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Topic Title: Reg 433.1 spur from a radial
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Created On: 15 April 2014 08:44 PM
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 15 April 2014 08:44 PM
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Grumpy

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On the one hand we have a regulation that dictates a CPD on change of cable size or type. On the other hand we have Fig 15B in the BGB showing a 2.5mm spur supplying one double socket off a 4mm radial with a 32A CPD.
Doing an inspection today and found a 10mm radial via a 40A mcb supplying a 45A dp isolator for a hob. Off the supply side of the isolator 3" of 2.5mm T&E fed an FCU supplying the built in oven.
My initial reacton was that this was wrong but on further reflection I thought that the fixed load via a 13A FCU was never going to overload the 2.5mm T&E and what could come to pass that the 3" of 2.5mm would ever need the protection of the 40A mcb.
Is there a case for common sense here?
If that's an emphatic no, what would be the best resolution given that it's in the back of a cupboard with very little slack in the 10mm?
 15 April 2014 08:50 PM
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geoffsd

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We also have 433.3.- omission or position of overload protection devices

and 434.2 - position of fault current protection devices.
 15 April 2014 08:56 PM
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Grumpy

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Were you thinking of 433.3.1 (i) Geoff?
Should you feel the urge to reply in English long hand I would be very keen to listen!
G
 15 April 2014 11:32 PM
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phantom9

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I agree. 433.3.1 (i) covers it. The 13A fcu is the protective device installed on the supply side of that point

Even without this it is clear that the fcu is the overload and short circuit protection for the built-in oven and 2.5mm2 is adequate to be fused at 13A.
 16 April 2014 12:22 AM
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geoffsd

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Yes. 433.3.1(i)

They could probably have employed 433.3.1(ii) and done away with the FCU.
However, I suppose the MIs would have stated 13A fuse.

It doesn't affect the reasons above but surely the 2.5mm spur would be better on the load side of the switch. Is it a switched FCU?
 16 April 2014 06:40 AM
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normcall

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And the designer says?

-------------------------
Norman
 16 April 2014 08:36 AM
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aligarjon

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If the spur had been connected to the load side of the switch then a fault on the hob would have meant the oven would have to be isolated as well so i would think better as it is so long as the terminals are good for conductors larger than 10mm and that the smaller cable is tight.

Gary

-------------------------
Specialised Subject. The Bleedin Obvious. John Cleese
 16 April 2014 09:24 AM
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AJJewsbury

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I might want to see a calculation to say the 2.5mm²'s 1.5mm² c.p.c. was protected by the 40A MCB for faults - which probably means showing that the PEFC at the supply end of the 2.5mm² was no more than about 3kA. Probably unlikely in a domestic, but maybe worth a quick check just to be sure. The weak point for shorts is more likely the terminations rather than the 3" of 2.5mm² itself.
- Andy.
 16 April 2014 10:11 AM
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Grumpy

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The NIC helpline take on this is that the arrangement is "unconventional" but is covered by reg 432.1. (i).
Thanks all for your replies.
 16 April 2014 10:19 AM
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davezawadi

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That's a curious reply Andy!
The usual adiabatic equation will check that the 2.5/1.5 has adequate short circuit capability (You only need to do the 1.5) to trip the breaker. If RCD protected the 2.5 is obviously the limiting conductor to calculate. A low PFC is really the risk as it may lead to a longer trip time, more heating and thus needing larger conductors. It is low currents which lead to extended tripping times and therefore may be a problem.

The basic circuit is perfectly sound, it can meet both the short circuit protection requirement, and the overload protection requirement (the protection for this can be anywhere in the circuit).

I just saw the NIC reply, it seems that they are less sure of the regs. than one might reasonably expect!

-------------------------
David
CEng etc, don't ask, its a result not a question!
 16 April 2014 11:01 AM
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AJJewsbury

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A low PFC is really the risk as it may lead to a longer trip time, more heating and thus needing larger conductors. It is low currents which lead to extended tripping times and therefore may be a problem.

Not entirely - unlike fuses - MCBs generally have a higher energy let-though at higher fault currents. E.g. for a generic B40 - 21,600 A²s at 3kA, 38,400 A²s at 4.5kA, 54,000 A²s at 6kA and 108,000 A²s at 10kA (and a little higher again for C types). Yes you need to ensure that Zs is low enough for basic disconnection time, but that's not the whole story...
- Andy.
 16 April 2014 04:38 PM
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davezawadi

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Yes Andy, but you are getting carried away a bit. Domestic premises are very unlikely to have a PFC at the head of anything like 10kA, let alone at the end of the final circuit (which is the point we are considering). Even in an exceptional case at the end of of domestic final circuits it would be unusual for it to be more than 1-2 kA. If it were we would need much more thought than the usual on cable sizing for all the final circuits, as you will immediately see that a lighting circuit with a 1mm2 CPC is much more fragile than the case under discussion, and would probably be unsatisfactory. Whilst energy let through is important it tends to be at the larger end of installations where it makes a difference to the design, thus the limitation of OSG standard designs to 100A supply. In any case the I2t to raise 1.5 to 70C is about 30,000, so we are very unlikely to have a problem with any domestic installation using this method.

