Right, my turn again.

You have read in BS7671 and heard from your tutors that cable ratings and fuses or breakers are absolute, but they forgot (as BS7671 does too, probably to its shame) that all the “magic numbers” are for 24/7 continuous use. They do not take any account of short-term loads and suggest derating factors based on ambient temperatures of 30 degrees. Whilst this may be reasonable for a large industrial plant, it very rarely applies in domestic properties, and if it did you would not be able to afford it anyway!

Looking at our twin socket as Mike did above, how many people would plug in 2 13A consumption appliances for 24/7? Realistically they might for a short period say 30 minutes to heat a cold room, but as the ambient is then well below 30 degrees the 2.5 mm cable would not get more than somewhat warm, and certainly not melt or catch fire! In fact, I will point out that almost all of you will NEVER have seen a domestic cable that is rather hot to touch, probably only at 50C. Fire cannot occur unless temperatures get to at least 250C, the temperature of a hot oven. Even if you leave the bread in here it will only char, but not burst into flames unless you put too much oil into the mix!

Danger of damage comes very specifically, and this is a long relatively small overload, not for the odd hour, but weeks at a time. This time allows the cable to reach thermal equilibrium with the environment but is insufficient to open the CPD (fuse or breaker) because of its fusing factor. Breakers have lower “fusing factors” than fuses, so,in fact, provide more protection from this situation than fuses.

All of this means that discussions (which we often have here) on the effects of overload are generally misplaced, and lead to erroneous conclusions. A 13A socket strip loaded to 20A will probably pop the plug fuse at some stage, not immediately but perhaps half an hour as the fuse local environment gets hot due to the fuse resistance, and poor ventilation inside the plug, but it will take much longer than this to raise a 1.5mm cable above the 70 degrees rating of the cable, and even if it gets to 80 degrees will not cause any great problem as PVC melts at around 140 degrees C. Even if melted it still cannot start a fire in solid materials, although if you drop cellulose thinner on it a fire might result! This would clearly be unwise. Actual melting is very likely to lead to a short circuit and immediate disconnection.

The outcome of all this is to not worry about these minor domestic overload problems, as one can see from the normal use one sees, they rarely cause any trouble at all. Loose connections are the cause of problems, not overload of cables. 

Your life should now be changed for the better because not understanding this subject is often the cause of worry. It is not well understood by the average electrician, or most college tutors I have met. If you want to look further you may get into the complex study of thermodynamics and more physics, but at least you should now see that there is much more to “overload” and that the TIME characteristic is very important indeed.

Right, my turn again.

You have read in BS7671 and heard from your tutors that cable ratings and fuses or breakers are absolute, but they forgot (as BS7671 does too, probably to its shame) that all the “magic numbers” are for 24/7 continuous use. They do not take any account of short-term loads and suggest derating factors based on ambient temperatures of 30 degrees. Whilst this may be reasonable for a large industrial plant, it very rarely applies in domestic properties, and if it did you would not be able to afford it anyway!

Looking at our twin socket as Mike did above, how many people would plug in 2 13A consumption appliances for 24/7? Realistically they might for a short period say 30 minutes to heat a cold room, but as the ambient is then well below 30 degrees the 2.5 mm cable would not get more than somewhat warm, and certainly not melt or catch fire! In fact, I will point out that almost all of you will NEVER have seen a domestic cable that is rather hot to touch, probably only at 50C. Fire cannot occur unless temperatures get to at least 250C, the temperature of a hot oven. Even if you leave the bread in here it will only char, but not burst into flames unless you put too much oil into the mix!

Danger of damage comes very specifically, and this is a long relatively small overload, not for the odd hour, but weeks at a time. This time allows the cable to reach thermal equilibrium with the environment but is insufficient to open the CPD (fuse or breaker) because of its fusing factor. Breakers have lower “fusing factors” than fuses, so,in fact, provide more protection from this situation than fuses.

All of this means that discussions (which we often have here) on the effects of overload are generally misplaced, and lead to erroneous conclusions. A 13A socket strip loaded to 20A will probably pop the plug fuse at some stage, not immediately but perhaps half an hour as the fuse local environment gets hot due to the fuse resistance, and poor ventilation inside the plug, but it will take much longer than this to raise a 1.5mm cable above the 70 degrees rating of the cable, and even if it gets to 80 degrees will not cause any great problem as PVC melts at around 140 degrees C. Even if melted it still cannot start a fire in solid materials, although if you drop cellulose thinner on it a fire might result! This would clearly be unwise. Actual melting is very likely to lead to a short circuit and immediate disconnection.

The outcome of all this is to not worry about these minor domestic overload problems, as one can see from the normal use one sees, they rarely cause any trouble at all. Loose connections are the cause of problems, not overload of cables. 

Your life should now be changed for the better because not understanding this subject is often the cause of worry. It is not well understood by the average electrician, or most college tutors I have met. If you want to look further you may get into the complex study of thermodynamics and more physics, but at least you should now see that there is much more to “overload” and that the TIME characteristic is very important indeed.