MCB temp rise

Simple question.

How warm (or not!) should a 40 A MCB to get when carrying around 35 A for 10 mins? RCD one side, 32 A MCB the other carrying about 3 A.

Reply to
Dave Liquorice
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Well it will get warm, qualifying how much is a bit harder!

Can you find a data sheet for the MCB in question - it might give you the expected dissipation at full load.

Also is there a gap either side of it? (handy for cooling on some of the higher trip devices)

Reply to
John Rumm

It is a little worrying though as any heat is surely energy wasted that you are paying for, no matter how you look at it. Brian

Reply to
Brian Gaff

Remember that MCBs use a bimetallic strip for thermal tripping so they cannot work without getting hot when around full load. I think BS

60898 limits the bits you can touch to someth> It is a little worrying though as any heat is surely energy wasted that you
Reply to
Robin

Clearly you'll have some temperature rise due to the sensing and actuating elements but it should be at ambient or very close otherwise you'd have to automatically derate it.

(page 17 of this doc for example)

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Never measured contact resistance on a domestic breaker but the big stuff used on the grid is in the order of low 10's of micro ohms. so at say 1000A it is around 10 watts per phase across the entire breaker.

Reply to
The Other Mike

Then look at it in terms of it being winter, in an inhabited room.

Reply to
Graham.

If you touch it you think is that warm? You have to compare against MCBs unloaded MCBs away from it to be sure.

Good idea, looked, only gives a reference temp of 30 C. Which is good as that's about what to gets up to according to an IR thermometer. B-)

What prompted the question was taking advantage of the recent good weather and fitting a slate vent for the bathroom extract. SWMBO'd has since used the (electric) shower in that bathroom and complained of "singeing" smell. As the light unit hadn't been used for a while and was new I thought it was dust on that but it doesn't get that hot. Then the fan failed (timer type, electonics OK, motor open circuit, no sign of overheat but assumed thermal fuse). Fitted replacement fan. All working, SWMBO'd takes another shower, complains again of "hot smell" but not in bathroom just outside where the CU's are. Discover warm 40 A shower MCB... Not surprised because of the thermal trip side but haven't come across a warm MCB before so had no reference. The 16 mm tails on the E7 get warm but that's after 7 hours at 10+ kW.

No, RCD one side, lightly loaded 32 A the other. Not sure there are two empty positions to shuffle things along to give a space each side. Not sure it's worth the bother as it's a shower so only intermittent use and the devices each side aren't loaded.

It's not a lot of "wasted" energy. A Raspberry Pi in a case runs warmer, at around 3 Watts.

Reply to
Dave Liquorice

Depends on how warm really. If its a case of "that feels a bit warm" its likely fine. If its "ouch that hurt" or "I can see smoke coming out of that" then something is amiss!

With shower circuits it worth checking that all the connections are made really well and are clamped down tight. It only takes a slightly lose screw to result in a burnt out switch etc.

Reply to
John Rumm

It's time to tighten up all the screws in the CU. Including the bus bar and incoming tails.

Reply to
ARW

As I said I had to compare the "warm" 40 A shower MCB with other unloaded MCBs to be sure that it was warm. I feel we are in the realms of normal.

I will check the firmness of the connections, I'm well aware that it doesn't take much resistance to make things rather hot after the meltdown of a new SFCU feeding a 3 kW storage heater. That prompted swapping all the SFCU's to SCU's.

Reply to
Dave Liquorice

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