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.

--
Cheers
Dave.

On 22/06/2017 22:15, Dave Liquorice wrote:
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.

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)


--
Cheers,

John.

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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

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"John Rumm" wrote in message
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On 22/06/2017 22:15, Dave Liquorice wrote:
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.

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)


--
Cheers,

John.

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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 something 40 degrees over
ambient. That's pretty hot in summer. But it doesn't take much power
when it's dissipated in a nice lump of insulating plastic.

On 23/06/2017 09:00, Brian Gaff wrote:
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



--
Robin
reply-to address is (intended to be) valid

On Thu, 22 Jun 2017 22:15:10 +0100 (BST), "Dave Liquorice"
wrote:

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.

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)

https://library.e.abb.com/public/3c8..._15%20MCBs.pdf

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.
--

On Fri, 23 Jun 2017 09:00:15 +0100, "Brian Gaff"
wrote:

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

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

--

Graham.
%Profound_observation%

On Fri, 23 Jun 2017 09:00:15 +0100, Brian Gaff wrote:

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.

Well it will get warm, qualifying how much is a bit harder!

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

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

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.

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

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

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

--
Cheers
Dave.

On 23/06/2017 14:09, Dave Liquorice wrote:

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.


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.


--
Cheers,

John.

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\================================================= ================/

On 23/06/2017 14:09, Dave Liquorice wrote:
On Fri, 23 Jun 2017 09:00:15 +0100, Brian Gaff wrote:

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.

Well it will get warm, qualifying how much is a bit harder!

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

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

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.



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


--
Adam

On Fri, 23 Jun 2017 15:43:59 +0100, John Rumm wrote:

Depends on how warm really. If its a case of "that feels a bit warm" its
likely fine.

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.

--
Cheers
Dave.