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Topic Title: 100ma main switch, can i put a 30ma before it
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Created On: 08 January 2009 04:51 PM
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 08 January 2009 04:51 PM
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Grandfortune

Posts: 348
Joined: 10 April 2007

HI all,

DOing some jobs on a house and the main switch is a crabtree 63amp 100ma RCCB main switch in a metal clad board, i've told them it needs changing for a 30ma rated one, but i cant get one, try'd e bay but they are not the same and the wholesaler says you cant get them anymore. thing is can i put a 30ma rcd before the 100ma one in a enclosure and run the tails there first then to the 100ma main switch. or any other options, just dont want to go to them saying i'll need another £300 odd for a new board.

CHeers Grand
 08 January 2009 04:56 PM
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AJJewsbury

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You could, but you might want to consider the effect of a single fault...

As another possibility, if RCBOs are available to fit, could you simply replace the MCBs on the non-30mA protected side?

- Andy.
 08 January 2009 05:19 PM
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chuckleelec

Posts: 56
Joined: 07 January 2009

Originally posted by: Grandfortune

HI all,



DOing some jobs on a house and the main switch is a crabtree 63amp 100ma RCCB main switch in a metal clad board, i've told them it needs changing for a 30ma rated one, but i cant get one, try'd e bay but they are not the same and the wholesaler says you cant get them anymore. thing is can i put a 30ma rcd before the 100ma one in a enclosure and run the tails there first then to the 100ma main switch. or any other options, just dont want to go to them saying i'll need another £300 odd for a new board.



CHeers Grand


I would (if I may) suggest an upgrade to a MCB/RCBO consumer unit.

I would not suggest a 30mA RCD/MAIN SWITCH at the origin due to loss of all power in the event of a fault.

The cost should be rewarded by the upgrade and increased safety.

Best wishes
 08 January 2009 05:32 PM
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jasons6930

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With 100mA main switch, would this not be TT system?

If so, i thought that the RCD needs to be in the tails before they enter the m/clad DB?
 08 January 2009 06:43 PM
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sparkingchip

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And me.

Andy Betteridge
 08 January 2009 10:41 PM
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AJJewsbury

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If so, i thought that the RCD needs to be in the tails before they enter the m/clad DB?

While that's the usual solution, some manufacturers produce an insulating kit to provide double insulation between the incoming tails and the metal box for just this situation - e.g. MK do 5802s - have a look in http://www.mkelectric.co.uk/PD...hnical/SENTRY_prod.pdf Crabtree probably do something similar.

- Andy.
 08 January 2009 10:46 PM
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Testit

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I'm not exactly clear what your view of the issue is... why are you saying to change the 100ma for a 30ma?

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Experience can sometimes show that cost prevails over quality and safety, such little self-value that people hold.
 09 January 2009 02:43 AM
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Grandfortune

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i have put some extra lighting and sockets in and want to 30ma protect them, the best solution was a simple change of the main switch, i know its not ideal to have it as the main switch but it was the cheapest solution, i did give the two options when quoting change main switch and that is was better of to change the board, i just feel stupid now asking for another £200 to £300 for a new board change, oh yeah the system is TNS
cheers
grand
 09 January 2009 04:43 AM
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Testit

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I wouldnt worry too much then... Stick the 30ma in and just put recommendation for CU upgrade on the certificate. I would check the Ze as would wonder why 100ma was there.. it may look like TNS but have high Ze and may actually be TT. Ideally you should price the 2 options at onset. I tend to ask about finances and give informed advise tailored by a subjective estimate of their means... Many installs are on 30ma main switch, units were sold as such. Nowadays its a deviation from the regs but sometimes if you explain the safety aspects of everything going off with a fault sometimes people will consider the extra outlay, but sometimes not of course..

