UK HOUSE WIRING Question

On Wed, 5 Oct 2005 10:21:28 +0100, "Bob Watkinson"

indeed. If the circuit is all perfect though then the neutral *should* be at the same potential as ground, so no current should flow. However since the neutral is definitely a current carrying conductor you shouldn't ever touch it in the first place, just in case.
C
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Charlie wrote:

Arggh, whenever people post wikipedia links I always waste hours clicking on hyperlinks and end up learning bizarre things.
On that topic, I have a theory that in order to create the ultimate AI computer, all that is needed it to write a program that allows the machine to cross reference unknown words in text files. Then simply feed the entire contents of the OED into it and let it site there for a bit while it cross references every word in it. It will then be able to reccursively cross-reference every word and gain total and utter comprehension of the English language.
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wrote:

30 years ago that's what we thought. Then computers started to get fast enough to do a very small amount of that task and we realised that each word didn't need simple hooks to other words but a whole word-specific program of their own. AI (true AI) is now further away than it was in the late seventies. ;-(
I don't know how she worked it out - or how much it was deliberate - but my 14 month old granddaughter calls our small dogs 'dog' and her own (much larger) dog 'dog-dog'. One of us swears that, on seeing a horse, she called it 'dog-dog-dog'. Now tell me how you'd program a computer to produce the possible grammar behind that!
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Sadly not. If I say "I went up the river in a canoe", is the canoe in the river or is the river in the canoe? The only way to know which way it is, is to "know" what rivers and canoes are.

W-e-e-e-e-e-e-llll. Have a look at the CYC project, now at http://www.opencyc.org /

I'm certain this is common in some real-world languages. Sadly, my machine translation days are long behind me, so I can't recall which ones!
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Thanks for that. I'm *very* rusty with my AI reading but I know when I start I'll want to contribute - and that will be a full-time project! When I retire ...
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John Cartmell john@ followed by finnybank.com 0845 006 8822
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John Cartmell wrote:

I suppose the problem is that even if it could cross reference all the words, it still wouldn't actually know what any of them mean.

You'd have to create an algorithm that took factors such as the dimensions and mass of an animal and output the word dog x number of times as a result!
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wrote:

That's easy enough. The problem is how the easily-communicated idea developed in the first place. No one has taught her the concept of multiple words bigger and, at the stage of hardly even having a communicable vocabulary (whilst she talks plenty I don't pretend to understand any of it - except to her!) she appears to have generated a grammar of her own. This isn't a new observation: very young children brought up in a family using (deaf) sign language will probably acquire a formal, slow, limited, grammar but - where able to converse with other, similar kids at a young enough age, develop a natural fast, open-ended grammar of their own making. <this is uk.d-i-y.grammar isnt it?> Organic life is far more proficient than any artificial equivalent that we can (yet) design.
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AC is usually regarded as not having positive and negative.

You can get electrocuted by holding any of the wires in a mains installation (even if its only a fault that does it). Treat all wires as though they could be live and you will survive for longer.

Not strictly true as some circuits don't need wire to complete.

Its the *most important* wire. Its there to stop some common faults killing you.
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Yep
You shouldnt do, becasue back at the electrcity sub station the nuteral is connected to the earth (and in the brown stuff outside) so everything is connected to nutral all the time.

The ground, or earth wire is only there for protection, if there was a fault in an appliance where the live or the neutral wire somehow came into contact with the metal casing of something, the current will travel down this wire instead of down the neutral wire. Now, if the wire that has become connected to the metal casing of the appliance is the Live, this is effectively shorting the live to the neutral, causing a lot of current to flow (because neutral is connected to the earth) so your fuse or little MCB (Circuit breaker) will blow/trip.
Also, if you have an RCD, this checks the current going out of the Live, and returning back via the Neutral, if there is an imbalance (because some is going to earth, maybe through a person, or the earth wire etc.) then the RCD trips.
Sparks...
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Sparks
if the live wire comes loose and touches the metal case of my toaster then the current flows from the generator down the live wire , through the metal case, through the earth wire to ground, through the ground back to the generator where it goes up the earth wire there to neutral ?
when the neutral wire touches the metal case is the circuit simply totally grounded with the neutral wire at the same potential as the ground to which its connected ?
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That's right - but as there isnt much resistance in this circuit, lots and lots of current flows (as it is effectivly a short circuit) so the fuse or circuit breaker will brlow/trip

yes - if you dont have an RCD, then this fault will remain undetected, but as long as the earth is all connected properly, then the toaster will still finction and you shouldnt get a shock.
If you did not have an RCD, and you connected somthing to Live and Earth, the appliace should work and nothing should blow or trip
If the circuit has an RCD, then some of the current will return via the nutral wire, but some will return through the earth. The RCD will see that there is an inbalance between the current flowing from the live and returning via the nutral (It doesnt care where it has gone, it just knows it is missing!), and it will trip. Sockets will usually have a 30mA RCD - this will trip if the inbalanace is 30mA or greater.
This means, if there was a fault with your toaster where either the live or the neutral was in contact with the metal casing, but the earth wire was not connected, if you touch the metal casing, some current will be conducted through you to earth - the RCD will spot this missing current, and trip, hopefully saving your life :-)
Sparks...
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or
not
Just curious. How safe is an RCD. If I stood bare footed on a damp concrete floor and grabbed the Live wire, would it still be a nasty shock but not kill me, or does it switch off so quick that I would not even notice it.
Simon
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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'.
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live
was
conducted
trip,
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
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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...
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Simon wrote:

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:
http://www.tlc-direct.co.uk/Products/EMWP3NM.html
or if you want to use it as an ordinary light controlled from a switch as well then:
http://www.tlc-direct.co.uk/Products/EMWP3M.html
See also:
http://www.tlc-direct.co.uk/Technical/DataSheets/EmergencyLighting/Emergency_Lighting_Guide.pdf
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Cheers,

John.

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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
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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.
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Thats exactly what I have - and have never managed to trip the lights!
Sparks...
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Simon wrote:

Nothing wrong with multiple RCDs. If you are on a TT install then that is the way to go anyway. The ultimate solution you could argue would be a RCBO for each circuit that needs one.
The argument against RCDs, is the problems of unwanted trips as a result of "normal" leakage (some types of appliance are prone to it because they have mineral insulated heating elements, or capacitive filters on their mains inputs). For circuits that present very little real risk of electrocution the negatives may outweigh the positives.

Why not add a non maintained emergency light then? About 20 from TLC. Wire it to a feed on the lighting circuit in there. That will keep it charged. When the power goes off it lights up.
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John.

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