-------------------------
David
CEng etc, don't ask, its a result not a question!
 16 April 2014 05:01 PM
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AJJewsbury

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so we are very unlikely to have a problem with any domestic installation using this method.

I agree it's unlikely - but not impossible. We've had reports on here with domestics with no front garden having a PFC> 7kA so given a short final circuit in 10mm² I wouldn't want to assume that the fault current would always be below 3kA.

In any case the I2t to raise 1.5 to 70C is about 30,000

I take it you mean from 70 to 160 degrees?

Hopefully a 1.0mm² c.p.c. wouldn't be on a 40A breaker - more likely a 6A or 10A one - so BS 7671 would be satisified as Iz >= In so we wouldn't be obliged to check using the adiabatic. (I guess the theory is that the thermal element should always protect the cable where In <= Iz, so the performance of the magnetic element isn't a concern - although I'm not necessarily convinced of the mechanics of how the MCB might achieve that in practice). Maybe that's the thinking behind the 1.5mm² limit of table 52.3 (presuming the long term intention is to harmonize with Europe and have a 1.5mm² limit for everything).

- Andy.
 16 April 2014 07:24 PM
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davezawadi

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Andy, there is a slight lack of realism beginning to creep into your posts! Whilst almost anything is possible......

The concept of a CPC at 70C (even in a T&E cable being at 70C) is ridiculous. Even if it were, it will still not fail at 160C, so your comment is...., but then I think you know that. We need to be realistic in these posts because there are many people who actually learn useful stuff here. It is not useful to make them frightened about all kind of "well possibly" scenarios which are very unlikely. If you were to carry out EICRs on 100 domestic installations, how many would cause you this worry? Have you ever seen a domestic cable heated 30C above ambient? Probably not. Have you ever burnt your hand on an SWA (not just noticed its hot!)? Probably not. I am much more concerned that many see danger and problems where there are none, report this, and are open to legal action for professional liability for the cost of correcting the alleged "defect". My experience is that the standard of EICRs is terrible, and that the inspector then does the repairs. In my view this should never be the case! Inspect, report, be paid, end of job. This is the professional way, not this MOT style "faults = cash" scheme. Do we have numbers of problems with domestic installations with burnouts of conductors? No.

So as Grumpy says it is very important to be realistic.

-------------------------
David
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 17 April 2014 02:08 PM
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perspicacious

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"Doing an inspection today and found"

Did you also find that the supply to the 100 A cut-out was in 35 mm2 Al spurred off a 185 mm2 Al supplied via a 315 A fuse?

Regards

BOD
 17 April 2014 07:44 PM
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Grumpy

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No I didn't.
And your point is?
G
 17 April 2014 11:32 PM
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davezawadi

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Nice one BOD, standard practice by the DNO!

-------------------------
David
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 18 April 2014 12:47 AM
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dg66

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Originally posted by: perspicacious

"Doing an inspection today and found"



Did you also find that the supply to the 100 A cut-out was in 35 mm2 Al spurred off a 185 mm2 Al supplied via a 315 A fuse?



Regards



BOD


Not covered by BS7671 , so not applicable to the OP's inspection

-------------------------
Regards

Dave(not Cockburn)
 18 April 2014 10:57 AM
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AJJewsbury

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The concept of a CPC at 70C (even in a T&E cable being at 70C) is ridiculous. Even if it were, it will still not fail at 160C, so your comment is...., but then I think you know that. We need to be realistic in these posts because there are many people who actually learn useful stuff here. It is not useful to make them frightened about all kind of "well possibly" scenarios which are very unlikely. If you were to carry out EICRs on 100 domestic installations, how many would cause you this worry? Have you ever seen a domestic cable heated 30C above ambient? Probably not. Have you ever burnt your hand on an SWA (not just noticed its hot!)? Probably not. I am much more concerned that many see danger and problems where there are none, report this, and are open to legal action for professional liability for the cost of correcting the alleged "defect". My experience is that the standard of EICRs is terrible, and that the inspector then does the repairs. In my view this should never be the case! Inspect, report, be paid, end of job. This is the professional way, not this MOT style "faults = cash" scheme. Do we have numbers of problems with domestic installations with burnouts of conductors? No.

Hang on Dave! All I'm saying is that we have a situation that is a little out of the ordinary (1.5mm2 protected by 40A OPD), where it's not obvious that it complies with 543.1.3 - so perhaps we should do a quick check to make sure. While I agree it's unlikely to be a problem in 99% of domestics, I don't think that's a good reason to ignore it in every case. We're meant to be EICRing the properly we're in, not making assumptions based on other ones - that's the province of the drive-by EICR.

For sure BS 7671 can be a bit "conservative" for domestic installations, but that's the standard we usually work to. The OP didn't ask if it was likely to likely to lead to the end of the world, just whether it was "wrong" - and BS 7671 is our usual yardstick for that. Do the check - if it says all is well then you can sleep soundly, if not then you've got some information to start thinking about how dangerous it's likely to be and if to code it.

- Andy.
 19 April 2014 11:22 AM
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perspicacious

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"And your point is?"

Basic electrical awareness..........

"Not covered by BS7671 , so not applicable to the OP's inspection"

But the basic electrical theory is......

Regards

BOD
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