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Topic Title: Sub main question
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Created On: 19 April 2017 01:07 PM
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 19 April 2017 01:07 PM
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goldenboy1818

Posts: 812
Joined: 22 February 2011

Hi guys went to see a job today and wanted to check something. Its a flat and the guy wants a new fuseboard but my issue is with the submain. I cannot see it run only where it is terminated, its 16mm tails from old 60amp sub main. I think where this is not swa the submain switch fuse should be a 63amp s type 30ma rcd and then change the board as normal.
 19 April 2017 02:31 PM
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OMS

Posts: 22359
Joined: 23 March 2004

An S type doesn't provide additional protection (for presumably cables buried less than 50mm deep)

Suggest you look a bit harder to determine what the tails presented to the existing consumer unit actually comprise - and what the earthing is

OMS

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Let the wind blow you, across a big floor.
 19 April 2017 06:25 PM
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goldenboy1818

Posts: 812
Joined: 22 February 2011

Seems like a 16 mm twin and earth. What would you suggest to cover sub main cable
 19 April 2017 06:51 PM
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sparkiemike

Posts: 1630
Joined: 24 January 2008

Cover the sub main with what & why?

Depending on the earthing arrangement do you think the 16mm T&E with a 6mm cpc is going to meet your bonding requirements?
 19 April 2017 06:57 PM
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OMS

Posts: 22359
Joined: 23 March 2004

Well, ideally nothing other than a fuse or circuit breaker - as in not an RCD - but the RCD is only required if the cable is unprotected and buried less than 50mm deep in a wall

If you must have RCD protection then the only compliant type would be a standard 30mA unit - and at that point any further RCD's in the replacement consumer unit become pointless as you can't get discrimination

So, if that's what you do, then it's no worse than a typical "front ender" but with the disadvantage that the flat occupier needs to get to the origin for a "reset"

The alternative is to run the submain surface or in armoured or MICC and thus not need RCD protection

Given it's a flat I wouldn't faint dead away at the thought of a single RCD - but not BS 7671 compliant generally

Note the T&E CPC might well be a touch on the skinny side for bonding purposes (depending on the earthing arrangements)

Not that easy to be compliant in these circumstances - you might b tempted to just change the consumer unit and note the deviation for the supply cable on your EICR under observations on existing installation

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 19 April 2017 07:00 PM
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weirdbeard

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Originally posted by: goldenboy1818

Seems like a 16 mm twin and earth. What would you suggest to cover sub main cable


Earthed steel conduit, and a normal off the shelf 30mA upfront RCD just after the meter, should cover most bases with regards electrical safety

-------------------------
:beer)
 19 April 2017 07:40 PM
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peteTLM

Posts: 3715
Joined: 31 March 2005

I hate t and E submains. Why oh why were they so popular in the 1980's???? Oh yeah, there cheap and rough as a badgers.....

Back in the day when the local Dno done the connection and turned a blind eye to everything for the price of a bacon sandwich.

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Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry
 19 April 2017 09:14 PM
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goldenboy1818

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Would it be a acceptable to just make a note of it ? It is for a rental property so landlord probably wouldn't want that as he wouldnt comply
 19 April 2017 09:28 PM
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OMS

Posts: 22359
Joined: 23 March 2004

Don't do it then

Me - I'd talk to the landlord about why things need to done - offer a menu

Is it safe now ? - will it be less safe when you finish performing ?

Try a bit of engineering judgement and stop taking on duties that aren't yours to own

Regards

OMS

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Let the wind blow you, across a big floor.
 19 April 2017 09:35 PM
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spinlondon

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Is there a requirement for you to provide RCD protection for a cable you did not install?
 19 April 2017 09:37 PM
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Parsley

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What have you been employed to do? If it is only to replace the consumer unit and you're happy that the T&E submain is suitable for continued use you could as OMS already suggested note the lack of mechanical or 30ma RCD protection in the comments on existing installation section of the EIC. However make sure the landlord understands the installation isn't complaint with the current BS, even with the replacement consumer unit. This should be highlighted on your quote to avoid any future misunderstandings.

