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Long outdoor run to sockets from CU
Kevin Cordina 11001215392
Joined 22/05/2020 - 4 Posts
Question
Hi,

I have inherited some wiring that feels a bit odd, but I'm assured is fine, so am interested in opinions if it really is OK. I'm an Electrical Engineer so know the theory, but don't have any wiring qualifications so am vague on actual practice, but am always interested to learn (for interest, not to try and do things myself!).

The situation is this - an outbuilding has a CU fed with a large SWA cable from the house.  A 32A MCB in the CU feeds a 4mm^2 armoured cable which runs outside around 10m to a small brick hut.  At the hut the armoured cable goes into a rotary isolator switch for a swimming pool heat pump, and has a twin & earth spur coming off to 4 x 13A sockets, one of which is used for a water pump. There's no further protection in the hut.

Everything is running fine, but I'm surprised there's not a CU with RCD/MCB in the hut to provide local protection and I'm intrigued if there should be one.

Thanks for any information for my curiosity.

Kevin
12 Replies
Kevin Cordina 11001215392
Joined 22/05/2020 - 4 Posts
Thanks all for the interesting information.  Replying to some of the questions and comments:-

Alcomax‍ no, no-one has assured me it's fine - as per my enquiry it's a system that I inherited with a house purchase and which is operating fine but raised my curiosity, hence wanting to understand the arrangement and whether it's correct.  Based on the suggestions below I'll speak to an electrician to improve things.

AJJewsbury‍ more investigation shows the origin at the house has a 60A fuse in a switch unit as shown in the picture below. The meter feeds the switch/fuse unit and the output SWA runs about 30m to the outbuilding. The photo just about shows the paired wires.

Sparkingchip‍ will speak to an electrician to get it done.

Kevin


 
Alcomax 11001195572
Joined 01/02/2017 - 171 Posts
I have inherited some wiring that feels a bit odd, but I'm assured is fine,      and

I'm surprised there's not a CU with RCD/MCB in the hut      and

At the hut the armoured cable goes into a rotary isolator switch for a swimming pool heat pump, and has a twin & earth spur coming off to 4 x 13A sockets, one of which is used for a water pump. There's no further protection in the hut.

Out of curiosity did someone assure you that all this was fine? Not trying to be funny, but was this advice given before 1983 and the IEE reg 471-12 ?

Sparkingchip has been trying his best to make the obvious point.
ebee 81966746
Joined 02/12/2004 - 664 Posts
"If you're saying that this SWA has been directly connected to the DNO's supply without any overcurrent protection at all, "   I`ve seen a few like that I`m afraid.

the 3m figure I took to be from cutout, thru meter to CU rather than from meter to CU. I might be wrong though (I often am!)
AJJewsbury 77361768
Joined 13/08/2003 - 1433 Posts
I can't read the size - it seems to be 4 core as there's red & yellow into the CU live inlet in the outbuilding, and blue & black neutral. Total O/D is around 3cm.
That's pretty large - if it really is about 30mm diameter and 4-core that would put it somewhere around 35mm² or 50mm² - ether someone had the cable left over from another job or they were very worried about voltage drop - how long is this one?

 
so no protection in the house other than the main 100A inlet fuse.
If you're saying that this SWA has been directly connected to the DNO's supply without any overcurrent protection at all, that's a bit worrying. Normally you'd expect at least a fuse within 3m of the meter and the cables before that fuse to be as short as reasonable practical,  normally single core insulated & sheathed to minimise the risk of faults between conductors and placed to minimise the risk of damage (certainly not buried in the ground). In practice, if the SWA really is as big as it seems (and not too long), the actual dangers might not be that large in your particular case, but it's still way beyond what's normally acceptable.

I'm presuming you've got use of the DNO's earth terminal (TN) and don't rely on your own earth electrode (TT) - otherwise there'll be a stack of bigger problems too.

    - Andy.
Sparkingchip 72796851
Joined 18/01/2003 - 2247 Posts
Definitely time to upgrade and add RCD protection.

Andy B
mapj1 80733779
Joined 22/07/2004 - 1901 Posts
Tis a little odd to use 4 core cable with 2 cores each in parallel for L and N, and then earth only on the armour. However, it should not be a problem so long as the paralling of the cores has been done properly. I presume also there is a nice solid bond from the main earth terminal near the meters to the water piping that feeds the pool, or that the piping is plastic and does not need to be earthed.
Kevin Cordina 11001215392
Joined 22/05/2020 - 4 Posts
I can't read the size - it seems to be 4 core as there's red & yellow into the CU live inlet in the outbuilding, and blue & black neutral.  Total O/D is around 3cm.  Armour sheath is pealed back and joined to the earth connections at both ends.

Kevin

 
mapj1 80733779
Joined 22/07/2004 - 1901 Posts
can you read the cable size that is protected by the 100A fuse - that needs to be 16 or ideally 25mmsq, or fused down at the meter end.
mapj1 80733779
Joined 22/07/2004 - 1901 Posts
It is probably OK - a 32 A supply behind any number of 13A sockets is the normal way - the fuses in the plugs protect the flex to the appliance and the contacts and switch in the socket. If there is also a light in the hut it should be fused down but that could be a 13A fuse in a plug, or a fused spur as a light switch.
SWA needs to be properly terminated so the armoure wires are earthed at least at the origin end (at the load end the armour may be the way the earth is provided for the sockets or they may be "TT" earthed via an electrode and the armour stops )

As others have noted an RCBO or RCD at the origin would be good, and is mandatory on new work, especially with water and electrics in proximity.
 
Kevin Cordina 11001215392
Joined 22/05/2020 - 4 Posts
Thanks Andy & Andy B,

I had assumed the feed came from the CU in the house, but I've just checked and it doesn't - the meter outlet is split to the house CU and to the outbuilding, so no protection in the house other than the main 100A inlet fuse.  Outbuilding CU has a 100A main breaker and 32A MCB for this outlet. 

I guess the obvious question is whether it's worth adding an RCD?  The outbuilding itself also has a number of 13A sockets in it. Sounds like not legally needed, but certainly an area I'd rather adopt the best approach rather than the legal minimum!

Kevin
 
AJJewsbury 77361768
Joined 13/08/2003 - 1433 Posts
32A radial circuits in 4mm² are fine for feeding 13A sockets (it's one of the conventional circuit arrangements) - having it directly feeding a heat pump as well is perhaps a tad unconventional, but nothing fundamentally wrong with it provided the overall load is still within 32A and 32A protection is suitable for the heat pump itself.

There'd be little advantage in having another MCB between the 32A one and the sockets, except possibly to limit possible overloads from the sockets - for any faults MCBs most likely won't discriminate so you'll just as likely find the 32A opens as well as any downstream ones - cutting everything off anyway.

Sockets these days do require 30mA RCD protection though - so if there isn't one anywhere that might be an issue. Although it's not that long ago that the requirement was only for sockets likely to supply equipment outdoors, so if the installation is a few years old, it might have been acceptable at the time.

   - Andy,
Sparkingchip 72796851
Joined 18/01/2003 - 2247 Posts
Does the circuit and sockets have 30 mA RCD protection upfront at the consumer in the house? Do they have any RCD protection at all?

Andy B.

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