UK MCB's vs Cartridge fuses

clanky · Jun 8, 2024

Hi all, need to pick the collective brains as im concerned about a recent problem. We have a few freezers here on our small holding and one of them started playing silly buggers. Eventually the 20A MCB kept tripping so to make sure it was the freezer i put it on a different circuit with a 16A breaker and the same thing happened. When i pulled the freezer out, i was met with a massive mouse nest in the drip tray above the motor and the flex wire had been proper nibbled at. So quick clean up, new flex plus some mouse traps and all was cured. The problem is i dont understand why the 13A fuse in the plug didnt blow and why was a 13A fitted anyway when the original flex cored was 0.75mm2, which according to the tables, is only good for 6A. As i live in a wooden barn conversion, it scares the heeby jeebies out of me when i see things like this. Anybody shed any light on this?

[ElectriciansForums.net] MCB's vs Cartridge fuses

Chivers · Jun 8, 2024

It really depends how much the fault current was as a 13A fuse won't actually blow unless it's actually a little over 20A.

As for the cable supplying the fridge what power rating is the appliance? A short length of cable can carry quite a bit more current than tables suggest but a 5A would indeed be more acceptable, only got to look at DNO cables protected by 100A fuses and see how thin they are!

Google 13A Trip Time curve to see images

UNG · