17th Edition

[Oracle]

Hello,

I have been asked to fit a secondary CU for an extension that has been built - 1 ring, 1 light.

Client does not want existing non RCD CU touching.

They only want a small new CU (just for extension)

Would you just fit something like a 4 or 6 way, with an RCD as main switch?

Or CU with switch, RCD and MCBs?

Or I know the most expensive way is CU with main switch and RCBO's (but why is the above is sufficient)?

Thanks.

 

[NozSpark]

New ISO CU & RCBO's for me

Also check PEB's..

 

[SLIPSHOD & SLAPDASH]

I am carrying out a similar job at present and would have fitted a small C/unit with Rcd main switch and 2 breakers, but Building control wants smokes fitted to the house so this ruled out one Rcd. BS5839 ( Fire alarm regs) state that fire alarms or smokes must not be connected to an Rcd that also provides power to socket outlets. I do see this a lot though whist carrying out Pir's and code it 2.

 

[NozSpark]

Should you really be coding that as a 2!!

That's a BS5839 reg not a BS7671 reg; and as you're testing to BS7671:2008 it isn't a non compliance!!

 

[SLIPSHOD & SLAPDASH]

It was an Niceic assesor that pointed out to me as its connected to the electrical supply this still stands. My view is this is correct, other may not. One of those 50-50 debates. I am sure you would agree that as this thread is about a new install its valid here, and as such why not should you also expect it on all installs ?

 

[Andy™]

dont know 5839 regs much, but doesnt 7671 say something about equipment must be installed to appropriate BS, in which case, if its known that the smoke alarm doesnt comply with 5839, then surely it cant comply with 7671, and should be noted on PIR

but then it depends on what you think the regs mean

 

[SLIPSHOD & SLAPDASH]

dont know 5839 regs much, but doesnt 7671 say something about equipment must be installed to appropriate BS, in which case, if its known that the smoke alarm doesnt comply with 5839, then surely it cant comply with 7671, and should be noted on PIR/QUOTE]

Good point Applaud Smiley That would have saved me typing out the above post

 

[binky]

Thought the general consensus on domestic electrics is wire smoke to regularly used light circuit (AICO put out a statement to this effect about 6 months ago). Unfortunately that almost certainly means sharing a RCD with a socket circuit given dual RCD CU is pretty much standard now. If fitting fire panel type system iaw 5389 then would give it its own RCBO.

 

[Andy™]

but is it really that much of a bad idea having a smoke on the same circuit as sockets or lights? if its on its own redicated radial, and there is a fault tripping RCBO, it wont be noticed or repaired as soon as if it was sharing a more used circuit - if you go not lights/sockets most people will get that repaired soon as

 

[SLIPSHOD & SLAPDASH]

binky & andy, your missing the point. Yes i agree that its better to have the Domestic smokes 'connected to a commonly used lighting circuit' the point is it doesn't say '& sockets' which would happen if you use a common Rcd. Lights rarely trip Rcd's , but socket circuits do. As i have said loads of time this should not be down to money. High integrity boards are available for this reason. A Rcbo costs as little as

 

[betty swollocks]

Lights rarely trip Rcd's ,

until a bulb pops....

i get my dual rcd cu's for

 

[Andy™]

Lights rarely trip Rcd's ,until a bulb pops....

that would be MCB that trips. and if a lamp failing trips an MCB, then you have designed the system incorrectly (unless it has to be B type for EFLI). use a C6 and your done

 

[Andy™]

As i have said loads of time this should not be down to money. High integrity boards are available for this reason. A Rcbo costs as little as

 

[Oracle]

So a small CU with one RCD as main switch and two MCBs?

Oracle.

 

[SLIPSHOD & SLAPDASH]

Possibly ; \

 

[batty]

Surely if you give customer options and prices they can make there own minds up and if they go for the cheaper option and things go wrong at least you can say I told you so.

Batty

 

[SLIPSHOD & SLAPDASH]

So a small CU with one RCD as main switch and two MCBs? Oracle.

I suppose the cheapest option is a 'garage' C/unit with Rcd main switch.

The best option is seperate Rcbo's for each circuit. Your choice.

 

[Steve3948]

Health and Safety issue.

If RCD is used as main switch to CU you will lose both Sockets and Lighting in the event on of a fault on either.

So for me it would be two RCBOS.

 

[Oracle]

Thanks guys

On a related note....

If running armed from the cut out to a CU - distance less than 2m.... what's the consensus on terminating at the cutout end (won't be using a fused isolator)?

1) Terminate into a plastic box with henley's inside (that way you maintain double insultation on the SWA cores) and from the henley's in tails to the cutout?

Or simply terminate into a box (just something to terminate the SWA into) and run the cores straight out to the cutout or even an L shaped bracket to hold the SWA gland?

Obviously on both of the above, the actual armoured would be tagged and linked into the earth.

The service box is classed as an enclosure, and requires a tool to open it - So does it require the henley's to be in an inner plastic box?

Oracle (by name, not by nature)

 

[Andy™]

Why do you want to use SWA for 2mtr? split con can be used if you can get it. if its to avoid RCD's, you could terminate into enclosure and DI tails into enclosure to henley