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Online Services - http://propertydevelopment.org.uk

Experience can sometimes show that cost prevails over quality and safety, such little self-value that people hold.
 09 January 2009 06:49 AM
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normcall

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"Many installs are on 30ma main switch, units were sold as such. Nowadays its a deviation from the regs"
Whoops, I didn't realise they had been banned. Where in my big red book does it say that a single RCD on a TT installation (or any other for that matter) isn't allowed.
PS - don't quote the old chestnut otherwise we shouldn't have any protection in case it fails due to a fault and we can't have that, can we?

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Norman
 09 January 2009 07:24 AM
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Samuel Chan

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Why don't keep the old main switch w/ 100mA RCD. Then add a 32A or 20A w/ 30mA RCD for the added circuits?

Samuel Chan
 09 January 2009 12:42 PM
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Legh

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"Why don't keep the old main switch w/ 100mA RCD. Then add a 32A or 20A w/ 30mA RCD for the added circuits?"

IMO,that would probably mean running another set of tails through the CCU, from the output of the 100mA RCD to the 30mA RCD.
I think that would look messy and also require extra space.
Anyway, for a 100mA 'G' type RCD what would be the point?
Legh

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 09 January 2009 01:35 PM
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perspicacious

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"Where in my big red book does it say that a single RCD on a TT installation (or any other for that matter) isn't allowed."

314.1 would be a good starting point.

Regards

BOD
 09 January 2009 01:54 PM
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GeoffBlackwell

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314.1 would be a good starting point.


No mention of front end RCDs there - it just talks about dividing into circuits .

Regards

Geoff Blackwell
 09 January 2009 02:20 PM
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OMS

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Have I missed an amendment regarding 531.4.1?


Perhaps not, but this front end RCD could be a 300mA slugged device which would then allow you to comply with 314.1 with 100mA and/or 30mA downstream devices (Geoff's reluctance notwithstanding )

Regards

OMS

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 09 January 2009 02:26 PM
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GeoffBlackwell

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314.1 Every installation shall be divided into circuits, as necessary to, .....

No mention of fault protection here - perhaps that is somewhere else .

Well it is a boring friday afternoon init

Regards

Geoff Blackwell
 09 January 2009 02:36 PM
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OMS

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314.1 (i) specifically mentions faults (and teh requirement to consider the fault and its impact on safety and convenience.

314.1 (iv) mentions unwanted tripping as does 531.2.4.

I tend to agree with you that this particular reg is used and abused to make a case for lots of RCD's and in all honesty is probably not achievable in any commercially driven installation.

That said, the CDM regulations would place a duty on designers - subdividing an installation to mitigate against the single fault event for those parts of the installation that the designer has control over would seem to be a way of showing "reasonably practical"

Regards

OMS

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 09 January 2009 02:47 PM
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GeoffBlackwell

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314.1 (i) specifically mentions faults (and the requirement to consider the fault and its impact on safety and convenience.

314.1 (iv) mentions unwanted tripping as does 531.2.4.


Yes but the regulation is all about division of the installation into circuits - not the method of fault protection, or the device used. The fact that these things may be mentioned elsewhere is not relevant to any claim concerning the meaning of 314 - so I move for case dismissed my lud (not my fault the prosecution couldn't get the words right) .

Regards

Geoff Blackwell
 09 January 2009 02:58 PM
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OMS

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Definition of circuit any good in my defence - particularly the bit about protected from overcurrent by the same protective device.

It could be argued (and many will) that an RCD gives protection against overcurrent so a front end device essentially results in an installation comprising of one circuit and therefore not meeting the avowed intent of dividing the installation et seq.

I certainly can't disagree that the penmanship could have been at least an order of magnitude better - but if BS 7671 was clear and consice, where would be the fun

Regards

OMS

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 09 January 2009 03:03 PM
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AJJewsbury

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It could be argued (and many will)

moi?

RCD gives protection against overcurrent

Surely an RCD is an under-current device - it trips when not enough neutral current returns from the circuit!

- Andy.
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