Electric shower MCB went flash bang

matty_hunt

Hi. My Mira sport electric 9kw shower stoped working, tripped MCB. I changed the pull switch but it tripped again. Third time I turned on MCB, big flash bang from it. Haven't touched it since! Instructions recommend MCB 40a. What is fitted is a B50. I'll be getting an electrician to look, unless someone here shows its a simple job, but out of interest, is a B50 (I assume 50a) too high? Dangerous? Should it be replaced with a 40a or like for like 50a? Shower has been fitted several years with no problems. If shower is broken, would it trip fuse or just stop working? Any idea why this might have happened? Many thanks

bris

You have a short somewhere. It's never a good idea to keep turning it on when it's clearly tripping the switch.


Likely cause, which I have came across more than once is the live and neutral wires some where down the line have melted together over time.


40 amp is all you need 50 amp takes longer to trip causing heat to build up and cause fire.


Get an electrician in, electricity kills.

Risteard

bris wrote: »

40 amp is all you need 50 amp takes longer to trip causing heat to build up and cause fire.

Rubbish. We don't know the cable size which may well be adequately protected by a 50A circuit breaker. Also a shower does not require overload protection as it is a fixed load which cannot overload, and therefore only fault protection is required.

Ectophile

A shower circuit normally only has one load at the end (the shower), so that load is known and predictable. The cable needs to be sized for that load.

The breaker needs to be chosen so that in the event of a short circuit, the breaker will trip before the cable insulation melts. A 50A breaker may be fine if the cable is large enough.

It looks as though there's a short somewhere. Replacing the 50A breaker with a 40A one will do nothing to fix that.

matty_hunt

Thanks. The cable is 10mm. Just found paperwork minor electrical installations works cert dated 30/1/17 when we brought the house. B50 listed on that so I guess it was considered ok. Will see what the next electrician says. From what Ive been reading, 50A is fine on 10mm cable, allowing for 9KW+ showers. (about 8m run, wall mounted in plastic trunking, then loft, to pull switch then another 1.5m to shower) Since the cable size is large enough to handle the heat coupled with 50A, is there any benefit to having 40A rcd? (I don't ever see the need to upgrade shower about 9kw) would it be that little bit safer on 40A? Thanks

Lorian

I know a bit about the Mira Sport. I've just had to replace the power switch and the flow valve on one that froze and split open.

Best resource I found on the net is here:

https://www.showerdoc.com/shower-spares/mira

Make sure you look at the diagrams for the right model, there have been several mira sports.

I'd guess the only things capable of shorting are the powerswitch or more likely thermal switch and/or heater tank. All pricey enough parts. The site offers expert advice.

And if maintaining it yourself, as well as electrical isolation make sure you never undo the screws on the inlet valve or the joint between the flow valve and the tank without isolation the mains water first, or you'll crack the case or the inlet clamp.

If you decide you need to replace it completely then screwfix had a good price on the 9Kw sport last week when I looked.

edit - just seen the date - I think it has a 2 year warranty - if still in warranty maybe talk directly to Mira, I believe they may have engineers to send out for warranty works.

matty_hunt

Thanks Lorian. Unfortunately I think the paperwork is not the original installation but a 'recheck' needed to sell the house as original couldn't be found/didnt exist. Installed by the previous owners I don't know how old it is. I will read your link. Cheers

Lorian

If it's not the post 2011 model with the chrome bit at the bottom of the case personally I'd replace rather than repair anyway as the tanks don't last forever.

matty_hunt

Yes it does have a silver button so guess its the latest model. Im surprised how expensive it is! I only ever see electric showers in B&Q, Tritons for about £80

Lorian

not silver button - silver on the bottom of the case.

If you can get a 9KW sport for much less than £200 you are doing well.

https://www.screwfix.com/p/mira-sport-white-chrome-9kw-manual-electric-shower/59704

occasionally a bargain on ebay of course.

a new heater tank is best part of £100 on its own IIRC.