American electrics

On 06 Jul 2005 23:37:41 GMT,it is alleged that andrew@a17 (Andrew Gabriel) spake thusly in uk.d-i-y:

You are correct, I don't see any circumstances where a reduced earth conductor could be better than or even equal to a full size one.

So because someone else says they're old fashioned, the public should be inconvenienced?

Reply to
Chip
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OK then, please think of some likely scenario in which it is undersized.

Well, when I replace a CU, I don't use a B6 breaker on the lights. That's largely a question of how competent a designer your electrician is.

In my own home, I don't have any mains filament lamps indoors that I can think of at the moment, at least, none on the lighting circuit. I do have a couple of halogen ones outdoors, but they are on their own breaker.

Reply to
Andrew Gabriel

Agreed, There are not many electrical fires due to fixed wiring.

Just a reasonable engineering safety margin.

Ridiculous.

We may have more old or poorly maintained installations but safety has always been a primary concern.

Look up safety margin.

If underground the cables from the meter to the CU. If overhead the cables from the bottom of the drip loops (approx. 2feet from building anchorage) to the meter and into the CU. Yes, Copper!

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Similar technology for UK use.

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There isnt even a black

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Reply to
Jim Michaels

Ring mains have less than 20amp breakers?

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Reply to
Jim Michaels

No, Our supplies are also 240V

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Reply to
Jim Michaels

snip

Copper is the preferred from the meter to the panel, but Aluminum is not unheard of.

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Reply to
Jim Michaels

Nope -- you require regulation mostly at the 120V level.

Reply to
Andrew Gabriel

Why?

Reply to
:::Jerry::::

But given the frequency of power cuts in the US you'd have automatic emergency lighting anyway?

Reply to
Dave Plowman (News)

On Thu, 07 Jul 2005 09:48:34 +0100,it is alleged that "Dave Plowman (News)" spake thusly in uk.d-i-y:

[snip]

Seems reasonable yes, the frequency of power cuts *is* much higher in the US, but emergency lighting's a good idea anywhere.

Reply to
Chip

On 07 Jul 2005 00:24:17 GMT,it is alleged that andrew@a17 (Andrew Gabriel) spake thusly in uk.d-i-y:

I just don't like the concept of the protective conductor being a smaller size than the line conductors potentially feeding fault current into it.

[snip]

Sadly most are not 'designers' at all, they install a B6 because everyone uses 6amp for lighting, and if they just ask for a 6 amp breaker, a type 'B' is what they get.

To be fair we have mostly compact fluorescents. It's just the chandelier fitting in the living room which looks awful with anything but 25w candle bulbs, and the GU10 Halogens in the conservatory. I am intending to switch the GU10's to an FCU off the ring, but it's the candle bulbs that do the breaker tripping (possibly due to smaller lead spacing inside the lamps causing the plasma effect suggested elsewhere).

Reply to
Chip

In my kitchen, the extractor hood lighting is on a separate circuit to other lighting, and when cooking it's always on. So even if a bulb blowing

*did* trip an MCB, it would not plunge the room into darkness.

Same with stair lighting - it's on two circuits. But I think I'd find my way downstairs ok in the dark. ;-)

Reply to
Dave Plowman (News)

reasonable safety margin on wire sizes has been addrssed already. I cant help thinking youre perhaps not keeping up.

hardly, look at the stats, and the practices that are known to cause fires. Or dont.

Given what we've read in this thread, that conclusion is simply impossible to draw.

That doesnt answer the q at all. Im perfectly familiar with safety margins, and UK has more than big enough margins in its cable sizes. US cables are not large for that reason.

thats news?

thats news too??

I think time to end this discussion. Good luck.

NT

Reply to
bigcat

There's no logic to that. Protective conductors only have to be sized to withstand the earth fault current for the time it takes to clear the fault. Live conductors [1] have be able to withstand continuous full-load design current as well as (where relevant) occasional short-term overloads. The principle of using 'undersized' CPCs is very well established, both in theory and practice, and has been so for a very long time. The practice doesn't compromise safety; in fact it avoids over-engineering and saves copper.

