversity much easier, and final cable temperature in use better defined. It takes a 2.5mm T&E cable something like 1/2 hour or more to reach a final temperature at the tabulated maximum current, and even longer for large cables like 300mm. If you have a short duration load, like a washing machine and dishwasher which have thermostats and water heaters, one can see that the cable heating is not a problem, even if the current is somewhat high for a short period, say 15 minutes. The motors take much less current and the cable will gradually cool. The danger comes from "long and small" overloads (perhaps 25%), which do not trip the CPD but do heat the cable over a long period, typically many hours. It is often suggested that two electric fires on a ring spur is unsatisfactory, but this is not true, because any reasonable domestic space would become very hot indeed in that half hour, and their thermostats or a person would turn them off. It could be unsatisfactory if the circuit was feeding a lot of computers 24/7, although that would strongly depend on what they were actually doing, they take increased power with increased activity too, just like motors.


I know that I keep on about this subject, the reason is that it is often very badly taught in college, and overload seems to be the one subject everyone remembers as the suggested results may be "interesting"! This is rarely true unless the designer is very stupid, cables themselves are rarely subject to overload failure, even when buried in the street and operated at several times the aparent rating. Cable failure usually is due to mechanical damage or wear-out by water ingress or similar. It can be fairly spectacular with a big cable, and a really big fuse on grided distribution in a large city, particularly central London.

I have read the rest of this thread and we seem to be back to this ridiculous idea that currents of any duration are exactly the same as far as effects on parts of the circuit are concerned. This is not, and never has been the case. Taking the limiting case, a short circuit trips the CPD, at 50 x In. Has anything been "overloaded"? Is anything damaged by this large current, which exists for a short period? I would expect not, even if the fault was made by means of a switch. People here and probably elsewhere often worry about cable ratings, and exact compliance with the tables in the "Book" to the last amp. This is particulary common when ring circuits are discussed, for example the aparent 21A rating of 2.5mm cable above. In some way these tables would be better re-written with current ratings in Ampere-minutes, or something actually connected to the temperature rise caused by loading. This would make understanding of diversity much easier, and final cable temperature in use better defined. It takes a 2.5mm T&E cable something like 1/2 hour or more to reach a final temperature at the tabulated maximum current, and even longer for large cables like 300mm. If you have a short duration load, like a washing machine and dishwasher which have thermostats and water heaters, one can see that the cable heating is not a problem, even if the current is somewhat high for a short period, say 15 minutes. The motors take much less current and the cable will gradually cool. The danger comes from "long and small" overloads (perhaps 25%), which do not trip the CPD but do heat the cable over a long period, typically many hours. It is often suggested that two electric fires on a ring spur is unsatisfactory, but this is not true, because any reasonable domestic space would become very hot indeed in that half hour, and their thermostats or a person would turn them off. It could be unsatisfactory if the circuit was feeding a lot of computers 24/7, although that would strongly depend on what they were actually doing, they take increased power with increased activity too, just like motors.


I know that I keep on about this subject, the reason is that it is often very badly taught in college, and overload seems to be the one subject everyone remembers as the suggested results may be "interesting"! This is rarely true unless the designer is very stupid, cables themselves are rarely subject to overload failure, even when buried in the street and operated at several times the aparent rating. Cable failure usually is due to mechanical damage or wear-out by water ingress or similar. It can be fairly spectacular with a big cable, and a really big fuse on grided distribution in a large city, particularly central London.