Old 45A and new 40A MCB's get hot

Hi,

We've got an old style consumer unit, which had a 45A MCB for the electric shower in it when we moved in a few years ago. I've never tried touching the MCB or the CU after using the shower, so can't comment on whether it used to get hot or not.

However, a few days ago, while the shower was running I noticed a sharp electrical smell, which I'd never smelled before, and opening the cabinet in which the CU sits, it was clear the smell originated there. The 45A MCB was hot to the touch, as was the CU surrounding it.

I replaced the 45A MCB with a new 40A MCB (couldn't find a 45A one). A short run of the shower (60 seconds) proved fine, but a longer run eventually led to the MCB getting hot, and again, the faint electrical odor.

The CU did not get hot this time, but the shower didn't run for as long as it had the first time.

We have an electrician coming out next week to investigate anyway, but wondered if anyone had any experience of something similar and knew if I was over-reacting, or if there's likely to be an issue somewhere in the shower or the wiring between it and the CU.

Nothing has been changed in the last 8 years, but that doesn't mean wiring hasn't degraded, etc.

Reply to
Tony
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MCB's of any current rating will get warm when running at or near their max rating. However, they shouldn't smell.

Shower circuits do often fail at the electrical connections, because the connections are often not made well enough. The sequence is that a connection or a contact has a slightly higher resistance than it should and heats up. This heat might start either in the contact or the terminal, but it gets conducted to both and causes both to deteriorate, increasing the resistance and rapidly increasing the operating temperature. The smell results from an insulator nearby or in contact which is overheating and breaking down.

If this is an old Wylex unit (of the type which originally took rewirable fuses), the blade contacts on the plug-in breakers can start increasing their contact resistance, causing the bakalite support and/or cable insulation to overheat and break down. That's a common cause for these old CU's to fail.

What's the power rating of the shower? A 40A MCB would be OK for up to 9kW, but it also depends on the size and routing of the cable.

Reply to
Andrew Gabriel

Yeh, if it was just heat, I would be less concerned.

Thanks, it is a Wylex, and that's one of the things I was concerned about.

I don't actually know the power rating of the shower, it was in when we moved in I've not been able to find anything in plain site on the shower unit which gives me any clues (the name of the unit is mostly worn away). I do intend to keep investigating that as well however.

Thanks for your response, it gives me additional background info when the electrician arrives.

Reply to
Tony

Yes a cooker doing this to a unit. Eventually traced to a very badly secured wire end in the unit. Got it in time to stop it melting though. Guy told me he did not like the wire they had used to fit the cooker as it was aluminium and very soft and corrosion prone. However we never had the problem again. Bet you would not get that smell from old ceramic ones, they probably just burn the house down instead.. grin

Brian

Reply to
Brian Gaff

The old wylex fuseboxes are prone to this on 45A circuits. Once the metal bits get hot they can lose their springiness - once that happens the game is over. Don't use the shower on hot until its fixed, its a real fire risk. Probably time for a new CU.

NT

Reply to
meow2222

Thanks, appreciate the response. I'll actually be happy if it's only the CU that's the issue.

Reply to
Tony

Thanks for everyone's comments. This turned out to be an issue on the supply side, incorrect fuse due to previous owner not having it updated when the power shower went in, plus some loose connections and arcing, had melted some insulation.

All fixed now, no cost in the end.

Reply to
Tony

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