HRC fuse

How would changing to an 80A fuse add value?

John

Yes, BS88-3, 80 and 100amp are the same physical size, but they do come in smaller sizes too, so it will need measuring before you order one. Why not go down another step to 63amp? More chance of the 63 amp blowing, rather than the 100 amp at the source if a large fault develops.

Possibly:

But why would you want to do such a thing?

It reminded me of a discussion I had with somebody who was moaning about the utilitarian designs of consumer units and meters and their limited range of colours. After all, its not really like a bathroom suite is it? Just shove a pretty cover over it all. As for fuse size, one has to ask quite what the point is.If one is worried about consumption, a fuse reduction is not really the way to go! That is unless somebody in the house has a canherbis factory going while you are away on holiday!

Brian

I had assumed (dangerous I know) the OP wanted a smaller fuse on his side of the supply than the electric company's fuse so his would go first.

Am I right in thinking this is a fuse after the meter that you need when the consumer unit is located far away (I think >3m) and so protects the supply in case somebody drills through the buried tails?

In which case I'm not sure what the point of a 100A fuse is, if the cutout fuse is going to blow first?

Theo

It might be fine depending on which one of them blows first. I wouldn't bother swapping it for a lower rated one unless it fails.

An 80A slow blow fuse might easily survive something that takes out a higher rated 100A fast blow fuse first. Most fuses will handle a 50% overload condition almost indefinitely unless they are very old.

Serious faults in mains wiring draw currents in the thousand amp range.

Fuses have very nominal ratings so an ordinary 100A fuse will probably tolerate a 200A load for a couple of minutes before it blows. It will get pretty hot but it won't actually blow at that.

I have seen a 13A fuse on an extension lead unhappily support two 3kW kettles running flat out for an extended period the plug starts to soften and melt after while. There is just enough recovery time for the fuse to cool down a bit whilst they refill the kettles.

The church tea ladies are devils for doing that.

The point is that you're not allowed to rely on the "company fuse" to protect your installation. There's a pragmatic exception to allow less than 3m of tails to be protected by it (otherwise how do you connect your fuse to the supply - the tails to your fuse need a fuse!).

Presumably this is because the distributor could change the fuse - and/or if the distributor wants to change the fuse they don't want to be having to inspect a long sub-main, or worse a branching system of tails and sub- mains. In technical terms it adds nothing, of course, as a second fuse of the same rating gives no extra protection. It's all about enforcing the boundary between the distribution network and the installation.

Mike

But a 100A fuse behind a 80A cutout fuse explicitly *is* using the company fuse as the first level of protection. The 80A is likely to blow first.

Although perhaps the fuse was put in without pulling the company fuse, and it was assumed to be 100A.

Agreed that it makes sense that the cutout protects the company wiring, the second fuse protects the installation's wiring. But not in this particular case.

Theo

I think that is a misconception. The usual reason a switchfuse is installed is to provide fault protection to an extended run of tails from the cutout fuse. To a large extent that makes the actual nominal fusing current irrelevant, since under fault conditions it would be common to have >1000A flowing.

This is in much the same way that a 13A fuse in a plug can give fault protection to a 3A flex, or a 32A MCB can give fault protection to a spur cable with a installed rating of only 21A etc.

It could, although I can't see any real benefit in doing so unless the tails feeding it are <= 16mm^2

Its main job is fault protection not overload protection, so the value is not that important. Under fault conditions either or both could blow depending on the magnitude of the fault current.

UK power ( I think that is what they call themselves) changed mine to a

I’d had an EV charger installed and the installer recommended upgrading the fuse.

They changed mine to 100A in order than the short in the cable would be revealed by it burning out, which it duly did.

We then got the digger driver who had sliced through it to dig it up again and the chaps spliced in a new bit... And left the 100A fuse in place.

Actually I do. My earth loop is 0.00 and PFC is greater than 12kA.

I have 6kA breaking MCBs

But I'd have thought that the lower rated fuse would heat up faster and tend to blow first? Not guaranteed of course, but supposing the 80A fuse was

I managed to miswire live to earth in some equipment the other day: it was protected by a 6A MCB in the enclosure, then a plug fuse, a 10A fuse in the extension lead plug, then an RCBO in the CU. Three of them tripped (the first plug fuse remained), which was interesting in that I might have expected one of them to go first. But maybe the thermal/magnetic effects had enough momentum to keep them following through even after the first disconnect?

Theo

Would mean 16mm2 cable could be used for circuits rather than 25mm2, that was the consideration for me.

The Switch disconnector was fitted to comply with 2 requirements ..... the ability to isolate the build during construction and the need to have a switch within 3m of the meter stated requirement for meter tails not being more than 3 m form a point of disconnection. Being 100A when Supply fuse was 60A made fuse part pointless I agree.

That is exactly why it was installed.

Thanks for link