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.

 

[telectrix]

so that's that one put to bed. now what's all this GS38 nonsense? on my bench. no RCD. i'm still breathing fumes. xcuse the beer focus.

 

[Gavin John Hyde]

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

 

[telectrix]

only once blew the DNO fuse when fitted electric hob. however the fuse was a 30A rewireable from 1926. talk about how things used to last for years back in the day. today nothing lasts much more than the warranty period.

 

[James]

The only time I have. Known a 100A DNO fuse to go.

Large house 3phase supply

Big tent in garden and a party for 300 guests.
It was a family friend so I was lucky enough to have an invite.
As the guests were arriving one phase went down.

I never go anywhere without my tools in the pickup, so I am testing and checking things in the plant room dressed in a tuxedo as the party is starting .

Transferred the tent to standby generator
Re jigged important circuits to remaining phases and left off most upstairs sockets.
Turns out that 20 young ladies can pull over a 100A just using hair driers and other beautification products.

DNO turned up about an hour later, he said that it was rare to change a fuse with no fault to blow it.

 

[Vortigern]

Not only the above but selectivity, meaning what fuse will blow first, is a factor which is surprisingly hard to meet. Even with an intermediate fuse between the main fuse and consumer unit there is no guarantee one will blow before the other, it could still be the main fuse blowing first! However we can be content in the knowledge that each of the circuits in the consumer unit have their own fuse which will in most circumstances blow before the main fuse. That is how selectivity, or discrimination is assured. Also there is diversity, an allowance for the fact that not all circuits will be "revved" up to full capacity at the same time. So for instance you wont be showering while have all the sockets occupied with heaters and all the equipment such as dishwasher, washing machine microwave and cooker on full pelt simultaneously. So we calculate an amount for each circuit accordingly. This translates to around 50% max drawn at the highest load point.

 

[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.

 

[rogerdean]

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.

That's really helpful, thank you! I have set up something like this via my home automation setup - right now if it goes above 22kw then some of the storage heater are turned off. But it's a system that relies on the WiFi behaving so your suggestion is really welcome...

 

[Avo Mk8]

Not to encourage ignoring overload, but the fuse characteristics indicate that a 100A fuse won't blow until sustained current reaches around 130 - 140A. (I guess there's a specific minimum value in the standard, but I don't know what it is!)

 

[rogerdean]

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.

Is there such a thing as a relay controlled by a CT clamp? So a device could be disconnected based on the overall draw? Can't find one but don't know what the right name would be. Thanks again

 

[Mike Johnson]

As DefyG said, it's a load shedding relay, I don't know if they are available in the UK, but in the EU they are available as multiple units that will switch off loads in progression if required to maintain the load below the incoming supply.

 

[nicebutdim]

As DefyG said, it's a load shedding relay, I don't know if they are available in the UK, but in the EU they are available as multiple units that will switch off loads in progression if required to maintain the load below the incoming supply.

Should be Garo units available in the UK. It seems their products have been adopted by quite a few wholesalers since their shower priority board came in for some demand.

 

[rogerdean]

As DefyG said, it's a load shedding relay, I don't know if they are available in the UK, but in the EU they are available as multiple units that will switch off loads in progression if required to maintain the load below the incoming supply.

Thanks Mike, that's what I need. Only things I could find seem just to monitor another single circuit rather than the overall load. A link would be brill if you can... Thanks

 

[rogerdean]

Thanks Mike, that's what I need. Only things I could find seem just to monitor another single circuit rather than the overall load. A link would be brill if you can... Thanks

Thanks for the support folks. Turns out the best way for me is simply to upgrade my EV charger to one that has load management built in, like the Zappi or newer Ohme. Talk about making it complicated for myself eh?

 

[DefyG]

The Schneider A9C15908 is another, not cheap at £500 ish

Available at UK wholesalers (Edmundson & Yesss) but link to;

A9C15908 | Schneider Electric iC60 Series Contactor, 230 V ac Coil, 1-Pole, 90 A, 230 V ac | RS

uk.rs-online.com

 

[Avo Mk8]

Is there such a thing as a relay controlled by a CT clamp? So a device could be disconnected based on the overall draw? Can't find one but don't know what the right name would be. Thanks again

Yes.
Google something like "split core ct current monitoring relay 100A"
This one has the disadvantage you have to feed the tail you're measuring through the hole. You really need a split core version:

https://docs.rs-online.com/5d25/0900766b813e168e.pdf

Came across this, which when set up to the current you want (say 100A) could switch a relay or contactor in part of your installation:

CS-625 AC Current Switch (high output)

Measurement Systems provides CS-625 AC Current Switch (high output) for your business

www.measurementsystems.co.uk

But sounds like you won't need it now