Consumer units - split or dual RCD

[dal5band]

I plan to replace two old CUs - 2 X Wylex 6way, wired fuses. I have to replace the CUs with two modern ones. Should I get two units with twin RCDs ? or two units each with one RCD and a non protected section.
I would like to have the fully RCD proteced option but could this cause a lot of false trips ?
( this will be contracted out to an electrician - just planning at the moment).

Current set up CU1 - R1, R2, L1, L2, Immersion, Garage Power (original house)

CU2 - R3, R4, R5, R6, L3, Cooker (extension)

A lot of ring mains I know but I can't change the set up as the cables are in walls and under floor. Cables are all PVC.

Thanks for your thoughts

 

[PrenticeBoyofDerry]

You could consider going down the route of having all circuits protected by RCBOs then any trips would be easily diagnosed to a particular circuit, instead of taking a number out.

 

[tonyelectric]

RCBos are the way to go, maximun protection and no problem with whole house trip, prices have come down a lot too. I can honestly say that in the last 2 years and at least 100 CU replacements I havn't used anything else.

 

[ericmark]

As long as house is not on a TT supply the RCBO is the best option. However not cheapest option.
Three things cause a RCD to trip.
1) Faults.
2) Too many items with filters all which allow a small amount of leakage which builds up and causes the unit to trip.
3) Spikes on the line.
With overhead supplies spikes are more likely, although overhead lines often are TT anyway. And computers tend to have filters so where one has many computers more likely to trip.
With a fault of course it should trip so not too worried about them.

Now the next point to consider is what happens if it does trip. If all it means is you have to reset then no real problem. But if it plunges you into darkness and you have to feel ones way down the stairs and across the living room with toys on wheels and garage with stuff everywhere then it is a problem. You need to do a risk assessment. Then consider the cost of emergency lights v RCBO. There are other options. RCD's with indicators that warn likely to trip soon and even auto resetting however the latter at £350 each unless disabled not really a consideration.

 

[BSBS]

As long as house is not on a TT supply the RCBO is the best option.

Why is using RCBO's not the best option for a TT installation ??

 

[ericmark]

As long as house is not on a TT supply the RCBO is the best option.

Why is using RCBO's not the best option for a TT installation ??

Although you can get twin pole RCBO's the standard consumer unit sold in the UK will not really take them.

537.2.1.1 Every circuit shall be capable of being isolated from each of the live supply conductors. In a TN-S or TN-C-S system, it is not necessary to isolate or switch the neutral conductor where it is regarded as being reliable connected to Earth by a suitably low impedance.
Provision may be made for isolation of a group of circuits by a common means. if the service conditions allow this.

The reverse is of course true with a TT system all live conductors shall be capable of being isolated and live means both line and neutral.

I have in commercial premises used a three phase board for a single phase supply and not used third connection and used twin pole RCBO's but with domestic one needs to use a type tested board unless an electrician is living there and that limits ones options.

I see nothing wrong with a 100ma S type RCD feeding RCBO's but one would normally use RCD's feeding a number of MCB's using a stacked board one could get 4 RCD's so you have 4 circuits but other than that one would need to source a consumer unit which is designed for twin pole RCBO's would be interested to hear where you can get them in UK?

 

[ban-all-sheds]

The reverse is of course true with a TT system all live conductors shall be capable of being isolated and live means both line and neutral.

Nobody ever seemed to think that the traditional split-load board with SP MCBs was a problem - the RCD(s) could be used to do DP isolation on some or all of the circuits.

I see nothing wrong with a 100ma S type RCD feeding RCBO's

Why not a simple switch feeding RCBOs?

one would need to source a consumer unit which is designed for twin pole RCBO's would be interested to hear where you can get them in UK?

Buy a type-tested board from a manufacturer who makes 2-pole busbars, and swap them. Yes, I know you'll have "modified" it, but we need to keep a sense of perspective....

 

[plugwash]

IIRC some manufacturers do twin pole RCBOs that are connected in the same way as single pole ones (that is with live picked up from the busbar and neutral on a cable.

 

[bernardgreen]

IIRC some manufacturers do twin pole RCBOs that are connected in the same way as single pole ones (that is with live picked up from the busbar and neutral on a cable.

How can an RCBO be single pole ?. They have to have the live and neutral passing through the sensor to be able to detect the un-balance when there is a earth fault.

Maybe some only switch the Live pole but they must still have the neutral feed through them.

 

[ban-all-sheds]

It is the switching that's referred to.

 

[dal5band]

Thanks - got the idea !!!
Seems the best bet is use a split load CU which gives you the option of using RCBOs in the unprotected slots for those circuits that might give trouble.