How to install RCBO in Hager CU

[snzero]

Hi,

I currently have a Hager CU with just a MCB for our shower circuit. I need to replace the MCB with a RCBO (Hager ADN140) as I am installing a new shower and I have no RCD on the shower circuit at all.

This is a photo of our CU as it stands now:

Hager CU

• snzero
• 9 Jan 2010
• 1

This is how I imagine the RCBO will connect up:

1. Connect "Supply Neutral" fly lead to N busbar
2. Connect "Functional Earth" fly lead to E busbar
3. Connect Live from current MCB to Live on new RCBO
4. Connect new N cable from N terminal on RCBO to N busbar.

Does anyone on here see an issue with this? I have never installed a RCBO before so I just want to check I'm not going to do anything that will cause problems.

Thanks,
Wayne

 

[ColJack]

4 = NO!!!

the neutral for the circuit needs to come out of the neutral block and be put in the RCBO N terminal..

 

[mfarrow]

Everything looks fine except number 4.

You should disconnect the neutral for the shower circuit from neutral busbar and connect that to the neutral terminal on the RCBO.

EDIT: Too slow!

 

[snzero]

Thanks to you both!

It's a good job I asked on here first. It all makes perfect sense now.

 

[conny]

While you are at it I think you need to re-make those live feeds which have bare copper showing.

Don't forget to isolate first!

 

[Chri5]

Unless my eyes deceive me the current shower circuit is rated at 32a and is either the furthest RHS circuit or the 2nd furthest RHS circuit.

So are you replacing the 32a MCB with a 32a RCBO /30ma B and are you certain the new shower will run at less than 7.3kw (32 x 230v).

You may need to confirm shower rating and then consider if the cable in situ can cope with a large rated RCBO if the new shower needs more than 7.3kw

 

[mfarrow]

While you are at it I think you need to re-make those live feeds which have bare copper showing.

I think I'd leave them well alone - unless you've got an isolator at your supply origin just make sure you steer well away from them. Don't be tempted to pull the DNO's fuse.

 

[Spark123]

The RCBO you have quoted is a 40A one, what size is the shower cable, how long is it and how is it installed?
Is the isolator switch large enough?
What size shower?

 

[snzero]

Hi,

It's a 9.5kw shower. The twin and earth is 6mm which the manual for the shower says is adequate. The length of the cable is about 15 metres.

The isolator switch is a 50amps.

Will this be ok?

 

[kai]

On a 9.5kw circuit, 6mm cable will only just cope with the loading, if its mounted on perforated cable tray throughout and it won't look nice as a result. Run 10mm cable in white plastic trunking (the 50 by 50 mm size will be adequate).

 

[snzero]

I was hoping you wouldnt say that...

The cable from the CU disappears in to the wall and then appears in the loft so might be a bit tricky to replace. Would it be safe to disconnect the 6mm at the CU and attach it to some new 10mm and then pull it through?? I'm a bit concerned as there are a few cable bunched together and I dont want to disturb them. It would be even trickier to do a brand new run without re-decorating the living room!

Also, as I will be doing this myself I will also need to inform the building regs department at the council will I not? Will they need to come out and inspect the work once its done and if they do how much will that cost me?

Sorry for all the questions - it looks like a simple job of replacing an electric shower is just getting more and more complex!

 

[snzero]

How about if we exchange the shower for an 8.5kw one? Would that be ok on 6mm cable?

I also overestimated the length of the cable from the CU to the shower. It's approx. 6m in total to the isolator switch.

The cable from the CU runs up a concealed channel with other cables up to the loft. It then runs straight for couple of metres to the ceiling mounted isolator swich in the bathroom. The run from the isolator switch to the shower itself is about 1.5 metres.

 

[kai]

Maybe if the 6mm cable was not grouped with other cables, it would have worked with the 8.5. kW shower, but it's safer to abandon the 6mm, and swap it with a 10mm instead, due to Grouping factors etc.

 

[bongos]

How many other cables? 6mm² is rated as high as 47A, before derating takes effect.

6mm² will possibly be ok with a shower rated at 9.5kW (@240V) on a 40A RCBO.

10mm² will be better though...

 

[snzero]

Thanks for all your help.

I have decided to get an electrician in to change the 6mm for 10mm. Also, i have since found out that the CU had been moved by the previous owners and all the cables have joins in them somewhere (white sheath on cables leaving CU and grey sheath when they re-appear under floorboards and in loft).

I would rather make sure this was all safe so I am calling in an electrician to take a look.

Thanks again!