Wiring Regs - Rationale

Hi
I've installed a new kitchen sink and have put supplemental bonding from the incoming cold water pipe across to the hot water pipe and also to the sink. The regs don't require this but I can see no harm in it, and it reinstates the position before the works on the kitchen. I wonder why this was dropped from the regs?
I also noted that the main bonding from the water pipe to the block adjacent the CU is 6mm.sq not the 10mm.sq that the 16th ed. specifies. I condsider that there's no need to uprate this to 10mm.sq since the regs are not retrospective. Also it would be almost impossible physically without taking the wire round the outside of the house! Again, I wonder what was the rationale for the change?
Finally, I know it's been done a while back on previous threads but what 's the view re 2.5mm.sq unfused spurs from 2.5mm.sq ring mains (Type B CB at 32A). My personal view is that it's not acceptable on grounds of cable rating - but it used to be allowed. Anyone know why this changed?
Regards Jx
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An electrically conductive (metal) appliance that can be touched, or can come into contact with a live supply conductor, even due to a fault condition such as damaged insulation, should always be bonded to an earthing point. That hasn't changed.

The bonding to that point may be created in 6mm csa cable, but is the bonding to the actual mains water supply pipework the same. From the mains electrical supply, the bond must be created in 10mm csa conductors, but any further supplimental bonding can be reduced to as low as a 1mm csa conductor, totally depending on what rating of fault current is expected on that particular part of the circuits. That hasn't changed.

Many times it had been found after serious electrical fire incidents, that the occupants had connected large loads to circuits which had been added to the mains ring supply by smaller gauge conductors and so causing them to form overload condition in that particular section of cable. So this meant that the requirements had to be changed in the form of which methods were used in making tapped circuits only take the current rated for that particular section of cable.
Therefore, if a spurred circuit is supplied with a cable of 2.5mm csa, then this spur should be restricted by the use of an automatic breaker between it and the rest of the ring mains supply and which only allows that section of cable to take its maximum load rating. Connected this way, the spurred section is not allowed to rely only on the mains ring breaker back in the consumer unit to stop it going into overload condition and failing. This bit has changed from the old requirements, but many, many years ago.

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In a kitchen situation, it is possible to see that supplementary bonding may actually reduce electrical safety. The regs in general prefer complete isolation of metal parts to cross bonding. The situation in a kitchen is different to a bathroom due to portable electrical appliances (such as kettles) that can be dropped in the sink.

I have not heard that it has changed. I am still under the impression that a 2.5mm cable is fine for a spur to a single or double 13A socket. Overload protection is provided by the plug top fuse. Short circuit (and earth fault) protection is provided by the MCB, so earth loop impedences (including the reduced cross sectional area spur) have to be calculated. Why do you think this situation has changed? Have I missed something?
Christian.
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On Tue, 4 Nov 2003 09:30:50 UTC, "Christian McArdle"

I'd like to know too. I can see that (diversity aside) it would be possible to plug in two 3kW appliances into a double socket on a spur, each with 13A plugtop fuses. That would generate a moderate overload for 2.5mmsq. cable.
I have one spur wired with 2.5mmsq....now wondering!
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wiring unfused spurs in 2.5mmsq in a couple of ways. Firstly, 2-core 2.5mmsq is rated to carry (a) 18.5A, (b) 23A, and (c) 27A when mounted (a) surrounded by thermal insulation, (b) in conduit, (c) clipped direct respectively. (And no, I don't have the buried-in-plasteer figure to hand, sorry). So for some cable routings, even the hypothesized 2x13A loading will be OK.
More to the point, though, is the Regs' insistence (from 15th Edn onwards, if not earlier) that there be only one socket, whether twin or single, on such a spur. The likelihood of one double socket actually feeding a sustained 26A load is very low (indeed, many informed posters here allege that the rating of a double socket is only 13A). Of course if you *know* that a twin socket is intened to feed two heavy loads - a Burco water heater and a 3kW super-toaster running nearly-always-on in a builder's-brekkie-bar - you'd be negligent to feed it with a 2.5mm single, rather than putting the socket - or better, two singles, or better still, an FCU for such more-or-less-fixed equipment - direct on the ring or a dedicated radial. Chances of overload increase if there are two singles at more widely separated locations (e.g. in different rooms): hence that configuration being no longer permitted.
This is about *overcurrent*, i.e. "will the cable insulation start to melt because of sustained heating due to resistive losses with a sustained high current"; it's *not* about "will the cable still be OK after a big short-circuit". The latter is taken care of by the 32A MCB or 30A fuse; back in http://groups.google.com/groups?selmk1pl1%241b0%40cucumber.demon.co.uk Andrew Gabriel was kind enough to do the worked example to show the time characteristics of even a slower-to-react fuse (let alone an MCB) limits the temperature rise under fault conditions to safe values; "safe" here means a 20 or 40 degree Celsius rise in temperature for 0.1 or 5 seconds, which even if you'd been running at 70 degrees before the short (worst-case assumption) won't damage the PVC insulation when applied for so short a time.

