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Topic Title: Multiple consumer units
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Created On: 21 July 2015 11:16 AM
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 21 July 2015 11:16 AM
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Lieuwe

Posts: 8
Joined: 21 July 2015

All,

We recently moved into a new house and I would like some advice.

From the meter I have tails going into a 100A 5x2 way block.

From that I have 1 cable going through a 25A RCD device and then underground for about 50 yards to the stables where there is a consumer unit.

From that 5x2 block I have another cable leading to the main consumer unit for the house.

There is an extension to the house and what they have done is run a cable from a 32A trip in the main consumer unit to the extension where it feeds another consumer unit. This one contains 4 16A trips, 1 lighting circuit and 3 mains (they are not rings). The mains circuits are very small only containing a few sockets each, but they do run things like the washing machine, tumble dryer and some power tools.

None of the consumer units in the house are split or have any RCD protection. I have currently no problems with the whole set-up though.

What would be your advice, upgrade the units? perhaps run the second unit from the 5x2 rather than a 32A trip?

Cheers,

Lew
 21 July 2015 11:41 AM
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AJJewsbury

Posts: 15811
Joined: 13 August 2003

Nothing wrong with multiple CUs as such - although many would like to see a single main switch (isolator) before the 100A JB to provide an single point of isolation for the entire installation.

From that I have 1 cable going through a 25A RCD device and then underground for about 50 yards to the stables where there is a consumer unit.

If your "25A RCD device" is a simple RCD (i.e. an RCCD - rather than an RCBO) - so contains no overcurrent function, then you've no overload or L-N fault protection for the underground cable, possibly no overload protection either (depending on the cable size and what's downstream) - basically someone's forgotten to fuse it - which would be a BAD thing.

None of the consumer units in the house are split or have any RCD protection.

Lack of RCD additional protection is less than ideal, and I'm sure you're aware - but it's your choice whether to improve that or not. RCBOs are sometimes preferred over a split unit approach.

There is an extension to the house and what they have done is run a cable from a 32A trip in the main consumer unit to the extension where it feeds another consumer unit.

So this one has overcurrent protection at least! Running a CU from an MCB isn't ideal as there will be little or no discrimination between that MCB and the ones in the downstream CU - i.e. a fault somewhere in the extension will likely take out not just the individual circuit (e.g. 16A MCB) but the 32A one as well, blacking out the entire extension. But that's probably similar to having a split-load RCD arrangement. Definitely do NOT connect cable direct to the 100A block! The ideal solution would be something like a 63A switchfuse for the extension CU's supply cable - but there's a lot of detail to get right (cable size, earth loop impedance and so on, RCD protection if the cable is soft-skinned and concealed in a wall) so not a change to do lightly.

- Andy.
 21 July 2015 11:46 AM
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mapj1

Posts: 9346
Joined: 22 July 2004

EDIT I misread it - if the house is not on an RCD at all, then it might be better if it was - especially if the rest is a bit rats tail - one up front is better than none, and easily added.
Also should be some fuse or similar between meters and any long sub main to house.

at least

Where is the metering located in relation to the house or barn -if "indoors" this may preclude plastic boxes for new works under AMD3 ?

Apart from that ... If its all working well, and you see no problems with voltage drops dimming the lights over the long length, apart from a long walk to reset the RCD if there is a problem in the house, then it is quite possible that there is no burning need to do anything really.

Please check the RCD (or 25A RCBO - check please - a pure RCD does not protect against overload or short ) actually trips, and has not frozen - they tend to get 'sticky', especially in out building, and (I presume) you don't know the test history, also confirm that the earthing is actually continuous, and if there are electrodes what state of corrosion they are in.

More generally inspect for signs of overheating, damage by mice and so on, but if its all clean and tidy, then I'd say wait until you are refurbishing, or you need more load or there is a real problem.
It may be messy, but that doesn't always mean dangerous.

-------------------------
regards Mike


Edited: 21 July 2015 at 04:56 PM by mapj1
 21 July 2015 12:35 PM
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Lieuwe

Posts: 8
Joined: 21 July 2015

No as I said, the line to the stables has just an RCD, it has no overload protection.

Basically the 100A fuse, meter, RCD & 100A block are in the porch and the consumer unit is down the hallway (about 5 mtrs away). The second unit is another 5 mtrs away from that.

Thanks for the replies
 21 July 2015 01:43 PM
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mapj1

Posts: 9346
Joined: 22 July 2004

Hmmm. 5 metres is getting a bit long to be considered a short meter tail and suitable to be protected by the company fuse alone. Most DNOs suggest 3m max.

Sounds like a switch fuse between meter and Henley blocks is in order, and maybe that solves both the barn submain and the house one at a single stroke, assuming the barn sub main is a sensible cable size to be short circuit protected by that. Or it might be better to be independently fused down to match the cable or the RCD if the load is not well controlled (you don't say if its a 16A socket circuit and 3A of lights, or some high powered machines and a heaters.).

