New CU Advice

[d1rty]

We are having our house rewired and the electrician has recommended using RCBOs for every circuit. I wanted some advice on whether this is overkill or whether it is sensible.

We will (eventually) have the following circuits

- 5 lighting circuits (3 floors, kitchen and security)
- 4 mains circuits (3 floors and kitchen)
- Cooker
- Fire Alarm
- Boiler/Immersion

12 RCBOs @ ~£35ea works out quite expensive, not that thats a problem if it really is the best way to do it.

Any advice would be appreciated
Chris

 

[THRIPSTER]

He is recommending that in anticipation of the new, 17th edition BS7671 - therefore probably a good move.

 

[ban-all-sheds]

Individual RCBOs for circuits which require RCD protection is a better solution than the traditional single RCD covering all the circuits, as a fault will only take out the affected circuit, not all of them. However it is expensive, and with some CU makes needs a bigger CU as RCBOs can be wider.

BUT - it's not usual to put RCD protection on every circuit - in your list I'd only put them on the 4 socket circuits. Any idea why your electrician wants to put them on everything? (Do you have a TT supply?)

What did the other people you got to quote propose to do?

 

[JohnD]

It is usual NOT to put the fire alarm on an RCD or RCBO as this increases the chance that it will not work when needed.

I suppose if you have a good back-up battery, and it alerts you in case of power failure, it is not so bad.

One of the addvantages of the RCBO route is that it is not much trouble to later convert an existing circuit from MCB to RCBO. Personally I would not have put the lights or freezer on one either.

If the Immersion Heater is on one, then it will eventually start to trip (immersions and electric cookers are prone to minor earth leakage) but I suppose that will just prompt you to get a new immersion heater fitted.

I think RCBOs are wonderful, but ask the electrician to talk you through the reasons for putting everything on them. £35 is not a bad price, what brand is this?

Remember to test then all once a month (or at least quarterly) and keep the test dates and results written on a chart. RCDs and RCBOs that are not regularly tested are much more likely to stick closed and fail to trip under fault conditions

p.s. It is an advantage to have the Immersion and the Boiler on separate circuits. Then if one fails you can still use the other. One day you will be glad of it.

 

[GaryMo]

BUT - it's not usual to put RCD protection on every circuit - in your list I'd only put them on the 4 socket circuits. Any idea why your electrician wants to put them on everything?

Probably future proofing (17th BS7671) as all circuits which have cabling at a depth of less than 50mm will require RCD protection unless it's mechanically protected. This will pretty much mean every circuit.

If you don't want fire alarms RCD protecting then either install mains cable deeper than 50mm, mechanically protect it if less than 50mm or run it in mini-trunking.

 

[d1rty]

Thanks for the replies so far, all very helpful!

Do you have a TT supply?

Sorry, I don't know what that is, can you explain or point me in the right direction to find out?

If the Immersion Heater is on one, then it will eventually start to trip (immersions and electric cookers are prone to minor earth leakage) but I suppose that will just prompt you to get a new immersion heater fitted.

So would it be sensible to use a standard MCB for these circuits, and maybe for the fire alarm also as GaryMo suggests? Can i run my fridge on the same supply as cooker to avoid having it on the RCBO, or is this a bad idea?

It is an advantage to have the Immersion and the Boiler on separate circuits.

Yep the boiler will be on the kitchen ring, the immersion will have its own supply.

All the new wiring will either be under floor boards or in cavity walls, theres very little that i can think will be chased into walls. Will the new BS7671 regs have any affect on me?

Cheers, Chris

 

[JohnD]

Thanks for the replies so far, all very helpful!

Do you have a TT supply?

Sorry, I don't know what that is, can you explain or point me in the right direction to find out?

Have a you got an earth Spike or a Supplier's earth? Have a look round your Supply Head, Main Fuse and cables to your consumer unit. Can you see a Green & Yellow cable, as thick as a pencil? Where does it go?

If the Immersion Heater is on one, then it will eventually start to trip (immersions and electric cookers are prone to minor earth leakage) but I suppose that will just prompt you to get a new immersion heater fitted.

So would it be sensible to use a standard MCB for these circuits, and maybe for the fire alarm also as GaryMo suggests? Can i run my fridge on the same supply as cooker to avoid having it on the RCBO, or is this a bad idea?

Not impossible, but you'd need extra fusing down. If you're splashing out on a top-quality rewire, have a 20A radial Circuit provided for Freezer and Fridge. The sockets (but not the switch) should be inaccessible or non-standard to prevent you plugging your lawn-mower in to the non-RCD circuit.
Or, like me, have a cooker radial at each side of the kitchen, so you can remodel it at whim, and use one for the freezer with a 10A fuse

p.s. re "TT Supply" see "Earthing Arrangements" on
http://www.diynot.com/forums/viewtopic.php?p=141310#141310

 

[d1rty]

Have a you got an earth Spike or a Supplier's earth

The earth is connected to the sheath of the supply cable coming into the building. We're having the location of the cable moved by the power company and the electrician told me to get the earth upgraded, but i forget the name that he used to describe the type we should have - possibly PME?

Thanks for the info, i'll be sure to discuss it all with my electrician before starting anything.

Cheers, Chris

 

[GaryMo]

Sounds like TNS.

 

[ban-all-sheds]

If it's TN-S, why should the DNO change it to TN-C-S?

Unless he knows that the sheath is really flaky?

 

[luminaire]

Also you could do with some spare MCB slots for any additions you might have in the future.