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 ?

 

[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