Green Building Forum - Electric cables

Electric cables

tony — Mon, 15 Feb 2016 17:18:59 +0000

Why do we still have twin and Earth?

Then fiddle about with earth sleeve?

Should be all three inners are insulated, blue, brown and green/yellow.

When will we progress?

Electric cables

ringi — Mon, 15 Feb 2016 17:48:34 +0000

It will put up the cost for not much benefit AND make the cables bigger.

Electric cables

Paul in Montreal — Mon, 15 Feb 2016 17:57:33 +0000

Posted By: tonyWhy do we still have twin and Earth?

The only reason the non-earth conductors have insulation is to protect them against shorting to each other or shorting to earth. The earth conductor is already at earth potential so there's no impact if it shorts to earth.

I now like the North American colour codes (which work well for colour blind people). Earth is uncovered, white is neutral and black and/or red are live. So even if you can't see red, you can see that it's not white and anything that's darker than white is live. The colour coding carries over to devices such that the silver screws are for the neutral connection and the brass screws are for the hot connection.

Paul in Montreal.

p.s. plus the outer sleeve insulation is nylon so there's no worry with passing through polystyrene insulation

Electric cables

Gotanewlife — Mon, 15 Feb 2016 23:45:40 +0000

Got some T&E in for the first time last month because I needed a flat cable for important aesthetic and construction reasons and I needed small size with external temp range - it was solid core, have ripped solid core out before but never worked with it - terrible stuff compared to multi-core. All insulated 3 core seems to me to be a more advanced system.

Electric cables

Simon Still — Tue, 16 Feb 2016 10:18:11 +0000

It does puzzle me that domestic electrical standards still vary so much between countries - particularly within Europe I'd have thought we'd be much closer to a common standard by now.

Electric cables

djh — Tue, 16 Feb 2016 10:48:14 +0000

The design of T+E makes sense to me at a technical level as well as economic. If the earth was insulated then if the insulation was damaged on the live or neutral there'd be no fault detection, whereas with T+E there's at least a chance that the live would touch the earth. Plus of course it makes it easier to strip the wire. The outer insulation is so the wire can be run without a conduit, because it's already double insulated.

I confess I've never seen a house with the wiring in conduits. It's sort of appealing from an over-engineering neatness point of view, but if I think back to wiring our house I think there are a lot of places where having to route conduits would have been awkward.

Electric cables

Simon Still — Tue, 16 Feb 2016 12:33:40 +0000

Posted By: djh

I confess I've never seen a house with the wiring in conduits. It's sort of appealing from an over-engineering neatness point of view, but if I think back to wiring our house I think there are a lot of places where having to route conduits would have been awkward.

My guess is that the reason you never see it in the UK is that it's next to impossible to pull solid core T&E cable through (15 or 20mm) conduit. You can get it through before it's installed but once a conduit has a few bends in it you can't. On the continent, if they're using stranded singles it's much easier.

Electric cables

ringi — Tue, 16 Feb 2016 12:41:10 +0000

Using conduit costs a lot more unless it is seen as a "normal" part of the build process. It is an option on same timber frame kits.

Remember on the continent, their plugs don't have fuses, so each socket has to connect back to the consumer unit with its own cable.

Electric cables

Simon Still — Tue, 16 Feb 2016 12:54:33 +0000

Posted By: ringiRemember on the continent, their plugs don't have fuses, so each socket has to connect back to the consumer unit with its own cable.

I'm pretty sure that's not the case. They use radials rather than rings but there will be a number of sockets on each radial.

The principle of fuses is that they protect the cable beyond the point of the fuse. A UK radial on 2.5mm will normally have a 20A MCB at the Consumer Unit. That protects the 2.5mm cable between the CU and the socket. The fuse in the plug protects the cable between the socket and the appliance.

Even if a continental radial has a 16A MCB that's still more than many appliance cables could handle. That said, most of the plugs I've looked at in the UK have the wrong fuses in (including extension leads clearly marked as 5A with 13A fuses in the plugs).

Electric cables

djh — Tue, 16 Feb 2016 13:27:20 +0000

Thankfully there aren't too many electrical failure modes that lead to a failure current greater than the fuse in the plugtop but less than the fuse/MCB in the CU and which also don't also blow something in the appliance itself.

Electric cables

Ed Davies — Tue, 16 Feb 2016 13:59:29 +0000

Part of the point of plug fuses, though, is to blow quickly. It's fairly easy to have a fault which makes the case of some appliance high voltage but, particularly with the resistance of thin flex, will take a considerable time to trip an MCB (more than a few tens of milliseconds) so delivering enough energy (power for long enough) to stop somebody's heart.

AIUI, this is the reason there's a higher minimum conductor CSA for flexes plugged into radial circuits than there is for rings for with the same fuse or MCB.

