DIY Posted Question for You All

[Paul.M]

Ok a DIYer posted this up yesterday and I think it is a good talking point for us lot.


"Hello,

I have an 8.5kw electric shower I am replacing like for like. Upon reading the instructions the manufacturer stated the need for a RCD. The only RCD device I have in my system (2 bed house) is a 63a main RCD. This supplies all MCB's including the 40A shower MCB.

Is this adequate protection?

I know this isn't work that should be carried out by an unqualified person, however I have 8+ years experience as an electrical engineer so feel I can undertake this safely. I'm just not up to scratch with domestic wiring regs.

Thank you in advance"



The TS Mentors have already made their points on that thread and there is a lot to consider for us Electricians. Home owners normally only see a box on the wall that we change "like for like" BUT what do we have to do as professionals to undertake this "like for like" swap?????

Have fun and lets see how many points you can pick up on????

 

[rolyberkin]

Why do I feel like I am putting my head above a parapet!?;-) Left my onsite guide on my desk at work but here goes anyway!

1. The work is notifiable to the local authority or via Part P approved contractor as it is an addition/alteration to an existing circuit in a special location. Simplified in April 2013, so that now regulation 12(6A) of the Building Regulations 2010 identifies notifiable work as comprising the installation of a new or replacement consumer unit, the installation of a complete new circuit connected to the consumer unit, or alteration work in and around a bath or shower.
Could argue that it could be done D.I.Y with notification to the LBA and payment of the fee and an agreed inspection process ( but probably not economically viable!)

2. We need to know what size the existing cable is and that it has a suitable double pole isolation switch, how far is the cable run?, I left my on site guide at work but I would specify 10mm, it is more suitable for that rating shower and future proofs the cable run for any future unit. 6mm would in my view be insufficient and a false economy.

3. The consumer unit looks to be under specified at 63amp, should be 80 or 100 to run the whole house and an 8.5kw shower, could run a separate shower connection unit or replace board.

4. What is the ma rating of the existing RCD, is the manufacturer specifying a RCD specific to that that circuit, if so one should be fitted. A shower circuit needs to be protected by a 30ma RCD (I think I recall). I personally would be protecting with a 40amp 30ma RCBO in any case. That way I would have complied with the manufacturers specification for an RCD protected circuit and the onsite guide.

5. All wiring should be compliant with BS 7671.

Go easy on me!

 

[Richard Burns]

One input given, well done!
Always hard to be first.
Does anyone else have their thoughts or perhaps expansions, contractions, or other considerations of the first response.
I will not comment as yet.

 

[Marvo]

Can I also make a comment as someone who's not a qualified sparky?

If the existing RCD is 30mA and effectively a main circuit breaker for the entire installation (assuming it's not a 100mA upfront addition to a TT system with inadequate Ze to achieve disconnect times) how would you achieve discrimination if you installed a 30mA RCBO for the shower circuit?

 

[rolyberkin]

Can I also make a comment as someone who's not a qualified sparky?

If the existing RCD is 30mA and effectively a main circuit breaker for the entire installation (assuming it's not a 100mA upfront addition to a TT system with inadequate Ze to achieve disconnect times) how would you achieve discrimination if you installed a 30mA RCBO for the shower circuit?

Yep good point, unfortunately hadn't got that far in my studies yet! interesting though, you learn something every day.

 

[Resu]

Can I also make a comment as someone who's not a qualified sparky?

If the existing RCD is 30mA and effectively a main circuit breaker for the entire installation (assuming it's not a 100mA upfront addition to a TT system with inadequate Ze to achieve disconnect times) how would you achieve discrimination if you installed a 30mA RCBO for the shower circuit?

By changing the upstream RCD to a time delayed one.

 

[Resu]

Why do I feel like I am putting my head above a parapet!?;-) Left my onsite guide on my desk at work but here goes anyway!

1. The work is notifiable to the local authority or via Part P approved contractor as it is an addition/alteration to an existing circuit in a special location. Simplified in April 2013, so that now regulation 12(6A) of the Building Regulations 2010 identifies notifiable work as comprising the installation of a new or replacement consumer unit, the installation of a complete new circuit connected to the consumer unit, or alteration work in and around a bath or shower.
Could argue that it could be done D.I.Y with notification to the LBA and payment of the fee and an agreed inspection process ( but probably not economically viable!)

2. We need to know what size the existing cable is and that it has a suitable double pole isolation switch, how far is the cable run?, I left my on site guide at work but I would specify 10mm, it is more suitable for that rating shower and future proofs the cable run for any future unit. 6mm would in my view be insufficient and a false economy.

3. The consumer unit looks to be under specified at 63amp, should be 80 or 100 to run the whole house and an 8.5kw shower, could run a separate shower connection unit or replace board.

4. What is the ma rating of the existing RCD, is the manufacturer specifying a RCD specific to that that circuit, if so one should be fitted. A shower circuit needs to be protected by a 30ma RCD (I think I recall). I personally would be protecting with a 40amp 30ma RCBO in any case. That way I would have complied with the manufacturers specification for an RCD protected circuit and the onsite guide.

5. All wiring should be compliant with BS 7671.

Go easy on me!

1. Agree. (Rules are different here in Scotland so can't really add much).

2. I'd be inclined to disagree with this. Would obviously depend on cable route, etc. but chances are the 6mm would do the job.

3. Also agree, easiest option would be to uprate RCD (but really just guessing from info supplied).

4. Chances are it would be a 30ma RCD given that it is a domestic installation. If you were to use an RCBO you would have to fit an uprated, time delayed RCD to replace the existing one to prevent nuisance tripping (as per Marvo's prod!).

5. Can't really argue with that!

 

[Simonslimline]

I have an 8.5KW shower in my flat supplied by a 32Amp B type MCB on 6.0mm2 and no problems.

It seems to me that the circuit in question for this shower has RCD protection Although whether it is 30ma and serves as additional protection is not entirely clear from the post.
It could quite possibly be a 100ma upfront, which is not uncommon in older installs i have seen on numerous occasions (TN).

Even though this is in a special location, is the circuit being altered! I Would say it is not.

Is the shower the same rating as the old one? This is a key factor i think.

Technically if it is a straight like for like swap then there is no problem really.


I am sure people will have different views on this. testing the circuit would certainly be a wise idea before connecting anything to it IMO.

 

[rolyberkin]

Can I also make a comment as someone who's not a qualified sparky?

If the existing RCD is 30mA and effectively a main circuit breaker for the entire installation (assuming it's not a 100mA upfront addition to a TT system with inadequate Ze to achieve disconnect times) how would you achieve discrimination if you installed a 30mA RCBO for the shower circuit?

A bit off topic but trying to get my head around the whole discrimination thing on a circuit design level. My own house has a garage consumer unit with it's own RCD which is from a split board with one side without RCD (no problem there!) On a new style split board if you wanted to put a supply elsewhere say a shed or garage, would you simply use a non RCD unit or should you feed it bypassing the upstream consumer unit RCD (can you even do this, I have not even looked at a new style high integrity board yet!?)

Thanks

 

[telectrix]

A bit off topic but trying to get my head around the whole discrimination thing on a circuit design level. My own house has a garage consumer unit with it's own RCD which is from a split board with one side without RCD (no problem there!) On a new style split board if you wanted to put a supply elsewhere say a shed or garage, would you simply use a non RCD unit or should you feed it bypassing the upstream consumer unit RCD (can you even do this, I have not even looked at a new style high integrity board yet!?)

Thanks

a hi-integrity board will have 1 or more non-RCD ways so that , as long as the cable feeding a garage/shed does not require RCD protection itself, you can have the RCD at the garage/shed.