Replacing an MCB

[jeffturl]

I am looking to replace the electric shower, the current one is a 8.5kW the new one will be 10.5kW. The current cabling from the shower to the consumer unit is 10mm (i checked before i bought the shower!). The MCB in the consumer unit is rated at 40A. I have picked up a 50A MCB from my local merchants.

My question is about how to safely replace the MCB in the consumer unit. The consumer unit has an RCD switch that covers all MCBs, is it safe to flip the RCD to off and replace the existing 40A MCB with the 50A? or do i need a professional in to safely isolate the consumer unit to carry out this work?

The main fuse before the consumer unit is rated at 80A.

Regards
Jeff

 

[scousespark]

The work has to be notified to your LABC.
The safest option you have is to use a Part-P registered electrician.
If you need to ask the questions you have asked, you shouldn't think about doing this yourself.

 

[plugwash]

My question is about how to safely replace the MCB in the consumer unit. The consumer unit has an RCD switch that covers all MCBs, is it safe to flip the RCD to off and replace the existing 40A MCB with the 50A?

It SHOULD be if the consumer unit is installed correctly.

However since you don't know whether the consumer unit has been installed correctly you should take extreme care not to touch anything inside when first removing the cover and immediately after removing it you should check for any exposed metal on the incoming connections.

The work has to be notified to your LABC.

It's borderline depending on exactly how narrowly the "replacing fixed equipment" exception is interpreted.

 

[Chri5]

The rcd off still means that there is power in to the cu. you should turn off the main switch and turn off the power between main head and cu if you have an in line isolation between the two items.

The mcb should be tested, hence that tends to take the job on to a qualified spark. As for paperwork since the circuit is being upgraded rather than replaced like for like, a cert is required.

 

[riveralt]

I am looking to replace the electric shower, the current one is a 8.5kW the new one will be 10.5kW. The current cabling from the shower to the consumer unit is 10mm (i checked before i bought the shower!). The MCB in the consumer unit is rated at 40A. I have picked up a 50A MCB from my local merchants.

Before you go changing anything have you read the manufacturers instructions regarding the size of the MCB required.
Firstly, the 10.5kw shower will be the headline figure used to sell the product and will be calculated at 240v. The regulations use 230v as the standard figure thus giving the actual rating around 9.6kw. Check your instructions and you will see they give the standard equivalent figure as well.
You have 10mm cable so there is no problem regarding the size of the cable though 6mm T&E would have done.
The purpose of the MCB is to protect the cable and often the manufacturers will require a 40Amp MCB for this size shower.

If they require a larger size MCB then you're back to notifying your Local Authority Building Control in which case it might be cheaper to employ an electriican as previously stated.

As Chris5 said if you turn the RCD off (I assume it incorporates the main switch as well) then the inside of the Consumer will still be live in places and you need to be very careful, especially if the MCB is positioned next the the RCD.
If there is a double pole isolator between the meter and the cu then turn it off here otherwise, be very careful.

 

[scousespark]

Granted notifiability is open to interpretation.

However, there is no vagueness when it comes to advising the op to get an electricain in to sort this. I don't think I have ever seen an electrician give advice to open the CU up and 'be careful not to touch anything'.

Have you ever heard of safe isolation?

This was for the benfit of Plugwash.

 

[securespark]

The other issue is whether a 50A breaker is available for your board.

 

[ericmark]

If you look at the chart the rating of 10 mm² cable depends on the installation method. Even Method 101 # (above a plasterboard ceiling covered by thermal insulation exceeding 100mm in thickness) it is only rated at 47 amp.

As said the MCB is to protect the cable not the shower, and it depends on the cable run as to if the MCB can be up rated, you will need to get the loop impedance, it may be on the old installation certificate, the MCB is two devices in one, the B40 the 40 refers to the thermal trip, and the B the multiplier for the magnetic trip, so B = 3 to 5 times, so to trip on the magnetic part it needs 200 amp, only for 0.01 seconds but ohms law 230/200 = 1.15Ω add 5% for safety and that's 1.09Ω change the B40 to a B50 and then 0.87Ω the problem is the meter to measure this loop impedance costs around £250 so unless you already have the meter, simply not worth while to DIY, it is cheaper to get a scheme member electrician to do the job.

 

[EFLImpudence]

If you look at the chart the rating of 10 mm² cable depends on the installation method. Even Method 101 # (above a plasterboard ceiling covered by thermal insulation exceeding 100mm in thickness) it is only rated at 47 amp.

Actually 36A according to BS7671

The table in your link appears to be incorrect; some of the columns are mixed up.