Electric cables

Paul in Montreal — Tue, 16 Feb 2016 14:26:46 +0000

Posted By: djhThankfully there aren't too many electrical failure modes that lead to a failure current greater than the fuse in the plugtop but less than the fuse/MCB in the CU and which also don't also blow something in the appliance itself." alt="" src="http:///newforum/extensions/Vanillacons/smilies/standard/bigsmile.gif" >" alt="" src="http:///newforum/extensions/Vanillacons/smilies/standard/devil.gif" >

Actually, it is precisely this failure mode that, thanks to several fires, lead to the introduction of arc fault circuit interrupters (AFCIs) that are now mandated for all bedroom circuits here in North America. The failure mode is something like a lamp cord behind a piece of furniture where the insulation gets crimped/damaged over time. Arcs form between the now exposed conductors, but the current in the arc isn't enough to trip a normal circuit breaker, but it is enough to cause a fire. Earth Leakage Circuit Breakers (ELCBs / RCDs) cannot detect this fault either, but AFCIs can.

Paul in Montreal.

Electric cables

Simon Still — Tue, 16 Feb 2016 14:50:09 +0000

Posted By: Ed DaviesPart of the point of plug fuses, though, is to blowquickly. It's fairly easy to have a fault which makes the case of some appliance high voltage but, particularly with the resistance of thin flex, will take a considerable time to trip an MCB (more than a few tens of milliseconds) so delivering enough energy (power for long enough) to stop somebody's heart.

AIUI, this is the reason there's a higher minimum conductor CSA for flexes plugged into radial circuits than there is for rings for with the same fuse or MCB.

So continental circuits just don't have any protection over this type of fault? A 16A or 32A MCB isn't going to make any difference there is it (or is that situation covered by the RCD in both countries now?)

Electric cables

Ed Davies — Tue, 16 Feb 2016 15:29:06 +0000

If the fault current's, say, 40â€¯A then a 16â€¯A MCB would trip a lot quicker than a 32â€¯A one, I'd think. It takes a surprisingly long time (many seconds) for a fuse or MCB to blow/trip for currents which are not quite a few multiples of their rated current.

Electric cables

ringi — Tue, 16 Feb 2016 15:37:14 +0000

A working RCD will trip if the current going out in the L is more then what is coming back in the N. Therefore it will trip if someone touches a live case.

If the case is earthed the RCD will trip as soon as the case becomes lives.

However RCD do fail more work more often, hence they should be tested every 3 months.

Electric cables

bot de paille — Tue, 16 Feb 2016 22:32:18 +0000

You can have up to 8 double sockets on one 20A MCB if the wires are 2.5mm and 5 double sockets on a 16A if they are 1.5mm. I put everything on 2.5mm wire so that the circuit isnt limited to 5 sockets.

Conduit costs about 25 euros for 100m, peanuts.
Earth, Phase and Neutral costs about 20 euros each for 100m.

The MCBs are grouped together under and protected by RCDs which are rated to 30milliamps.

When you have this modern setup there really is no need to for a fuse in the plug as far as I can tell. the slightest leak to earth or short circuit and the RCD will trip within fractions of a second.

I bought all Schneider for the fuse board components, sockets, switches. Really nice gear and everything is push fit.

All circuits runs back to a central hub/box in the loft where it is all individually labeled and connected together with WAGOs.
Sockets and lighting can be daisy chained or have their own conduit run depending on the layout of the house, as long as you stay within the 8 or 5 double socket limits for each circuit you can do what you want.

lighting circuits can be run using either 3 core double insulated cable or conduit with insulated individual insulate wires.

This includes the wires for any two way light switches so that hey are just connected as needed as you kneel over the hub.

Old French wiring is beyond frightening. SO glad my family dont have to live in a house with the kind of wiring now that was considered acceptable then....

Electric cables

Gotanewlife — Wed, 17 Feb 2016 00:41:17 +0000

Broadly, a cable (ie 3 wires) costs the same as a conduit with 3 wires run in it. I only use 4 colours brown, Blue, Earth and Grey for switch lives but there about 10 colours here and each electrician has his own 'code' for what colours do what - saves on labelling!! If it's going in a wall or in screed I always use a conduit and separate cables and my main hub on each floor has 5 or so radials in it so one earth and 5 pairs of N and L with lighting all 1.5 and sockets all 2.5mm2. I use 16A MCBs for sockets and 6amps for lighting and then make enough circuits to allow this but I didn't do any sums. I have a good smattering of UK sockets about and lots of UK switched fused spurs, Over here to create a switched fused socket you have to use a 3 gang box and and connect up a double pole switch to a fuse to a socket (circa 25 Euros all in) - only did it once as extra protection for my whole house water pump. I made a lot up as I went along - I put the electric oven on a unique MCB (even though it doesn't draw that much) and put separate radials in for the kitchen, my sons bedroom (partly so I didn't have to turn him off when i was working on that floor).

This is our forever home, I just hope if I ever sell (or my son comes to sell!) so many years have passed that there won't be any problems 'cos it's all done DIY - no supervision or documentation beyond the 3 consumer units. I was able to buy the house with outrageous in-situ electrics so I hope this indicates it will all work out fine in the end.

Electric cables

djh — Wed, 17 Feb 2016 10:19:42 +0000

Posted By: bot de pailleWhen you have this modern setup there really is no need to for a fuse in the plug as far as I can tell. the slightest leak to earth or short circuit and the RCD will trip within fractions of a second.