Unless you know that the spur feeds substantially heavy loads, you're in good shape; even if it does, they'd have to be unusually demanding loads ("pottery kiln" is the canonical wiring-advice-book example!) to draw over 20A together over a *sustained* period.
You're much more likely to find serious problems in other bits of your previous owner^H^H^H^H^Hbodger's wiring, than worrying about whether Regs-conformant Conventional Final Circuits are suspect...
HTH, Stefek
(Googling back to the words of Wade and Gabriel points one at several relevant posts. Try a search at groups.google.com with the query
13A socket double "current rating" group:uk.d-i-y
and you'll find 6 messages, with the three Wade or Gabriel ones being directly relevant.)
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On Tue, 4 Nov 2003 11:38:06 UTC, snipped-for-privacy@hp.com wrote:

(etc.)
That's quite reassuring.
The spur concerned is one double socket; I don't like doing spurs and only did so here (yes, it's part of my new wiring) because it was a pig to get the cable to it. The spur cable is short (no more than a couple of metres). It's also not likely to have a heavy load; it's in a hallway and really intended for vacuum cleaners and the like.
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"Getting a SCSI chain working is perfectly simple if you remember that there must be exactly three terminations: one on one end of the cable, one on the other end, and then the goat, terminated over the SCSI chain with a silver-handled knife whilst burning *black* candles."
You'll need to update the sig, Bob: a single black goat is adequate only for single-ended SCSI (well, duh!) For low-voltage differential, you need a *pair* of pushme-pullyou style two-headed goats, engaged in Crowleyesque sexual practices, but only on alternate full moons. For high-voltage differential SCSI, they need in addition to be standing on a black rubber mat and have a 12V car battery connected to their cloven hooves...
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You're grabbing the bull by the horns....which is also needed to make a triple SCSI chain correctly. :-))
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On Tue, 4 Nov 2003 12:37:32 UTC, snipped-for-privacy@hp.com wrote:

You won't see if often; it's my 'posting from work' sig! But time for a chnage anyway...
P.S. Just had a letter from the IEE asking if I want to join....
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there must be exactly three terminations: one on one end of the cable,
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wrote:

LOL
I always like to read the part that says "Not likely to have heavy loads connected to it" and would really like to know what exactly that means. If it is a convenient socket for a builder to connect his supply too when carrying out work on the outside of the house, then it is likely to have a heavy load connected to it. But as it is only really for the vacuum cleaner, then it should be OK.
:-))
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On Tue, 4 Nov 2003 12:37:54 UTC, "BigWallop"

Well, they were the thoughts I was having! I guess it's still OK unless the builder has two max load appliances... or perhaps I'll just review that bit anyway.
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--
Chris Green ( snipped-for-privacy@x-1.net)

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The Acme Inc Bucking Bronco Rodeo Machine....
-- Richard Sampson
email me at richard at olifant d-ot co do-t uk
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Nah...a big kettle!
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Bob Eager
rde at tavi.co.uk
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"Johnny Boy" wrote in message

It wasn't. Supplementary bonding never has been required in kitchens.

You're going back to the 14th ed. here. The 15th & 16th editions both require a minimum main bonding conductor size of at least half that of the main earthing conductor, with a minimum of 6mm^2, or 10mm^2 for PME. If the meter tails are 16 or 25mm^2 then the earthing conductor has to be 16mm^2, hence the minimum main bond size becomes 10mm^2.
The rationale for the change was (a) the 15th ed.'s generally more rigorous approach to earthing and conductor protection across the whole range of expected fault current, and (b) the introduction of PME (TN-C-S) supplies to the UK. 10mm^2 (or larger if the supply authority so required) was written into leglislation affecting suppliers - originally as the PME Approval, then in the 1988 Electricity Supply Regulations. I don't think it's in the current ESQC Regulations (which replaced the ESR this year) but these cite "British standard requirements" (i.e. BS 7671) instead.
Another change which took place at around the same time was to BS 6004 and increased the size of the CPC in 2.5mm^2 T&E cable from 1.0 to 1.5mm^2. Someone realised that - under some conditions with a rewireable fuse - the CPC was not properly protected by the fuse.

It didn't. Unfused spurs in 2.5mm^2 are allowed, as they always have been (provided that the 'as installed' cable rating is 20 A or more). Provided that the design complies on voltage drop the MCB will give fault and earth fault protection to the cable. Overload protection is provided downstream by the fuse in the BS 1363 accessory (plug top or FCU). The load is limited to 13 or 20 A by the restriction that an unfused spur may only feed one single or double socket, or one item of fixed equipment. (The assumption that the load on a double socket will not exceed 20 A is a diversity rule which has stood the test of time.) The 14th edition was more lax, and allowed two single sockets on the spur, but that anomaly was removed when the 15th ed. came into force, which is over 20 years ago now.
--
Andy



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