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 ?

 

[brianmoooore]

Q

only main fuse I have seen blown is where they had a 60amp fuse and had installed own ev charger, coupled with 2 electric showers, range cooker and induction hob. one night it went pop as they were busy getting ready for a family get together, lots of cooking, washing etc.. I was called to sort out the charger only.
the funny thing was the dno said that had they contacted them they would have changed the cut out and given them an 80 amp fuse free of charge but instead relied on diy dave down the road who did it all cheap and watched a few videos to fit car charger themselves. never seen a 100amp fuse blow

I've had quite a few DNO fuses blow amongst my customers. The rural area I'm in was 'electrified' in the mid '60s, and all the farmers, who up to then had been managing with 2.5 or 3.5 kVA generator sets, thought that a 15kVA supply would be more than enough to satisfy all their future needs.
Forwards a couple of decades or so, and many of them were running 100 cow dairy herds and associated equipment off of this same supply, as well as large farmhouses, and popped DNO fuses were a regular occurrence, along with the odd meter board burn up.
I still have one customer, currently installing a £1 million+, 500 cow dairy set up, that I've only just manged to persuade that he needs to upgrade from 15kVA single phase.

 

[Lister1987]

Q

I've had quite a few DNO fuses blow amongst my customers. The rural area I'm in was 'electrified' in the mid '60s, and all the farmers, who up to then had been managing with 2.5 or 3.5 kVA generator sets, thought that a 15kVA supply would be more than enough to satisfy all their future needs.
Forwards a couple of decades or so, and many of them were running 100 cow dairy herds and associated equipment off of this same supply, as well as large farmhouses, and popped DNO fuses were a regular occurrence, along with the odd meter board burn up.
I still have one customer, currently installing a £1 million+, 500 cow dairy set up, that I've only just manged to persuade that he needs to upgrade from 15kVA single phase.

I suspect part of the issue would be NFU and the like not advising farmers properly in terms of what is actually required onges of power to run large scale operations.

 

[brianmoooore]

More to do with how tight farmers are!

 

[Lucien Nunes]

This actually reinforces the original point. If you can just about run not only a farmhouse but also a farm c/w milking plant, with only 60A total, then 100A ought to serve most domestic requirements.

 

[brianmoooore]

This actually reinforces the original point. If you can just about run not only a farmhouse but also a farm c/w milking plant, with only 60A total, then 100A ought to serve most domestic requirements.

I had one a couple of years ago where I measured 170A peak on the supply to the milking parlour only.
This one was interesting, because the DNO cut out was a large one, marked 300A, but had been running happily for years through a 100A rated old type meter. Meter had been replaced by a modern one which burnt out in weeks. Meter replaced again, and then the DNO fuse started blowing.
What had happened, of course, was that the DNO fuse had been 200A for years (seal fairy confirmed this), and a good old 'quality' meter could happily live with a 70+% overload, unlike the modern junk that replaced it. Simple solution for the supplier was to downrate the DNO fuse to 100A and walk away. They knew the fuse would likely blow, but their meter would survive, so not their problem.
This all led to months of wrangling, denials from the DNO that the supply was rated at anything more than 15kVA, in spite of the customer producing a 40 year old receipt for "upgrading of electricity supply", but unfortunately not the actual invoice.
The eventual 'solution' from the DNO was to fit a three phase head, with two of the fuse holders paralleled with the incoming single phase supply, fitted with two 100A fuses, a second meter, and to feed the dairy from one and everything else, including the all electric farmhouse, from the other.
So far, so good, but I fear it's only a matter of time.....

 

[snowhead]

So far, so good, but I fear it's only a matter of time.....

Instead of straw and Camel, it'll be Cow and grass

It'll be that 1 cow that ate a bit too much grass and produced an extra 1/2 litre of milk causing the milk unit to run for that extra few minutes.

 

[pc1966]

Even with an intermediate fuse between the main fuse and consumer unit there is no guarantee one will blow before the other

With BS88 series fuses you get selectivity at ratios of 1.6:1 or more. So if the incomer is a 100A fuse then next one has to be 63A (or less) to achieve selectivity.

If you mix fuses and MCB (or MCCB) down stream it gets far more complicated and typically you get 3 regions:

• Current below MCB rating = all happy
• Above MCB rating, but fault let-through (I2t) less than pre-arcing of the fuse = MCB trips, fuse survives.
• Fault current such that fuse pre-arcing I2t exceeded and both fuse and MCB go.

 

[Ziggie]

I have a 80 amp RCD after the meter fitted by an electrician.
Unfortunately the type used does not allow for differentiation from other RCD in the house.

my CU is old wylex so at best if one of my house RCDs triggers so does my main RCD but at least I can withdraw the fuses from the CU , reset the RCD and install the fuses one at a time to identify the circuit at fault.

 

[littlespark]

I have a 80 amp RCD after the meter fitted by an electrician.
Unfortunately the type used does not allow for differentiation from other RCD in the house.

my CU is old wylex so at best if one of my house RCDs triggers so does my main RCD but at least I can withdraw the fuses from the CU , reset the RCD and install the fuses one at a time to identify the circuit at fault.

The rcd won’t trip at 80A, would need to be an rcbo.
Cant you remove that one if there’s rcd’s in the CU?

 

[Ziggie]

The rcd won’t trip at 80A, would need to be an rcbo.
Cant you remove that one if there’s rcd’s in the CU?

No RCDs in the CU. I fitted them on circuits that serviced the bathroom and outside sockets.

 

[littlespark]

gotcha, @Ziggie
and i wasnt sure if you implied the 80A RCD was acting as an overcurrent device or not.

 

[rogerdean]

Hi all. Reassuring that this is an uncommon issue. Here's my use case. I have 2 CUs with total 180A on their main switches on a 100A DNO fuse. I load shift as much as I can into a 6 hour window on Intelligent Octopus.
That means 10kw of storage heaters, 7kw car charger, 3kw immersion, 3kw battery charger. 23kw already. Add to that the washing machine, tumble dryer, dishwasher, and then heaven forbid someone makes toast at night!
So in deep winter I could run somewhat over 24kw for several hours. I've not done this yet. But this, surely, would blow the fuse, right?
Thanks in advance

 

[OLDBOY]

24kw is barely over 100a, it'll take another 20% or more for brief periods. I doubt very much you ever get anywhere near 100a. Dont forget your storage heaters only operate at night.
A 100a service head will inevitably show heat damage if it has been subjected to maximum loads constanty for long periods , they are not designed for this. I maintain a large property with extensive electric UFH, a large electric AGA, electric water heating, and a kitchen with every coceivable appliance. It has been running on a 60a head for decades.

 

[rogerdean]

Thanks for that. Yes the point is that everything I listed is on during the same 6h cheap electricity window. So I think I risk running over 100A for some hours at a time.
I suppose I need to look at pricing up 3 phase...

 

[DefyG]

A cheaper option than have a 3ph supply installed might be installing a load shedding relay that will isolate heavy loads if total gets near 100A.