Prefered RCD arrangement for TT these days

[Adam_151]

Basically my question is as per the title

At first it sounds clear cut... plastic cased CU with dual RCD and no none RCD ways, but someone on here expressed doubts about this arrangement in the past, bringing up the possibility of a fault between the internal wiring of the CU and a CPC ... seems a remote possibilty though, as long as the board is tidy and the CPC sleeving is correctly fitted (rather than leaving half hanging out)

Obviously RCBOs are out... SP ones no good on TT as a fault could knock it into a dodgy hybrid TNCS and not get disconnected, and anyway they are too expensive for a standard domestic

 

[GaryMo]

I read a post somewhere suggesting the use of busbars with reinforced insulation when fitting an all insulated dual 30mA protected consumer unit.

The only boards I fit are Wylex and their busbars certainly wouldn't come into that category.

 

[Adam_151]

The busbar is not a problem, both busbars will be RCD protected, its the tri-rated links between the iso and the RCDS which are not RCD protected and thus my concern

 

[electronicsuk]

The busbar is not a problem, both busbars will be RCD protected, its the tri-rated links between the iso and the RCDS which are not RCD protected and thus my concern

No matter how you go about it, there will always be a small unprotected section of the installation. I can see your point, but where do you stop with this? I would imagine a fault with an RCD causing it not to operate under fault conditions is more likely than the scenario you propose, yet we don't install multiple RCDs inline to provide redundancy.

 

[NotHimAgain]

I raised the question.

It originally came about because of some advice given by the Electrical Safety Council (ESC) that suggested that a 'front end' RCD was no longer needed provided all circuits were protected by split bar RCDs or RCBOs.

There is nothing wrong with this in principle, but there is a risk if a fault between a line conductor and earth occurs upstream of the RCDs - this would not be detected and could cause all exposed-conductive-parts and bonded extraneous-conductive-parts to become live with respect to earth.

This situation was first addressed years ago when a contractor installed a large number of metal clad domestic distribution boards on TT systems. There is a risk of a fault between the incoming tails and the metal clad CU causing the same problem as above. This resulted in a change to the Wiring Regulations [531.4.1]. A particular solution was developed by one manufacturer involving 'insulating kits' being retro fitted. This was later adopted by others and these kits were, and might still be, supplied for new works.

The problem with an insulated CU has a similar result but a slightly different cause. I identified it after looking at some of the early so called "17th Edition" CUs. There was (and still is with some) a real risk that a cpc could make contact with an unprotected connection.

This risk can largely be eliminated by good workmanship and designing out points of contact.

I raised this issue with the ESC and following a meeting of the great and the good, they decided to modify their advice.

Go here - http://www.esc.org.uk/forum/forum_nd_thread.html - and look for NDQ 14 & NDA 14.

EDIT - text added
The change they have made is to include the comments about 'all insulated construction'. IMO this is not very clear, but that is what they decided.

If you want more comfort you could ask a particular manufacturer for confirmation that their unit is suitable for this use.

 

[bernardgreen]

This is soemthing I have been thinking about.

In my opinion there is a significant improvement to safety by having a means to automatical cut incoming power if the CPC of the installation rises more than 50 volts above true ground.

Safety of life matter such as the outside water tap bonded to the CPC being at a lethal voltage to a person on the ground. In properties where two houses have metal pipe work connecting them a fault that raises the CPC to a high voltage in one will affect both houses

Hum and other disturbances to telephone wiring and cable services coming to the house.

So I would always want a voltage operated isolator on a TT supply. I have also considered it a worthwhile addition to those PME systems where the network neutral is at risk of being disconnected leaving the CPC at the mercy of the the phase unbalance in the area. ( overhead supplies for example )

 

[ericmark]

From ESC
NDQ14
Is an RCD main switch (such as a 100 mA time-delayed device) still required in the consumer unit of a new domestic installation forming part of a TT system?
NDA14
For a domestic installation complying with the 17th Edition where all the final circuits are RCD-protected, an RCD main switch is no longer required, provided that the consumer unit is of all insulated construction.
314.1 Every installation shall be divided into circuits, as necessary, to:

(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit.
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation.

531.2.4 An RCD shall be so selected and the electrical circuits so subdivided that any protective conductor current which may be expected to occur during normal operation of the connected load(s) will be unlikely to cause unnecessary tripping of the device.

Both bring into dispute if even two 30ma RCD’s will be allowed. Leakage from EMC filters do add up and although back in 1990 when I fitted my twin RCD protection tripping was rare as years have gone on this is no longer the case.

As already pointed out it would be nice if the RCD opened all poles but looking through BS7671:2008 only marinas need all pole RCD’s for socket outlets. Or IT systems which are not allowed anyway.

422.3.13 Except as permitted by Regulation 537.1.2. every circuit shall be provided with a means of isolation from all live supply conductors by a linked switch or a linked circuit-breaker.

Only refers to protection against thermal effects so I can’t find anything which prevents the use of single pole RCBO’s even though I don’t like the idea.

Personally I feel to use just two RCD’s needs a risk assessment and one needs to document why you have selected that method. Boats and Caravans often only have one RCD or if two they are in series. So there is nothing that says you must have more than one or less than one per circuit and to try to simplify the regulations to say that is rather a blinkered approach.

He who designs the system should do just that. And the “This is what I always do” approach is no longer good enough.

 

[bernardgreen]

Both bring into dispute if even two 30ma RCD’s will be allowed. Leakage from EMC filters do add up and although back in 1990 when I fitted my twin RCD protection tripping was rare as years have gone on this is no longer the case.

Back in 1981 when we had two RCDs the meter installer refused to connect our new supply. "" Two RCDs are not permitted "" A phone call to the engineer explaining one RCD for the building site tools and a separate one for the caravan meant we would still have lights in the caravan for first aid etc if an accident on site had tripped the site RCD.

Meter fitter returned the next day to connect us.

We now have 4 RCDs Two for the house in general. One for the washing machine and sewage pump and one for the garage.