Regards
 19 April 2017 11:18 PM
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mapj1

Posts: 9383
Joined: 22 July 2004

It is only non-complaint without an RCD if

it is concealed in a wall or partition at a depth of less than 50 mm from a surface of the wall partition shall:

(so surface wiring , or not a concealed route, like surface trunking is OK)

It does not
incorporate an earthed metallic covering which complies with the requirements of these Regulations for a
protective conductor of the circuit concerned, the cable complying with BS 5467, BS 6346, BS 6724, BS7846, BS EN 60702-1 or BS 8436,
(so SWA, pyro and most mains rated screened cables are OK)
and is not
(ii) enclosed in earthed conduit complying with BS EN 61386 and satisfying the requirements of these Regulations for a protective conductor,
(metal conduit would make singles or T and E OK)

(iii) enclosed in earthed trunking or ducting complying with BS EN 50085 and satisfying the requirements of these Regulations for a protective conductor,
(metal trunking would make singles or T and E OK)

and is not

(iv) mechanically protected against damage sufficient to prevent penetration of the cable by nails, screws and the like,
(so if it is behind armour plate or similar hard to drill stuff, even if not earthed, it is OK)


so actually there are quite a few ways of complying - making the route not concealed may be easiest.

-------------------------
regards Mike
 19 April 2017 11:19 PM
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goldenboy1818

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I had been asked to look at the small flat where various additions will need to be made, this meant realistically the board needed changing and upon investigation the sub main issue. Im sure he will want and need all work done to comply and cover himself
 20 April 2017 07:07 AM
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OMS

Posts: 22359
Joined: 23 March 2004

What's the means of earthing - from there you have all the methods of providing a compliant sub main

OMS

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Let the wind blow you, across a big floor.
 21 April 2017 08:06 PM
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goldenboy1818

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Its not easy to run a new sub main though
 21 April 2017 10:10 PM
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AJJewsbury

Posts: 15855
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Im sure he will want and need all work done to comply

But the sub-main cable is pre-existing, so won't compromise the legitimacy of the work you're doing.

It would be perfectly correct to just replace the CU as requested, and note the lack of additional protection to the submain cable on the certificate in the 'comments on the existing installation' box.

If the customer wants the submain bringing up to current standards then you can offer him the choice of either locating 30mA RCD protection at the source end, or replacing the cable with SWA. (For the sake of argument I wouldn't offer trying to encase the existing cable in earthed steel conduit/trunking or add mechanical protection etc as that's likely to be far more costly than a simple swap to SWA).

- Andy.
 21 April 2017 10:26 PM
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mapj1

Posts: 9383
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if its not buried in a wall or otherwise hidden, you don't need to do anything.
It complies by dint of not being concealed.
This is also how it is OK to have cables on RCDs >30mA for things like generator connections of hundreds of amps trailing over the floor at outdoor events.

-------------------------
regards Mike
 23 April 2017 08:05 PM
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goldenboy1818

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The cable in concealed so i cant see it and how its run. If i make a note of it then the landlord is going to have a flat which does not comply or more than likely not, then he may feel he is not covered by insurance. Would a 16mm twin and earth with 6mm earth be suitable on a 60amp switch fuse anyway ? will the earth be too small ?
 24 April 2017 01:50 PM
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AJJewsbury

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The cable in concealed so i cant see it and how its run. If i make a note of it then the landlord is going to have a flat which does not comply or more than likely not, then he may feel he is not covered by insurance.

Does his insurance really require him to upgrade all his flats electrical installations every time BS 7671 changes?

Would a 16mm twin and earth with 6mm earth be suitable on a 60amp switch fuse anyway ? will the earth be too small ?

It usually would be fine (there'd be little point in making cable like that otherwise) - as long as Zs (Zdb at the flat) isn't more than about 0.53 Ohms (presuming a BS 1361 fuse) it should be OK. Presuming it's not also being used as a main bonding conductor of course.

- Andy.
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