Circuit Breakers

[Chris]

Hi,

What current typically is sufficient to break a 32A breaker? I've heard
they don't cut at exactly 32A but usually some amount higher. Can someone
clarify?

cheers,
chris

[Tim Watts]

Very good question - and answered by the use of the trip curve:

http://wiki.diyfaq.org.uk/images/d/d4/Curve-MCBTypeB.png

50A through a type B 32A breaker takes around 1000 seconds to trip.

160A will trip it "instantaneously" for all practical purposes.



This is not as insane as it might first seem - happy to say more...

[Rod Speed]

Chris <c...@noreply.com> wrote

> What current typically is sufficient to break a 32A breaker?

Varys with how long the current is there for.

> I've heard they don't cut at exactly
> 32A but usually some amount higher.

Always at some higher amount.
They will allow 32A forever.

> Can someone clarify?

Its complicated. It you try pulling 1000A, it will trip immediately.

If you are taking 35A it will take a while to trip and may not ever do.

[Chris]

On Tue, 06 Jun 2017 22:30:45 +0100, Tim Watts wrote:

> This is not as insane as it might first seem - happy to say more...

Shoot!

[tabb...@gmail.com]

summary:
1. wire has thermal capacity so can be overcurrented for a short while.
2. pvc wire current ratings can be exceeded by a sizeable margin without any problem occurring. Indeed this is routinely done with ring circuits - and radials - which often supply 50A or so for short times.


NT

[John Rumm]

Also worth noting that a typical MCB has two distinct trip mechanisms
designed to cope with two different fault scenarios.

The first is a thermal trip, that uses a bimetal strip not unlike a
thermostat. This is quite a good model for the thermal effects on the
circuit cables as the overload starts to rise. It will allow a fairly
significant over current for a short time, or a lower one for more
extended times. The basic objective being to try and keep the cables
inside the thermal envelope that ensures insulation does not happen.
(conductors temperatures of up to 70 deg C continuously are the limit
for most PVC insulated cables). It avoids unwanted trips due to switch
on surges, or small overloads for extended periods.

The second mechanism is a magnetic trip. This is basically a solenoid
that will react and pull the breaker open when there is a very high trip
current. For a type B MCB this will be set at 5 times its nominal
breaking current. This mechanism will protect a circuit from a hard
short circuit (line to neutral or line to earth). The kind of thing
caused by nailing through a cable etc. Its designed to stop cables
bursting into flames or vaporising in the presence of very high fault
currents (which could be 1000's of amps)

(Type C breakers have and instant trip at 10 x nominal, and type D at 20
x nominal - these types being use for equipment with high inrush currents).


--
Cheers,

John.

/=================================================================\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\=================================================================/

[tabb...@gmail.com]

70C is a very conservative 'limit'. In practice a pvc cable can run above that for years without damage. I found one that had been doing so, a 5A flex buried under insulation running a 13A load for 20-30m or so at a time. It got soft like silicone at that temp, but wasn't harmed.


NT

[Tim Watts]

Sure.

The breaker is trying to achieve 2 things:

1) 0.4sec disconnection time in the event of a L-E fault (short circuit)
- this is to limit the touch voltage between the affected metalwork and
other sources of earth potential to a time that is deemed likely to save
the life of anyone stuck across them.


2) Stop the cable from overheating to the point of damage (eg >70C for
PVC cable) in the event of slight overloads. This is precisely why it
does not trip hard at 32A.

HTH :)

[newshound]

I never knew that! Although with hindsight it obviously makes sense.

[GB]

On 07/06/2017 08:51, tabb...@gmail.com wrote:
>
> 70C is a very conservative 'limit'. In practice a pvc cable can run above that for years without damage. I found one that had been doing so, a 5A flex buried under insulation running a 13A load for 20-30m or so at a time. It got soft like silicone at that temp, but wasn't harmed.

I loaned an extension cable in a drum to a decorator. He ran his steamer
off it without uncoiling it, and he did melt the PVC to the point it was
just runny.

[tabb...@gmail.com]

On Wednesday, 7 June 2017 12:10:06 UTC+1, GB wrote:

Wikipedia fwiw:
PVC starts to decompose when the temperature reaches 140 °C (284 °F), with melting temperature starting around 160 °C (320 °F).

70C is the glass temperature, ie the temperature above which pvc can have its shape changed if 66psi is applied. 66psi is not normally present in a domestic install, but if you nailed some T&E down with clout nails all bets are off.


NT

[ARW]

Do you want a job training my apprentices?

Just to add a small note to your post about the thermal part of the MCB.
For circuits that could be expected to be running at close to maximum
load for a considerable time you may have to leave a gap in the CU
between that circuits MCB and other MCBs to prevent the thermal trip
triggering below the rated value (you need a bit of an air gap) or put
the MCB that is close to maximum load next to one that is at low load.

