house smoke alarm false warning

[Dave Liquorice]

There will be protection but it will be several hundred amps per phase and good old fuse not a fast acting MCB. You appear to be making wrong assumptions about how power is distributed. As Mr Rumm says in most cases several properties will share the same 230v feed with them evenly spread across the three phases. Great use of diversity is made, each individual house may have a "100A supply", with the service cutout rated at 100A but most of the time they will only be taking 5A or so (1kW), big loads like showers will only be on for (relatively) short times and not at the same time as the other houses. Being rural we have our "own" 11kV to 230v transformer up a pole outside, the fuse on that pole is 200A (or was they recently refurbished the stays and fuse and it's now 20' from the ground instead of the nice handy eight).

current as the substation could put through the wire feeding the house.

Like most fuses a 500A fuse takes a considerable overload (think several times it's rating to blow in anything under a minute. An temporary fault type short on the short on the end of a customers will not be able to sustain the power disipation required (>>100kW) without exploding and reducing the fault current. The substation fuse is to protect the distribution cables from gross faults not individual supplies.

One can't help wondering how the ends of a service become disconnected from the cut out and touch each other without external influence...

[The Natural Philosopher]

Odd. Ive got one :-)

[The Natural Philosopher]

AFAICT the short happened before that.

[Lieutenant Scott]

Oh I dunno, changing a lightbulb?

Besides, electrocution isn't all it's made out to be. I've had loads of shocks off the mains, never given me anything more than a fright and a warm hand. However I do know someone who fell off a ladder because he replaced a light when the circuit wasn't off like he thought it was. RCD didn't trip, "normal" breaker didn't trip. but do pose a significant safety risk when interrupted so having

Safety risk?!? Do you have circular saws in your living room or something? Darkness isn't the end of the world.

Which is why I'd never fit an RCD to my house.

You sound like you're speaking from experience!

[Lieutenant Scott]

If the fire brigade had had the sense to pull that out, my neighbour wouldn't have lost his roof to the fire. They told me they "weren't allowed to" - not that it was dangerous, but that the electricity board would get him into trouble. If it had ben my house, that fuse would have been out quick smart.

current as the substation could put through the wire feeding the house.

Why isn't there a 100 amp fuse at the point where the wire is divided to one property?

OH and actually, in his case, the wire to his house was overhead, and went straight to the transformer pole.

The wind caused it. It was an overhead cable, which incidentally I had pointed out to the guy 4 months previously had a rather loose bracket. His response was "that's the electricity board's problem". It wasn't, it was his insurance company's problem when they had to pay for the entire house to be rebuilt.

[Lieutenant Scott]

Why do people write "troll" when they don't believe something? I am telling the absolute truth - the thing came off it's mountings on a windy night because the brcaket had become detached from the eaves of his house.

[Lieutenant Scott]

At 100 amps? Away from your house?

[Lieutenant Scott]

At the junction box where it comes from the big ass cable under the pavement to go towards your property.

The short was at the point where the cable entered his roof. Before the main

100A fuse in his house.

[charles]

There isn't such a junction box. All the wiring is buried under the road.

[Lieutenant Scott]

At some point the wire going to your house must seperate from the bigass one which services several houses. Are you saying they just twist the ends together?

[charles]

I suspect it's a properly soldered joint.

[Dave Liquorice]

So he has this "meter fuse" mounted at the eaves of his house? I bet the meter readers love that. B-)

Inconsistency of story = troll.

[Bob Eager]

Yes, there is. It's under the road.

[Lieutenant Scott]

And at this point, adding a fuse would not be a problem.

[Lieutenant Scott]

No, it's my neighbour where I used to live, not my own house. Therefore I only know what I've seen and what he's told me. Why would anyone go to the bother of creating a fictitious story with that much detail?

I did his gardening for him when I was a teenager, and when doing his hedge noticed that the cable was loose - the bracket had come off and the wire was supporting itself. I told him he should get it checked out, but he said it's the electricity board's problem. About 4 months later there was the fire. I have no idea where his meter was, I assume the big feed wire came downstairs inside the house. The point of the fire starting, according to the fire brigade, was in the attic - consistent with the mess I saw of the roof after they'd put it out. Perhaps the wire was severed at the point of entry to the roof, or there was a joiner up there. Whatever happened, the phase and the neutral/earth must have got joined together.

[The Natural Philosopher]

Yup. I have a personal substation in the corner of the garden :-)

Used to be 60A but it kept blowing, so they put in 100A and that blew the cable as well..where the digger had hit it.

[Lieutenant Scott]

It shouldn't blow apart from to protect buildings from extreme shorts like the one I mentioned. So it would probably never have to be accessed.

Having said that, my parents' one needed access. The tarmac over the top of it was not substantial enough when a heavy lorry parked on the pavement above it. The tarmac sunk. The council simply added more tarmac. When a lorry did the same thing again, the power was cut off. I suppose they're lucky it didn't cause a fire, although probably underground it wouldn't have come to much - unless their house had managed to receive two phases instead of one - I've seen the effects of that and it ain't nice.

[Lieutenant Scott]

I'm not so sure your property has been well made.

[John Rumm]

Not much of a risk usually. A typical bayonet connector only has a small live surface you can touch, and not in a way you can grab hold of it easily.

Much depends on how good your connection to earth is at the time of the shock.

Until recently, the use of RCDs on lighting circuits was uncommon. So there is a fair chance that it was not even supplying that circuit. A 6A MCB will need 30A to trip "instantly", and there is little chance of passing that through a body at 240V, hence it offers no protection from direct contact at all (and is not supposed to either)

Depends on what you are doing when it occurs. Fewer than 20 people will be killed by their fixed wiring in any give year, however thousands will die as a result of trips and falls in their own home.

You would be monumentally stupid not to, since should you ever be exposed to a life threatening shock, there is a fair chance that one would save you.

However, they also need to be used sensibly. The days of a single a RCD protecting a whole installation are long gone. Any sensible design will use multiple RCDs with circuits grouped sensibly. Better still a whole RCBO installation that has one RCD per circuit is becoming cost feasible these days.

I am, I have mains powered interlinked alarms with internal rechargeable batteries. When I first installed them, and had not completed the mains connection, they ran for several months from their batteries.

[charles]

except that it's burried under the road with no access cover. Different in towns where the join is probably under a pavement.