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:

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...

[Chris]

[tabbypurr]

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).

[tabbypurr]

70C is a very conservative 'limit'. In practice a pvc cable can run above t hat 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]

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.

[tabbypurr]

ve 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.

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 i ts shape changed if 66psi is applied. 66psi is not normally present in a do mestic install, but if you nailed some T&E down with clout nails all bets a re 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.

[Graham.]

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

[Graham.]

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

[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]

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

[tabbypurr]

The opposite is happening where regulations rule.

NT

[The Other Mike]

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)

[ARW]

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

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")