Do you know if earthing is TT, TNS or PME ? If TT there should also be a (time delay type) RCD at the meter end of things, while if it is PME or TNS, then a simple switch fuse would be OK, and then RCD(s) could be more usefully located at the load end of the cables they supply so long as if the cable got spiked you could rely on the fuse to protect it.

-------------------------
regards Mike
 21 July 2015 01:51 PM
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normcall

Posts: 8514
Joined: 15 January 2005

It might be better to call in your local friendly electrician for a bit of on the spot advice.
With the number of assorted permutations, it will be difficult to provide meaningful advice without seeing exactly what already exists as from the answers already provided, you may have spotted the questions are almost as numerous as proposed suggestions/solutions.

-------------------------
Norman
 21 July 2015 02:01 PM
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broadgage

Posts: 2380
Joined: 07 August 2007

From the limited information available, it does not sound immediately dangerous, but some aspects could probably do with improvement, especially lack of RCD protection and possible lack of overload protection to at least some parts of the installation.

I would agree that an approved electrician should be consulted.
 21 July 2015 02:29 PM
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Lieuwe

Posts: 8
Joined: 21 July 2015

Earth is TT, with one rod (no idea how long). The cables between the block & the CU are actually the same type as the cables from where the overhead ones attach to the outside wall to the 100A fuse (about 8 mtrs probably) .

The barn has 2 16A trips .. one with 2 sockets, the other with 3 sockets & 3 striplights. No large machinery.

I will in due course have a chat with the electrician of course.
 21 July 2015 02:48 PM
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mapj1

Posts: 9346
Joined: 22 July 2004

Earth is TT,


Ah.

Then if the house is on TT, (not just the barn) then the lack of any RCD covering the house is really much more serious - bumps it from

' in due course'

to

'please do something, like make that phone call today'

On a TT system , depending on the electrode impedance and the weather, there will almost certainly not be enough current in the event of a fault to earth to operate a fuse or circuit breaker in any respectable time, certainly not on the cables only protected by the company fuse going into the house, and quite likely even the lighting circuits and so on after the consumer unit.

Then if ever there is a live to earth fault, although your plumbing may all come live or whatever, the current will not be automatically switched off by anything, and the only clues will be a larger than expected leccy bill, odd tingles off the metal work, and the possibility of an electrocution.

On a similar line, years ago I had the (mis)fortune to deal with a defective shower where the occupant had noticed a tingle from the water leaking out of the back of the shower and down the tiles, while standing under the more or less earthed main jet as it were. When the risk was pointed out to the (admittedly rather ditzy) female , the observation that there "ooh, there could have been a pile of bodies in the shower" was made. I suggested that I thought it most unlikely that a second person will get in if the first is already dead in the shower tray, and a third even less so. Still, no accounting for tastes. No RCD there either..

-------------------------
regards Mike


Edited: 21 July 2015 at 03:05 PM by mapj1
 21 July 2015 04:12 PM
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Lieuwe

Posts: 8
Joined: 21 July 2015

Thanks Mike, that's what I suspected. The CU itself is a plastic clad one, do you think protecting the circuits with RCBO's is sufficient or would I need an upfront one (which may be handy to switch off the electricity at that point)
 21 July 2015 04:27 PM
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broadgage

Posts: 2380
Joined: 07 August 2007

I would go for an up front RCD of the 100ma time delayed sort, and then also RCD/RCBO protection of either each consumer unit for cheapness or each sub circuit for best protection against nuisance trips.
 21 July 2015 04:31 PM
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AJJewsbury

Posts: 15811
Joined: 13 August 2003

But bear in mind that solid N RCBO's don't discriminate with upstream RCDs on N-PE faults (or some L-PE faults where a load is connected)
- Andy.
 21 July 2015 04:54 PM
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mapj1

Posts: 9346
Joined: 22 July 2004

There are at least two aspects. It is highly desirable to be able to kill the power at or as near as poss to the meter, even on a non-TT system
Things like The REC2 isolator are often used for that.
This is especially true if future work is expected as it saves a lot of aggro about company fuse seals and safe working and so on.
Then the network companies (DNOs) normally like to see a fuse and a switch within 3m of the meter board, so you don't use their fuse as sole cover for a long run, like to your barn - hence my earlier link to A switch fuse

Now in addition to that, a TT needs an RCD to give automatic disconnection of earth faults (so called ADS). For a TT that is more than just a few circuits like your barn or a caravan pitch (where a simple 30mA RCD covering the whole lot is just fine, as if it all goes off together then so what ) then it is common to find one or more
Time delay RCD of 100mA or even 300mA rating, as the first thing, so that there is earth fault protection of submains etc.
Then finer protection is provided further down, in the form of instant 30mA devices for final circuits or even local to final sockets, - the idea being to give some discrimination and avoid a situation of "one-out all-out" due to a minor fault.

I'm becoming less clear what you have there, rather than more, so this is very general, not all may apply..

-------------------------
regards Mike


Edited: 21 July 2015 at 05:01 PM by mapj1
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