Main fuse protection

[Bagendman]

In our house the main fuse is rated at 100A maximum. The individual circuits in the consumer unit, if all used simultanuously, add up to much more than 100A. Now I understand this is common practice as it is assumed that not all circuits will be used to their max at the same time (the diversity principle). The question is though, what if they are ? For example, there could be a shower (10KW=42A), cooker (30A if all hobs and both ovens on at once), 4 electric heaters (each 2KW) =30A total) + lighting, Water heater 13A, TV, fridge etc (say 3A). This easily exceeds the 100A. Although, I understand the diversity principle, it seems to me that the liklehood of this happening is not so remote.

So if we did exceed the 100A, I assume the main supply fuse would blow. Does this mean that the power company would have to be called out to replace the fuse or do they reset themselves automatically? Calling out the power company would likely take a while, so we'd be left with no power. Why is it that there is not a quick reset fuse inside the consumer unit, to make sure that the maximum power taken never exceeds the main supply fuse rating? This would seem to be a simple thing which all houses should have. Does anyone know why this does not exist ?

 

[telectrix]

a switch fuse can be added after the meter, usually 80A. this would (hopefully) blow before the DNO fuse and be your responsibility to replace. otherwise, never known a 100A fuse to blow unless there's a serious fault. remember that a BS1361 fuse can handle 1.45 x rated value for up to 1 hour. so that's 145A for quite a while. time enough for some of the heavy loads to be off.

 

[Dustydazzler]

In 25 years I have only ever come across one blown cut out fuse and that was when a builder cut through the meter tails with an angle grinder that were buried in the shallow plaster.

 

[7029 dave]

I have never seen any cut out fuse blow due to over current.

 

[snowhead]

Under normal conditions it's extemely unlikely a house would be able to create a 100a + load for long enough to blow the Main (DNO ) fuse.

If it had been a national problem over the many many years of electrical supply to millions of homes then something would have already been done about it.

If a house could regularly create a 100a + (or exceed the main fuse rating) load then the occupants would need to evacuate the house due to the excess heat created and find 2nd or 3rd jobs to pay for the electricity bill.

It's about the same likelyhood as all the cars registered in the U.K turning out on the road at the same time

 

[brianmoooore]

Basic mistake being made by the OP is that a 100A fuse (more likely 80A) blows at slightly over 100A.

 

[Lucien Nunes]

It is extremely rare for a domestic load with only one electric shower to reach 100A, and especially to remain at 100A for a long time. That amount of heat dissipated within an average house would soon become uncomfortable (noting that water heating can store the heat or remove it in wastewater). OTOH a 100A BS1361 fuse will carry 300A for around a minute, dealing with those brief odd coincidences where everything fires up at once.

I have proven that my 4-bed house (partly GCH, gas cooking) will run on a 30A fuse, including normal heating loads e.g. diswasher, washing machine, tumble dryer, two electric heaters, 2kW incandescent feature lighting (!), used indiscriminately.

The entire street of 70+ houses runs on three 315A fuses.

 

[Megawatt]

a switch fuse can be added after the meter, usually 80A. this would (hopefully) blow before the DNO fuse and be your responsibility to replace. otherwise, never known a 100A fuse to blow unless there's a serious fault. remember that a BS1361 fuse can handle 1.45 x rated value for up to 1 hour. so that's 145A for quite a while. time enough for some of the heavy loads to be off.

.
So many people add up all the breakers and realize that the math totals more than the main fuse or breaker. First off you have what’s called a calculated load which is the breaker amperage of the breakers, then you have what is your actual connected load. If you turned everything on you still wouldn’t pull 100 amps

 

[snowhead]

In the U.K the DNO allow around 7 or 8 amps or so per house. (I did have the guidance list somewhere once)
If they allowed 100amp per house cables sizes in the network would need to be increased dramatically and we'd need hundreds of Nuclear power stations to back it all up, or hundreds of millions of wind turbines.

 

[brianmoooore]

A few years ago I inherited two 2 bedroom fully electric cottages, each supplied by a 45A fused sub main. They were like this for 35 years with no problems.