"idiot proof" final circuit design

[Dave Plowman (News)]

No, they are in the adjoining cupboard.

Your under sink etc cupboards are all filled to the very top? What with? Not words of wisdom, surely?

[Dave Plowman (News)]

No - that's a spur. ;-)

But you're talking about power radials, and these are normally fused at 20 amps. Fuse one at 6 amps and it's no longer a power circuit.

Lighting circuits aren't rings. The minimum size TW&E - 1mm - is perfectly capable of dealing with a fault on a 6 amp circuit as it's rated at 14 amps.

The reason why rings aren't used is that you can guess more accurately the load, since all lighting outlets will near always have a load. A power ring might have many sockets with no 'permanent' load or just a tiny one, like say cordless phone base station. That's why it's such a versatile system which has stood the test of time.

Room lighting circuits tend to be fairly fixed in that context.

[John Rumm]

Based on the number of times I have had a kitchen appliance blow a fuse that equates to never. I can live with that rate...

[:::Jerry::::]

,

Who said anything about under sink cupboards...

And what if the unit next to the appliance is one full of draws?

[:::Jerry::::]

,

amps -not 30

fused at 20

Well I wouldn't call .75mm flex (2192Y type stuff) flex as a 'power' rated flex, but hay-ho if you want to supply a 3Kw fire off it....

lighting

We are talking about the physical layout of cables.

accurately the

tiny one,

versatile

You are talking about diversity, if you have a ring circuit that you know will be loaded heavily all the time you might have to consider installing more than on final ring circuit - just as happens with lighting circuits.

[PC Paul]

Power nailers have two thigs to do before they fire - the trigger and a spring loaded nose that has to be compressed before it will fire.

US roofers have been known to tape the trigger shut so that it fires every time they dab it on the wood.

Which is fine until one was climbing down a ladder with his nailer hanging by his side and a workmate walked past and got a 6" glued nail attaching his hard hat to his head. I believe he survived though...

[PC Paul]

I've used car jump leads to short out some large mains filters to remove ~6A of noise current on the earth line in a new building (which was doing all the phones in) - does that count?

And yes it did get fixed 'properly' after a few weeks.. ;-)

[Owain]

Even easier. Pull draw out, lift, wiggle, lift entire drawer and contents from unit.

Repeat four times.

Reassembly is the reverse of disassembly.

Owain

[Owain]

Not at all as happens with lighting circuits. Diversity is *not* used in calculating lighting circuit capacity. In fact lighting circuits are over rated as each point is rated at 100W min, even if it's a 9W fluoro.

Try reading the Wiring Regs.

Owain

[Dave Plowman (News)]

You did. Cupboards.

Yes - I always draw things out first. Using !Draw.

But if it was a nest of drawers, even better. You simply line up the socket with the back of one so no chance of a cupboard being piled high with words of wisdom.

My stopcock to cut off the mains pressure feed to everywhere other than the kitchen is situated behind a drawer between the microwave and double oven. Very convenient to get at, if needed.

[Peter Parry]

[:::Jerry::::]

And I can just see some being able to do that ! The real world is not were you live, obviously...

[:::Jerry::::]

,

Until you really do need to get to it quickly, it's quite amazing how a draw always jams shut at the most inappropriate moment...

[:::Jerry::::]

consider

I know, that's what I said!

In fact lighting circuits are

Try reading what is said by others before you make stupid comment that just make you look like a total f*ckwit.

[PC Paul]

[John Rumm]

Could be worse:

[Martin Angove]

Perhaps some definitions are called for.

1: A RADIAL circuit radiates out from the source (i.e. the CU) to the point(s) of utilisation, and terminates at the last one thus:

CU - fuse - fitting - fitting - fitting

2: A RING circuit travels out in two directions from the CU, meeting in the middle. Or to put it another way, it leaves the CU on one bit of wire and returns on another thus:

/- fitting - fitting - fitting -\ CU - fuse | \- fitting - fitting - fitting -/

3: A SPUR is a "branch" from the main backbone of a circuit, be it ring or radial. For radial circuits this isn't a problem, and radials are permitted to have any sensible layout you like. Ring circuits, being protected at a rating higher than the capacity of a single length of the circuit cable are limited to having either just one single fitting on the spur, or several fittings protected by a common fuse of maximum 13A:

unfused spur | /- fitting -+- fitting - fitting -\ CU - fuse | \- fitting -+- fitting - fitting -/ | fuse | fitting | fitting

(Gone cross-eyed with all those fittings!)

Sockets circuits are either radial or ring. In Britain, lighting circuits are always radial AFAIAA. The switch drops might confuse matters for the uninitiated, but the fact is that there is no second run of cable making the circuit into a ring:

CU - fuse - junction box - junction box - (etc.) L N | | \-sw-lamp-/

Confusion sometimes arises from the fact that most textbooks talk about "loop in" and "junction box" wiring systems. In fact both systems are electrically *identical*. The only physical difference is that for loop-in the junction box is part of the light fitting (the rose) while it is a separate item for the "junction box" system. Most lighting circuits are a mixture of the two, particularly if a lot of "fancy" light fittings are used which do not contain enough terminals (or wiring space) to perform the switch-loop function.

There is no need to wire it as a ring as the load on the circuit is fairly well understood, and is pretty light. Sockets circuits are rated at up to 7.5kW (30A), but even my house doesn't come close to 7.5kW of lighting. The ring circuit means that 7.5kW can be distributed using

2.5mm2 cable which is a *lot* easier to handle than the 4mm2 or 6mm2 which would be required for a similarly specified radial.

A lighting circuit, rated at 1400W (6A) doesn't even need 2.5mm2 cable.

1mm2 is more than adequate (rated at 11A worst case) and is very easy to handle, so there is no reason to use a ring simply to allow the use of smaller CSA cable. Some people use 1.5mm2 cable which is rated 14A worst case, and allows the use of a 10A MCB so long as all switches and fittings are 10A rated.

1400W is 14 * 100W lamps, which is more than enough for each floor of most normally-sized houses. Even with a load of energy-guzzling mains halogen downlighters in our kitchen, our house downstairs maximum lighting load is only 1050W, counting each standard bayonet fitting as

100W (which you have to do) even though most of them have 20W CFLs installed.

Erm... isn't this rather OT?

Hwyl!

M.

[Jim Michaels]

Shouldn't this be 2.34 ohms and about 102 amps?

Remove SPAMX from email address

[Jim Michaels]

It is impossible to reach something which does not exist.

Two 16amp radials equal one 32amp ring. House with 3 rings versus one with 6 (or more) radials. How do they really differ in capacity?

Small thermal only MCBs can be produced for pennies. and will fit in a British size plug cap.

an example of an off the shelf quality product.

Remove SPAMX from email address

[Jim Michaels]

So if I fit a BS1363 plug cap with a 32amp fuse and 4mm lead it would be acceptable to power an arc welder (or other device) drawing 30amps from any ring that is handy?

Remove SPAMX from email address