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Are SP RCBO’s compliant on a TT system?


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#1 OFFLINE   Tony

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Posted 23 February 2013 - 11:26 PM

Don’t blame me! Bert bought this up in 1SC’s thread http://www.electrici...work-yikes-lol/

My thoughts have nothing to do with BS7671 but I would say no based on theory.

Think I'm going to have to do a drawing.

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#2 OFFLINE   Tony Re: Are SP RCBO’s compliant on a TT system?

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Posted 23 February 2013 - 11:31 PM

Will posted this in the original thread

I thought they were as long as you have DP isolation, ie at the main switch?


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#3 OFFLINE   MDJ Re: Are SP RCBO’s compliant on a TT system?

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Posted 24 February 2013 - 07:20 AM

411. something haha explains RCD protection for TT systems, I know that on the travellers sites where we do maintenance we have to have double pole RCD protection, it doesn't say that in 411 though regarding TT systems, however it may be around that area somewhere, I have clucked the 411 section from memory due to looking at it recently.

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#4 OFFLINE   SPARTYKUS Re: Are SP RCBO’s compliant on a TT system?

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Posted 24 February 2013 - 08:10 AM

Would have to look it up to remember precise wording of whats required.

Remember pondering this a few years back. Think the conclusion was that as said as long as theres a Dp isolator (as usually there is) then you're ok......

#5 OFFLINE   Tony Re: Are SP RCBO’s compliant on a TT system?

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Posted 24 February 2013 - 10:35 AM

So when a neutral earth fault is detected it’s OK to just disconnect the live?

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#6 OFFLINE   Johnboy6083 Re: Are SP RCBO’s compliant on a TT system?

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Posted 24 February 2013 - 10:49 AM

I think that an RCD main switch is a very good idea, and one I would advocate.

#7 OFFLINE   Lenny Re: Are SP RCBO’s compliant on a TT system?

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Posted 24 February 2013 - 10:54 AM

There isn't anything really in BS7671 precluding them.

WRT to isolation of "all live conductors" of a circuit in a TT system due to the supply neutral not being reliably held "at or near earth potential", obviously this can't be achieved with a single pole device but, BS 7671 does state that it can be achieved by a common device if the "service conditions" allow. I interpret this to be the main switch of the DB for instance. Now does this comply with minimising inconvenience......in my opinion....no but, "service conditions" is a VERY open term and has different meaning in different situations i.e domestic, industrial etc.

Fault protection is another issue by itself.

Although a single pole RCBO WILL operate under a N-E fault, IT WILL NOT clear that fault as it doesn't open the neutral and without any other back-up protection such as an upfront T/D RCD, the fault will remain.
By having the additional up-front T/D RCD, in the event of the SP RCBO operating but not clearing a N-E fault the up-front RCD will operate thereby disconnecting the whole of the installation. Does this comply with BS 7671 and "minimising inconvenience in the event of a fault" in my opinion...no.

In my opinion SP RCBO's are unsuitable on a TT system if they are the sole means of fault protection. Coupled with an up-front T/D RCD will make them a safer option but as I said before, doesn't comply with 314.1

Again in my opinion, the current best option for a TT system is DP RCBO's. Getting the customer to fork out for them is another matter which brings us to our dual RCD CU's. Like it or not, they fit the bill by being the most reg compliant and the most cost effective at the moment.

#8 OFFLINE   Tony Re: Are SP RCBO’s compliant on a TT system?

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Posted 24 February 2013 - 11:16 AM

OK so we fit a front end TD RCD. You may as well fit a normal RCD and chuck the RCBO’s out.
It can be reasonably be argued that a single pole automatic disconnection device is not fit for purpose.

PS I asked this question elsewhere and it went around in circles :P

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#9 OFFLINE   RoB Re: Are SP RCBO’s compliant on a TT system?

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Posted 24 February 2013 - 11:22 AM

Schneider RCBOs come with a disclaimer not to be used on a tt system.

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#10 OFFLINE   Murdoch Re: Are SP RCBO’s compliant on a TT system?

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Posted 24 February 2013 - 11:30 AM

Surely a new RCBO board in a TT situation is safer than a BS 3036 board?

We are obliged to make installations safer.

Tin hat on

#11 OFFLINE   Tony Re: Are SP RCBO’s compliant on a TT system?

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Posted 24 February 2013 - 11:53 AM

Schneider RCBOs come with a disclaimer not to be used on a tt system.


To be honest RoB I wouldn't trust a single pole RCBO on any system.

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#12 OFFLINE   Lenny Re: Are SP RCBO’s compliant on a TT system?

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Posted 24 February 2013 - 11:53 AM

Surely a new RCBO board in a TT situation is safer than a BS 3036 board?

We are obliged to make installations safer.

Tin hat on


RCBO's over fuses, yes but, if the 3036 board had an upfront RCD also, that would effectively make it safer than CU full of SP RCBO's with a simple main switch.

#13 OFFLINE   Lenny Re: Are SP RCBO’s compliant on a TT system?

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Posted 24 February 2013 - 12:23 PM

OK so we fit a front end TD RCD. You may as well fit a normal RCD and chuck the RCBO’s out.
It can be reasonably be argued that a single pole automatic disconnection device is not fit for purpose.

PS I asked this question elsewhere and it went around in circles :P


It seems fairly open & shut to me, or at least the theory behind it anyway. BS 7671 is another beast altogether though. :ph34r: :D

#14 OFFLINE   RoB Re: Are SP RCBO’s compliant on a TT system?

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Posted 24 February 2013 - 12:34 PM

Won't be long before double pole single modual rcbos are the norm.

I bet it's pushed in the next edition of bs 7671.

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#15 OFFLINE   Murdoch Re: Are SP RCBO’s compliant on a TT system?

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Posted 24 February 2013 - 12:52 PM

RCBO's over fuses, yes but, if the 3036 board had an upfront RCD also, that would effectively make it safer than CU full of SP RCBO's with a simple main switch.


There are loads round my way without an RCD in sight!

#16 OFFLINE   Lenny Re: Are SP RCBO’s compliant on a TT system?

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Posted 24 February 2013 - 01:08 PM

There are loads round my way without an RCD in sight!


3036's without an RCD on a TT system......strewth!!! :o

I'd even settle for a VOELCB over nothing at all!!!

#17 OFFLINE   MDJ Re: Are SP RCBO’s compliant on a TT system?

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Posted 24 February 2013 - 01:53 PM

The regs definately state that RCD protection to caravan sockets have to be double pole and both poles including the neutral must be broken upon tripping, it is in the caravan site section in section 7 somewhere, I know it's there but cannot confirm the reg number from the armchair here, but that doesn't suggest TT systems have to be double pole regarding RCD or RCBO protection. I think it should be though, lots of regs need clarification IMO, Another which comes to mind is local isolation for fans and showers, another grey area.

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