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Topic Title: Dual RCD plus RCBO consumer unit
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Created On: 27 July 2015 11:11 AM
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 29 August 2015 12:41 PM
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leckie

Posts: 4389
Joined: 21 November 2008

Originally posted by: alancapon

Originally posted by: leckie

. . . Alan Capon has said that his employers, a DNO, would not connect such an installation because they require a method of neutral isolation by an RCD due to the possibility of a neutral to earth high impedance allowing a high voltage to be present within the installation should there be a fault. I agree with Alan. I don't think it does say this in BS7671, but because an SP RCBO will disconnect the circuit supply, an N to E fault still remains because the neutral is not disconnected, and in a TT system that is bad news. . .


Almost. With a TT presentation, you are going to have a relatively high impedance earth by design. The DNO need to get their earth rod on the transformer neutral down to 20 ohms or less, so the earth rod at a property will not be lower than the DNOs figure. In most cases, the reason for the neutral to earth voltage is due to a phase to earth fault either within the suppliers network, or more likely, a nearby property with a faulty / missing RCD allowing the fault to remain connected to the system.



. . . So good and sensible practice would be to fit either DP RCBO's so the fault is isolated or minimally a main incoming S type RCD, which does not necessarily have to be 30mA for ADS. So if good design should take precedence over cost then instead of installing all SP RCBO's with a DP switch as an isolator, fit DP RCBO's, then there a no nasty N - E voltages left on the system. . .


There is one more thing to consider in your design, particularly for a TT supply. If all you have is a double pole main switch, then RCBOs, the piece of busbar that supplies the phase connection to the RCBOs is not protected, and usually not insulated. Often, there is a shield that limits the ability to touch the busbar with the cover of the CU removed, but these are often open to the rear, allowing bare un-sleeved earth conductors from T&E cable the opportunity to make contact with the live busbar.



Regards,



Alan.


On the first piont, I thought that is what I said Alan? It's what I meant, probably my writing is not clear enough.

Good point about the busbar, I must admit I hadn't thought of that. The busbar I use when fitting DP devices is in fact insulated both sides, but it's a good point. I'm pretty sure the Hager busbars are fully insulated, but as an example control gear are not.
 29 August 2015 01:26 PM
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GeoffBlackwell

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Joined: 18 January 2003

Leckie said

Good point about the busbar, I must admit I hadn't thought of that. The busbar I use when fitting DP devices is in fact insulated both sides, but it's a good point. I'm pretty sure the Hager busbars are fully insulated, but as an example control gear are not.


When the then Electrical Safety Council first indicated that they would support the use of a consumer unit with either dual RCDs or all RCBOs without a front end device on T.T. I contacted them and expressed my concern that a fault within the consumer unit on the supply side to the RCDs could go undetected - and worse of all - might impress the supply line voltage on all extraneous- and exposed- conductive-parts via the earth bar.

They said they would consider my input and I eventually heard from them - their comment was that the manufacturers would ensure that this would be prevented by making the parts between the source and the input side of the RCD(s) 'Class II'. The quotes are deliberate as there was no mention of any testing to confirm that standards had been met.

I had in mind some of the, supposedly prototype, boards I had seen in which the flexible conductors used from the main switch to the input side of a dp RCD had a significant amount of exposed uninsulated live conductor at the termination. I could foresee that if a cpc got pressed against it and if the sleeving moved - bingo - bodies everywhere .

They disagreed and now we are where we are.

Regards

Geoff Blackwell
 29 August 2015 02:45 PM
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leckie

Posts: 4389
Joined: 21 November 2008

You see Geoff, you think of everything!

Now I've just got to get my fish pie out the oven, Mrs Leckie is visiting her mum and dad so I'm trying to be domesticated. In the words of Ringo Starr, it don't come easy
 29 August 2015 02:58 PM
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GeoffBlackwell

Posts: 3794
Joined: 18 January 2003

Fish pie - sounds good to me .

Regards

Geoff Blackwell
 29 August 2015 03:00 PM
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leckie

Posts: 4389
Joined: 21 November 2008

Right, back from the pie....

After reading the points by Alan and Geoff I have decided that when I do a TT job, not very often, I will do what I have always done in metal DB's, and fit an S type in an insulated enclosure before the DB. The only time I can recall not doing this was when fitting an insulated board with all DP RCBO's inside it. It was Hager, so it would have a fully insulated busbar. Still the earth bar concern,etc, but I will have to rely on my, hopefully good, installation practices!
 29 August 2015 03:45 PM
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Grobbyman

Posts: 366
Joined: 14 August 2005

IET on site guide 2.2.6 (a) (b) (c).
(a) class II metal consumer unit. Anyone seen one?.
(b) All rcbo. No mention of upfront 100ma S type main switch!.
(C) Split, class I with upfront 100ma s type main switch.

Around here TT is pretty normal.
So old wylex metal boards with a 100ma RCD protecting everything if you are lucky.

regards
George
 30 August 2015 12:16 AM
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mapj1

Posts: 9553
Joined: 22 July 2004

A paper with the Biegelmeier and Lee curve
may be of some interest. Not really new - original in the late 70s
The other half of the great unknown of course is the combinatioin pf skin resisitance and contact area in an accidental situation. Suffice to say almost never enough to blow a fuse - and for a large number of 'lawnmower lead' type accidents, the return path is like a TT one, aty least in terms of high impednace, even if the supply is presented as TN.

-------------------------
regards Mike
 20 October 2015 05:17 PM
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sunnymudhar

Posts: 10
Joined: 30 June 2015

Can I summarise and ask then that where from the meter I have an instance where there is a RCD already installed from which tails go to an old CU which I want to replace with a metal CU which will be all RCBOs in a TT system will be just dandy?
 20 October 2015 06:43 PM
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AJJewsbury

Posts: 15990
Joined: 13 August 2003

It depends on the detail of the existing RCCB - to discriminate with the 30mA RCBOs you'd need a 100mA time delayed or better. If the existing isn't time delayed (S-type) or is less than 100mA, it will be somewhat less than dandy. The metallic nature of the CU wouldn't be a worry though.
- Andy.
IET » Wiring and the regulations » Dual RCD plus RCBO consumer unit

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