It's not likely to happen often in a correctly wired domestic property
but it is worth mentioning. ie You would not put the kitchen ring MCB
next to the cooker MCB, you would space them apart or use a low loaded
MCB between them.

--
Adam

[Graham.]

On Wed, 7 Jun 2017 12:09:59 +0100, GB <NOTso...@microsoft.com>
wrote:

I once saw an uncoiled reel under a table powering the conveyor
toaster at breakfast in a Premier Inn.
--

Graham.
%Profound_observation%

[Graham.]

My electric shower MCB is at the end, the next one is a 6A that ONLY
serves the doorbell transformer!
--

Graham.
%Profound_observation%

[Fredxxx]

Is that a change of general policy?

I recall reading some instructions many years ago that said to put the
highest current MCBs closest to the incoming/isolation switch. This was
on the basis the bus-bar would dissipate the least heat through being
the shortest current path.

[John Rumm]

Yup they certainly used to say that, and its probably still worth doing,
although the busbars are more serious affairs in modern CUs anyway.

The comment about spacing MCBs is something that you normally find in
the device data sheets. It probably does not come into play that often
due to the diverse loads on most MCBs, but can certainly be an issues in
cases where there are lots of larger long term loads like heating circuits.

[GB]

On 08/06/2017 11:10, John Rumm wrote:

> although the busbars are more serious affairs in modern CUs anyway.

Why is that? Most things are being lightened and reduced to the minimum?

[tabb...@gmail.com]

The opposite is happening where regulations rule.


NT

[The Other Mike]

On Wed, 7 Jun 2017 20:10:38 +0100, ARW <aXXXwa...@blueyonder.co.uk> wrote:

>

>Just to add a small note to your post about the thermal part of the MCB.
>For circuits that could be expected to be running at close to maximum
>load for a considerable time you may have to leave a gap in the CU
>between that circuits MCB and other MCBs to prevent the thermal trip
>triggering below the rated value (you need a bit of an air gap) or put
>the MCB that is close to maximum load next to one that is at low load.
>
>It's not likely to happen often in a correctly wired domestic property
>but it is worth mentioning. ie You would not put the kitchen ring MCB
>next to the cooker MCB, you would space them apart or use a low loaded
>MCB between them.

There is a similar restriction with din rail 'mini' contactors, a spacer between
any contactor and any MCB rated above 10A No more than two contactors next to
each other without a spacer (ref MK 6220S instructions)

Bit of a pisser if you're adding to an existing installation and the only space
left is next to a 50A shower circuit. Everything shuffled, new labels and a
new busbar required.
--

[ARW]

Well just don't use the shower on mischievous night (some parts of the
country do not have that)

--
Adam

[ARW]

On 08/06/2017 02:26, Fredxxx wrote:

AFAIK it goes back to the Wylex fuse boxes. They even had a special
edition one with a fuse at the other side of the main switch to the
other fuses for 40A fuses.

The Wylex bus bar is not a solid bar connected to each fuse like the new
CU bus bars but has riveted on "blocks and wedges" to allow the fuse
blade to be inserted

https://www.flameport.com/electric_museum/wylex_fuse_boxes/brown_wylex_nofuses.jpg



In fact the whole site is worth a look. It just proved me wrong as the
extra way was for 45A fuses not 40A fuses.


(and swap "fuses" for "fuse wire")

--
Adam

[Mike Clarke]

But this is for "circuits that could be expected to be running at close
to maximum load for a *considerable* time". How long do you expect to be
in the shower.

--
Mike Clarke

[ARW]

Not as long as teenage kids.

--
Adam

[Clive George]

On 08/06/2017 21:30, Mike Clarke wrote:

> But this is for "circuits that could be expected to be running at close
> to maximum load for a *considerable* time". How long do you expect to be
> in the shower.

Blondie documented it as an hour.

[John Rumm]

On 08/06/2017 21:30, Mike Clarke wrote:

Depends on who is in there with you ;-)

[Mike Clarke]

On 08/06/2017 21:32, ARW wrote:
> On 08/06/2017 21:30, Mike Clarke wrote:

[snip]

>> But this is for "circuits that could be expected to be running at close
>> to maximum load for a *considerable* time". How long do you expect to be
>> in the shower.
>>
>
> Not as long as teenage kids.

Replace the 50A breaker with a 32A one and the kids will clear out
sharpish after about 15 minutes :-)

--
Mike Clarke

[ARW]

No need, it's an electric shower, just turn on the the cold tap in the
kitchen whilst they are in the shower.

--
Adam