UK friends, help me understand

[winterbadger]

A discussion on my UK editors list has degenerated (as the good threads usually do :-) into something totally unrelated to work, namely electrical wiring.

Specifically, and this has me completely confused, the assertion by several members that they've been told by electricians that to comply with the new ("new" = four years old) electrical regulation called Part P, all the fixed lights in their house have to be on a single circuit (and all the plugs have to be on a single separate circuit). Looking up Part P myself, I find that it's suggested model plan does nothing of the sort; it puts all fixed lights on each *floor* on one circuit and all plugs on each floor on one circuit. Now, that still seems like overkill to me; why not do what we do here in the US, and have them circuited room by room? That way if the lights go out in the kitchen, I don't trip over everything in the dark on my way to the fuse box.

Which brings up the part that REALLY confuses me. All these folks are claiming that every time one of the lightbulbs blows, the CIRCUIT BREAKER TRIPS! WHAAAAA? Why on earth would the circuit breaker trip when a lightbulb goes *out*. We're not talking explosive decompression or implosion here, just an ordinary incandescent lightbulb has the tungsten coil inside fail and it stops generating light. Why on EARTH would that make the breaker trip, ESPECIALLY if that means every light in the floor (or house, since I don't doubt that these folks are telling the truth, just that what their builders are doing is actually the requirement of the code). This makes NO sense to me.

Can someone wise in the way of volt and amp explain?

Comments

[janewilliams20.livejournal.com]

I can confirm that if one of my lightbulbs blows, the relevant circuit breaker trips, and I have to stand on a chair to switch it on again (why they had to put the fuse box in the top of a cupboard is beyond me).

I can also confirm that in my house (not new, and possibly not conforming to current specs), there are separate circuits for plug sockets and for lights, and for upstairs and for downstairs, plus an extra for the oven (much higher power) and extras for the extension (again, split between lighting and power).
That's already seven different circuits: trying to do that per room rather than per floor would mean a ridiculously large number of switches.

[winterbadger]

I can confirm that if one of my lightbulbs blows, the relevant circuit breaker trips,

Not only does that seem unnecessary to me, I don't even understand how it *works*. The function of a circuit breaker is to prevent too much current from going through a line, either because a surge is coming from the outside or because appliances are drawing more than the line should be carrying. When a light stops working, surely the amount of current being drawn remains the same or drops, not spikes. So I don't even get how the breaker knows to trip.

I can also confirm that in my house (not new, and possibly not conforming to current specs), there are separate circuits for plug sockets and for lights, and for upstairs and for downstairs, plus an extra for the oven (much higher power) and extras for the extension (again, split between lighting and power).

That's already seven different circuits: trying to do that per room rather than per floor would mean a ridiculously large number of switches.

Except that you're comparing apples and oranges. Using the American system you *don't* have separate circuits for lamps and plugs. You may even combine a couple of rooms on one breaker if you don't have a lot of outlets there. Plus of course our rooms are generally larger and fewer than yours, so there's probably a difference in number of rooms overall in the same given living space.

You're also making a value judgement based on your experience. To me it seem ridiculous to only have one circuit for all the lights on one floor, in fact quite dangerous!

[pisica.livejournal.com]

Just don't stick a metal knife in the toaster.

Um, no, of course I didn't do that a little while ago.

[winterbadger]

Well, a plastic knife might melt, so you wouldn't want to use that!

[aitkendrum.livejournal.com]

I thought you had your hair cut!

[kevinrtaylor.livejournal.com]

It's not as simple as that.
Part P regulations require that you comply with the latest edition of the relevant BS standard (currently 17th edition, issued in 2008).
The previous edition insisted that lights were to be on a separate circuit from power sockets, but not that they had to be on the same circuit.
The usual practice is to have one circuit for upstairs lights amd one for downstairs lights, plus at least two circuits for power sockets, plus separate circuits for high-current devices such as electric cookers and immersion heaters.

However, the 17th edition has changed all this.
You can now have lights on a power circuit, as long as the section of circuit powering the lights has a separate switch and fuse between it and the rest of the circuit.

As for why a blowing bulb trips the breaker:
240V bulbs use a long coiled-coil filament.
When this snaps due to a section burning out the free ends lash around inside the bulb and can make contact with another part of the filament, creating a momentary high-current path which trips the breaker.

[silme.livejournal.com]

Oh, good, you've done it, so I don't have to ask Ian. :)

[winterbadger]

And they were the two people I thought would probably know! :-)

[silme.livejournal.com]

Ian's B.Sc is in electronics. :)

[winterbadger]

Thank you! I knew that someone would be able to explain the technicalities tot he lay mind! :-) Much appreciated!

[winterbadger]

The only thing I don't get about the lightbulb thing is this: if the loose wire contacts another part of the filament, is it going to draw *more* current than would go through it normally? If so, how? If not, why trip the breaker.

I realise our circuits in the US are set up for much lower voltage, but I assume the breakers are set up proportionally. We use incandescent bulbs with coil filament--why doesn't the same thing happen here?

[kevinrtaylor.livejournal.com]

If the loose end of the filament makes contact elsewhere, the total length of filament will become shorter, and so the resistance will be lower and the current higher than normal.

Also, because the voltage is higher here the total length of filament required for the same wattage of bulb is higher. At least four times higher if the same thickness of wire is used, as there is a square law involved.

So there is more scope for the filament to wave around and short out.

And even if you do get the same filament behaviour, as your lights are installed on the same circuit as your power sockets the trip switches will be rated at a higher current and may not notice the temporary overload as much.

[winterbadger]

I think I may actually understand that! :-)

Thank you so much for the detailed explanation! Most appreciated.

[peaceful-fox.livejournal.com]

And he told ME it was the electricity fairy. Hrmph.

[winterbadger]

Maybe he's speaking to each of us in what he assesses to be our own ...worldview. :-)

[peaceful-fox.livejournal.com]

You crack me up! Of COURSE it isn't the electricity fairy. It's the electriticy gnome.

Anyone would know THAT! Sheesh.

:-D

[winterbadger]

Like the garden gnomes in The Full Monty? or Alan Ayckbourn's Norman Conquests?