Wiring Regs - Rationale

[Johnny Boy]

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

 

[BigWallop]

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?

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.

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?

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.

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?

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.

Regards
Jx

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[Andy Wade]

in message

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?

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

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?

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.

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?

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.

 

[Bob Eager]

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?

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!
--
"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."

 

[stefek.zaba]

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.

The 16th Edn tables and associated text support the common practice of
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/[email protected]
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.

I have one spur wired with 2.5mmsq....now wondering!

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.)

 

[Bob Eager]

The 16th Edn tables and associated text support the common practice of
wiring unfused spurs in 2.5mmsq in a couple of ways.

(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.

--
"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."

 

[stefek.zaba]

"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...

 

[BigWallop]

(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.

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.

)

 

[BigWallop]

"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...

You're grabbing the bull by the horns....which is also needed to make a
triple SCSI chain correctly. )

 

[Bob Eager]

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.

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.

--
"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."

 

[Bob Eager]

"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:

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....
--
"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."

 

[usenet]

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.

So what's a builder going to connect that will take a lot of current?

 

[RichardS]

So what's a builder going to connect that will take a lot of current?

The Acme Inc Bucking Bronco Rodeo Machine....

 

[Bob Eager]

The Acme Inc Bucking Bronco Rodeo Machine....

Nah...a big kettle!