Many electricians in the UK criticize the 17th edition wiring regs because of what they regard as its over-reliance on the 'miracle' of RCDs. As Ringi said:

Posted By: ringiHowever RCD do fail more

And sometimes in quite Byzantine ways! In my own experience, it's fairly common that a fuse in the plugtop will blow without tripping the RCD, even without the RCD being faulty. So I'm still happy with defence in depth.

The latest madness is the insistence on metal CUs; supposedly to fix a fire risk that everybody knows is actually due to the use of single screws on connections instead of two screws. Though a better connection scheme to replace screws altogether would be even better.

Electric cables

SteamyTea — Wed, 17 Feb 2016 11:06:22 +0000

A RCD and a fuse/MCB do different things.

Electric cables

bot de paille — Wed, 17 Feb 2016 11:10:21 +0000

RCDs are made to fail to "off" no?

Electric cables

SteamyTea — Wed, 17 Feb 2016 11:22:19 +0000

They go to the 'off' position under a fault condition.
Not the same as failing to 'off'.

Electric cables

bot de paille — Wed, 17 Feb 2016 11:25:48 +0000

Whats the difference?

Electric cables

Gotanewlife — Wed, 17 Feb 2016 11:28:16 +0000

Posted By: djhit's fairly common that a fuse in the plugtop will blow without tripping the RCD, even without the RCD being faulty.

Exactly right - no fault to earth just fault from live to N making for too much current and blowing fuse, if fuse wasn't there then either there would be catastrophic failure of electrical elements breaking bad circuit (and fire invariably) OR the consumer unit MCB would turn off - but this type of fault would never cause the RCD to trip. Though it might as melted wires and arcing caused some current to flow to earth, except perhaps not in a double-insulated laptop.

DIYers - You need to know these things if you do any electrics; they are on page 1 of the things DIYers must know before touching a screwdriver.

Electric cables

ringi — Wed, 17 Feb 2016 11:30:13 +0000

Posted By: djhAnd sometimes in quite Byzantine ways! In my own experience, it's fairly common that a fuse in the plugtop will blow without tripping the RCD, even without the RCD being faulty. So I'm still happy with defence in depth.

A RCD will not trip on a L to N fault, or a overload.

Electric cables

Paul in Montreal — Wed, 17 Feb 2016 11:33:27 +0000

Posted By: ringiA RCD will not trip on a L to N fault, or a overload.

The ones we have over here will trip on both ground faults and over-current faults. The breakers we have are a combination of over-current and earth-leakage (or arc fault detection). We don't have separate RCDs except for special sockets that go in places like bathrooms or outside where the overall circuit doesn't have earth fault detection. In my bathroom the socket has RCD protection and all the lighting circuits are fed from the downstream output of the RCD so that all the switches in the bathroom are protected rather than having the whole radial circuit that the bathroom sits on having an RCD breaker.

Paul in Montreal.

Electric cables

SteamyTea — Wed, 17 Feb 2016 11:33:38 +0000

Posted By: bot de pailleWhats the difference?

The fault condition is what it is designed to sense, a fault, which in the case of an RCB is an imbalance between the live and the neutral, and or earth.
An RCD failure is when the actual unit fails, so it may be unable to detect the imbalance or may be physically impeded from reacting to the imbalance. Then it may fail in either the on or the off position.
They are not intrinsically safe devices.

Electric cables

Paul in Montreal — Wed, 17 Feb 2016 11:58:29 +0000

Posted By: SteamyTeaAn RCD failure is when the actual unit fails, so it may be unable to detect the imbalance or may be physically impeded from reacting to the imbalance. Then it may fail in either the on or the off position.

This is why they (should) have a test button that should be tested regularly.

RCD breakers are still supposed to act as overcurrent breakers, just like regular ones - though maybe in the UK the RCD is a separate thing and isn't bundled into a "MCB"?

Now with mandated arc-fault breakers, the two functions (earth leakage and arc detection) have been integrated into one circuit breaker device, such as: http://www.schneider-electric.us/en/product-range/62274-qo-dual-function-circuit-breakers/

See also http://download.schneider-electric.com/files?p_Reference=RP19520116&p_EnDocType=Brochure&p_File_Id=1713692785&p_File_Name=Square+D+Advanced+Function+Breakers.pdf which gives more details about arc faults etc.

Paul in Montreal.

Electric cables

SteamyTea — Wed, 17 Feb 2016 12:04:49 +0000

RCBO is the one that does residual current and over current.

Electric cables

ringi — Wed, 17 Feb 2016 13:19:42 +0000

Arc Faults for some reason don't seem to be common in the UK.

It will be very hard to damage "twin and earth" in a way that gives a N/L fault without getting a L/E fault. But I would still expect Arc faults on cables attached to plugs.

Electric cables

ringi — Wed, 17 Feb 2016 13:21:41 +0000

Posted By: SteamyTeaRCBO is the one that does residual current and over current.

Why don't we have consumer units with 3 bus bars (N,L,E) that a RCBO can plug into, rather then the number of wires you have when you use a lot of RCBO in a UK consumer unit?