RCDs and the 17th

[Reg Man]

Just wondered what everyones thoughts were on this hyperthetical situation which has been bugging me:

If you were to install a ring circuit in a new extension using T+E in 20mm pvc conduit buried in wall 30mm deep etc. The house has a TNS supply, a Wylex fuseboard with cartridge type fuses with no RCDs at all and everything else in the build is in a good state.
What do you do about the RCD protection?

Tell customer they must have new RCD/RCD board even though there is nothing wrong or unsafe with current board?

Install 30mA RCD before main switch over whole system thus possibly introducing a nuisance tripping problem?

A sub circuit with small CCU?

Or what?

Its not a catch, the reason I'm asking is because be it whether its a ring circuit, one additional socket or moving a switch, the cost implications for a really minor job to the customer could be drastically different and see at the minute even more so, Joe Public are very wary when it comes to money.

So how have you all been tackling the situation, Or maybe NOT!????

 

[tony.towa]

My thoughts are that the requirement for the RCD will be on the new final ring circuit only. Assuming a suitable feed can be taken from the cu I would install a 32amp 30mA RCBO in a small plastic enclosure from which the final ring circuit would be run. The extent of certification would be the new final ring circuit.

That's my thoughts anyway, anybody else got any suggestions.

 

[Reg Man]

My thoughts are that the requirement for the RCD will be on the new final ring circuit only. Assuming a suitable feed can be taken from the cu I would install a 32amp 30mA RCBO in a small plastic enclosure from which the final ring circuit would be run. The extent of certification would be the new final ring circuit.

That's my thoughts anyway, anybody else got any suggestions.

Like your thinking. No spare ways though. Not allowed to double up. Can't get the feed. Not so easy without getting into more expense.

 

[PAUL M]

fit a seperate ccu for extension with rcd /rcbo protection.

 

[tony.towa]

Can you get to the outgoing side of the incomer?

 

[Reg Man]

fit a seperate ccu for extension with rcd /rcbo protection.

Has to be the simplest way I think. Trouble is these bloody customers just think your trying to rip them off when you tell them they have to have another board. Interesting to see how everyone would tackle it though.

Can you get to the outgoing side of the incomer?

What cable would you use? Cos it would have to be capable of carrying current rated to the main cut out fuse wouldn't it?

 

[tony.towa]

Realistically from the outgoing side (i.e. busbar side) you would have a maximum current draw of 32amps (RCBO rating) so could you use 6mm2 short tails into the stand alone unit through coupling and bushes.

Not sure whether it would be called good practice but would be interested to hear comments.

 

[Carter]

Extend existing ring and employ socket o/lets incorporating RCDs? Radial from RCD equipped FCU on existing ring but either option means an extension of or addition to the existing installation and so would therefore immediately throw up the issue of additional protection being required? But if you've no spare ways you can't run a seperate circuit anyway because although there's nothing in sec.526 (Electrical connections) mentioning the practice of doubling up 314.4 requires that....

...each final cct sall be connected to a seperate way in a distribution board. The wiring of each final cct shall be electrically seperate from that of every other cct, so as to prevent the indirect energizing of a final circuit intended to be isolated.

so no doubling up at the board. Gotta be recommending CU upgrade to something consistent with modern domestic requirements not those of the 1950s.
Sub-board to supply 4 ways with RCD protection for ring/up, ring/down, ring/conservatory as the intermediate cost option.

 

[tony.towa]

Hi Carter,

Hoped you would look in on this one.
A genuine question, if you took up your sub board option where would you take the feed to the sub board from?

 

[Reg Man]

Realistically from the outgoing side (i.e. busbar side) you would have a maximum current draw of 32amps (RCBO rating) so could you use 6mm2 short tails into the stand alone unit through coupling and bushes.

Not sure whether it would be called good practice but would be interested to hear comments.

See where you are with this one but would not the busbar side be on the main cut out fuse, with up to 100A protection?. Wouldn't the feed to the RCBO have to be done with 16mm tails min??

Hi Carter,

Hoped you would look in on this one.
A genuine question, if you took up your sub board option where would you take the feed to the sub board from?

The more I have thought about this now, the sub board does seem best option. Remove tag, pull main fuse, Split the tails, install service block and run new tails to say a 4way board with RCD or RCBO.

 

[Carter]

Hi Carter,

Hoped you would look in on this one.
A genuine question, if you took up your sub board option where would you take the feed to the sub board from?

From the ways made available from re-routing the tails of the existing ring cct/s (how many) which will end up in the new sub-board thus freeing up at least one/two ways. Or am I missing something?? Only issue would be ensuring that there is enough slack and wiggle room to make them into the new board, if not it might mean replacing one or two of the outgoing/return legs to gain that length to allow easy termination and to keep the routing and dressing nice and tidy.
This being theoretical I go on the number of old Wylex boards I've seen from which cables radiate in all directions so moving circuits isn't so straitforward as when everything exits from the top and you can usually just shunt them across. A lot depends on how its all laid out and how cramped for mounting space it is, sometimes the understairs board is so cramped that this is a real trade off between aesthetics and level of ballache expected when deciding on a mounting position.

 

[tony.towa]

Cheers Carter, I thought that might be where you were coming from just with an old Wylex board with fuses it would most probably mean removing the carrier and as you say a direct connection, assuming the inherent "rat's nest" will allow. I think Regs man has come up with the easiest way splitting the tails with a Henley block and then feeding the sub board.

Let's just hope that the seals are missing!!!