UK HOUSE WIRING Question

[Bob Eager]

No, it doesn't.

[AlexW]

I thought it did actually as Electrons flow from -ve to +ve IIRC, or the current flows from +ve to -ve in conventional notation.

The live is always energised though with either a +ve or -ve potential wrt the neutral.

Alex.

[Bob Watkinson]

Assuming the RCD is working correctly it would trip between 10 and 20 milliseconds while carrying 30mA (depending on type). To quick for you to feel it I would think but as they say 'Don't try this at home folks'.

[Sparks]

It depends on your heart really, if you were in good health, then I expect you would be OK, but dont try it!! You would feel it for sure. The RCD should trip extremly fast (I think it has to be at most .04 of a second)

Sparks...

[Bob Mannix]

You fell into his little trap! It changes direction 100 times per second, having a burst in both directions 50 times a second.

[Lurch]

On 5 Oct 2005 01:21:04 -0700, "blackboab" scrawled:

No, it's live and neutral you're after. Positive and negative are for DC. I'd forget about the swapping over bit, it sort of isn't relevant.

Yes

Technically.

There for protection, as others have explained.

[Rob Morley]

It changes direction twice on each cycle.

[Simon]

Thanks. Quite a useful thing to have then. Why don't they build one into the electricity meter and then all homes would automatically have one? An even better design would be to have a small light on the meter that came on when the RCD trips, then you would be able to see to turn it back on again.

Simon

[AlexW]

Trap?

It still changes 50 times second though for the UK supply, the limits would be a minimum of 99 to 101 times. AC current could actually change at any frequency.

[AlexW]

Yes.

[Bob Mannix]

So that would be 100, not 50

Well so it could but it *could* also be a 330,000 V AC supply, in which case I wouldn't touch the neutral! The context of the question implied a UK domestic supply of 24V 50Hz, where the current changes direction 100 times per second and the neutral isn't that dangerous.

[Sparks]

This isn't done because a whole house RCD is not a good idea...

Say you were working in your workshop at night and you tripped it, what would happen? (All the lights would go out too!)

Sparks...

[John Rumm]

On a modern install they almost do. The RCD will usually be present in the consumer unit where it protects *some* of the circuits (typically socket circuits where there is a good chance that the socket may be used to power and appliance used outside, or where there is another particular risk factor)

People used to install RCDs such that they protected the whole house (i.e. in the meter tails that fed the CU). The danger here is that you a swapping one hazard for another. You reduce the risk of death by electrocution, but *increase* the risk of death by trip or fall. (e.g. you are drilling into the wall at the top of the stairs on a ladder - hit a cable in the wall, and trip the whole house RCD which plunges you into darkness!).

(Note that installs with what is known as TT earthing (commonly found with overhead power wires) are usually covered by different requirements)

Since many many more deaths occur each year as a result of trips and falls than by electrocution, the regs were changed to require better discrimination between faults. Such that completely unrelated circuits (i.e. lights etc) should not be de-energised when a fault causes a trip on another circuit

This is worth doing anyway. What you require is an "emergency light" (cost about 20 to 30 pounds). These react to loss of power by turning on the lamp powered from internal batteries. I added one to my meter cupboard powered from the downstairs lighting circuit. That way if that MCB trips the light comes on.

Something like:

if you want to use it as an ordinary light controlled from a switch as well then:

also:

[Bob Watkinson]

The thing is you don't want to put everything on an RCD. For example if you had a fault on your lights you wouldn't want the power to be disconnected to your freezer. Better to have stuff like the immersion heater and shower etc on an RCD and other stuff not. That's why we have split CU's

[AlexW]

Yes, it could.

You snipped the context of the (rest of) the question though and the response did not really clarify things for the OP.

Why not assuming its at the same/similar potential as you?

In this context I agree that touching neutral should not be deadly or whatever unless part of you is touching something else at significantly potential to neutral.

Alex

[Bob Mannix]

I responded to the OP question in another posting which was crystal clear. In this posting I was responding to your assertion that "it did actually" change direction 50 times per second (which it doesn't)

Well I would make the assumption (for my own safety) that given a 240V supply might generate stray neutral voltages of up to 30V, a 330kv one

*might* generate neutral voltages of up to 30kv. Of course it might not, as such supplies would be three phase and arranged differently, but I wouldn't take the chance myself!

[Rob Morley]

It does in a way - given that it changes 50 times in half a second it must change 50 times in a second (then another fifty times as well), but I think he's just wriggling.

[AlexW]

Sorry Bob E snipped.

[Simon]

Everyone seems to be saying tripping out the lights is a bad Idea as you may fall over in the dark, so why not have two RCDs, one for lights and one for everything else. I must admit ours is old and protects everything. I have never found it turning the lights off a problem. I have only ever tripped it by accidentally touching the neutral, I never had the nerve to touch the live to see if it hurt. I have always liked the idea that everything in the house was protected so that I have to do something very foolish before I can electrocute myself. Just wish that there was one light in the garage that came on so that I could climb across all the junk in the garage to reset it.

[Sparks]

Thats exactly what I have - and have never managed to trip the lights!

Sparks...