switch-fuse and service-fuse discrimination

[Fash]

This is the final step in a design issue for my house rewire. The situation is that I end up with a CU which can potentially pull enough current to take out the company service fuse. Likelihood of this happening is small, however... I'd like to protect against taking out the service fuse with a 100A switch-fuse unit between the meter and CU. Clearly this is not much help unless the switch-fuse trips out before the service fuse. Simple - install a quicker blowing fuse, problem, I would rather this fuse didn't go before the MCBs in the CU. Solution, find a 100A fuse which will blow quicker than the service fuse but slower than an MCB.

Any suggestions/alternatives?

Fash

[hzatph]

Do you need 100A? Why not install say 80A? I can't comment on the speed of trip but no doubt others can.

[Fash]

I definitely want 100A, the point is that I need overload protection rather than fault current protection as this will be provided by the MCBs/RCBOs.

Fash

[Christian McArdle]

The alternative solution is to buy a supply of fuses that will fit the electricity cutout. Naughty? A little. Effective? Yes.

Christian.

[David Hansen]

On 18 Jul 2006 04:26:28 -0700 someone who may be "Fash" wrote this:-

One can generally have two out of three criteria, but not all three. In this case I think there is no way to have the sort of discrimination you are after. The "electricity board" fuses do not obey different laws of engineering to the ones you can buy.

[Fash]

I already have one! I do have a solution that is 'proper' but I think it's probably overkill and I should get over worrying about it. Solution would be: Cut-out --> Meter --> CU1 with 100A mainswitch and

2x 63A Type D MCBs ---> 2xCU with 100A mainswitch and mix of MCBs and RCBOs.

Resulting effect would be that the Type D 63A MCBs would give overload protection to the service fuse but would not trip ahead of the downstream MCBs. Each of the child CUs would then have overload protection as well.

It just feels a bit ridiculous given that there appears to be nothing wrong with relying on the service fuse and putting in a standard albeit large CU.

The current set-up is awful: Cut-out ---> Meter --> Henley block -->

split to 1. Rewireable Fuse CU, 2. Another Henley block which then splits to 5 switch-fuse units each with no more than two fuse-ways in them, all rewireable. Pretty terrible.

Unless someone comes up with the killer suggestion I'm going with Christian's proposal with the fuses.

Fash

[Fash]

True, but the service fuse is there to protect the company's equipment and has fairly slow tripping characteristics. I accept that I can't get two fuses with the same rating that will discriminate at very high fault currents, but it should be possible at lower overloads which is what I'm talking about.

Fash

[Christian McArdle]

What lower overloads do you anticipate?

How many electric hobs and showers do you have?

Christian.

[Fash]

Realistic overload condition: It's christmas, evening time. The cooker is running all 3 electric ovens (6kW), the lights in all 14 rooms are on (2kW), the wife has decided it's a good time to fire up the kiln (3.5kW) and the underfloor heating is on in the cellar (3.5kW). The washing machine and dishwasher are running (3.5kW), someone is drying their hair (1kW) The TV and all associated gubbins are on (1kW), Granny has decided it's cold in her room and turned on the 3kW fan heater. (We're now up to 23.5kW - 98A). Someone boils the kettle and we're now over our 100A. This assumes that the electric shower has been removed.

OK so it's not that likely it's just that as discussed in another post there will be 300A worth of breakers in the CU and somehow it feels like there should be something between all of that and the service fuse.

I am starting to get over it, particularly whilst racking my brains to think of more things to turn on to take me past 100A. Particularly given that I could probably pull 120A for quite a while without creating a problem for any of the components.

Fash

[Lobster]

What rating is the service fuse I wonder?

I'm asking because recently when we had our meter replaced routinely, the guy said our fuse was not up to modern standards and needed upgrading - I can't remember what ours is, and what he said it should be, but maybe you are in a similar situation and all you need to do is whinge at your supplier and they'll uprate your fuse FOC, end of problem?

David

[Christian McArdle]

That lot is easily dealt with by diversity. It is very unlikely that most of those heating appliances will be on simultaneously as they will largely be thermostatically controlled. The system is designed to cope with short term overloads where by fluke all the thermostats are engaged.

Christian.

[Christian McArdle]

They probably won't allow more than 100A on a single phase supply, which I gather the OP has already got.

Christian.

[The Wanderer]

Err, yes they do. Or to be more accurate, they will tend to have a different fusing characteristic. In very general terms, the Distribution Companies will be using fuses that are capable of passing a sustained overload without undue degradation but will be fast-operating in the event of a short-circuit.

Memory fails me now, but it's all to do with fuse-class ratings. I had a browse but info about it is sparse on the web.

[dcbwhaley]

Even an ordinary BS1361 fuse will carry its rated current indefinately, will sustain a 30% overload for an hour, will trip in one second on a

3 times overload and will trip 'instantaneously' only on a ten times overload.

Service fuses have an even more forgiving characteristic than this so OP has little to worry about. Except his electricity bill.

[David Hansen]

On Tue, 18 Jul 2006 16:59:05 +0100 someone who may be The Wanderer wrote this:-

That is different to other cartridge fuses in some way?

[The Wanderer]

Yes, it's a very long time since I had to know about fuse classes for interviews :-). I think the classes are P, Q & R, BICBW. Each fuse class has different operating characteristics.

[The Wanderer]

Err, yes, that was more or less what I said. The theory is many years behind me, sort of thing that was asked for a job interview.

[John Rumm]

I think you are overworrying!

Remember that a 100A cartridge fuse will supply 150A steady for at least an hour or two without blowing. That ought to deal with any overload scenario you can imagine.

Which the main fuse will happily supply indefinitely. ;-) (although glad I am not paying your bill!)

[meow2222]

as others have said, you dont have a problem to solve. Not many families run the kiln while cooking 6 turkeys simultaneously for christmas. But any that do certainly wont have any need to switch on extra electric heaters, not with most of 100A already heating the house up.

And as also said, you can use >100A anyway

NT

[Andy Wade]

I tend to agree - but in any case the most likely choice for the fuse in the main switch fuse is a BS 88 gG (non-motor rated) type. This will almost certainly operate more quickly under overload than the DNO's BS

1361 Type 2 service fuse.

For the time/current characteristics of the service fuse Google for "LR85" or "KR85".