For live conductors of up to and including 16mm^2, BS 7671 requires the designer to check the sizing of any intended under-size CPC using the adiabatic equation (see Reg. 543-01-03) [2]. However, for twin & earth cables, detailed calculation can be avoided by using the pre-calculated final circuits given in Table 7.1 of the On-Site Guide (OSG). Provided that you don't exceed the circuit lengths given in that table, for the relevant type of protective device, type of earthing and required disconnection time, and provided that the earth fault loop impedance tests out OK (see Appendix 2 of the OSG) then your design and installation should be safe from this POV.

If you scan through Table 7.1 you'll see that most circuits are voltage-drop-limited, so a larger CPC will confer no advantage in terms of allowable circuit length. Circuits where the length is limited by Zs could be stretched by using a full-size CPC, but that rules out using T&E cable, unless a separate CPC is run.

[1] This term means the current carrying conductors and therefore includes any neutral. [2] For 25 and 35mm^2 live conductors a 16mm^2 CPC can be used without calculation; for larger sizes calculation is only required if the CPC is less that one half of the live conductor size [BS7671 Table 54G].
Reply to
Andy Wade

The old style 2.5mm cable certainly used to be earth loop impedence limited in many circumstances, which is why the CPC was increased to 1.5mm, to enable longer circuits. As you suggest, there was no need to go larger, as voltage drop becomes the dominant limit to circuit length.

Christian.

Reply to
Christian McArdle

On Thu, 07 Jul 2005 17:25:28 +0100,it is alleged that Andy Wade spake thusly in uk.d-i-y:

That is where I have issues with the concept, I admit my calculations verge on the 'worst case scenario' but a 1.5xI(n) fault would not blow a 30 amp rewireable fuse in under a few minutes, and 45 amps flowing through 2x1.5mm earth wires seems a generally bad idea to me. I am at odds with the IEE and the prevailing opinion on this NG for that, and the worst case is highly unlikely to happen, but I DO overengineer wiring systems, for example I won't use 1.0mm2 cable, I use 1.5, always, as the cost difference is minimal. I see no negative safety implication of oversizing things compared to 'what is allowed'.

One of the things I am looking for is evidence of what is 'unsafe', so I can avoid doing it :-)

I think the differences are not as major as people at first think, in the US the neutral on the drop (TN-C-S system before the split) is often slightly undersized, they just don't extend it to final branch circuits, we in the UK do, and many other european countries disagree with this practice.

The saving in copper is minimal, and copper can be (and is) recycled after the cable reaches the end of its useful life.

A small 'anachronism' is that if wiring in conduit, you use full size earth conductors AND bond the conduit, which nobody feels is overengineering, and with MI cable, the sheath is several times the CSA of the conductors.

In short, I am not saying the 'UK ring circuits are dangerous' but more that 'under certain circumstances I can see that different arrangements could be safer'. It's all a matter of degrees I guess.

Reply to
Chip

45 amps will be generating some 10kW at the scene of the fault. It would be pretty impossible to even deliberately engineer such a fault which lasted more than a few seconds, without it either shorting completely or blowing itself out into an open circuit. The generation of 10kW will change the nature of a fault site very quickly.

There just aren't credible scenarios where large currents flow for an extended period in the earth conductor.

On a fully loaded US system, the neutral current is zero. Same is true of a fully loaded 3-phase system in UK, which is why 4-wire 3-phase circuits do sometimes have reduced size neutrals (need to watch out for 3rd-harmonic components though, which do add in the neutral rather than cancel out).

Reply to
Andrew Gabriel

On 07 Jul 2005 20:48:55 GMT,it is alleged that snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) spake thusly in uk.d-i-y:

Fair point :-)

Yep, many people in the US are now being caught out by needing larger size neutrals than phase conductors, especially in IT rich environments such as office space etc, where the PC and IT equipment PSUs generate nasty harmonics and cause local overheating of the neutrals, not good.

Reply to
Chip

Didn't stop someone stealing a 75MVA (? might be a factor out there, it was a few years ago) transfromer from the ex-steelworks at Templeborough. When British Steel handed the building over to the group which turned it into the Magna Science Adventure Centre (I worked there for a while) they left the last arc furnace transformer parked in the lot while they found somewhere to put it.

No-one knows what happened to it, and it must have involved a large crane and a low-loader, but when they came to claim it just before the centre opened it was no longer there. Magna ended up paying a lot of money to BS for having "lost" their transformer.

Hwyl!

M.

Reply to
Martin Angove

bit hard to resell that one...

NT

Reply to
bigcat

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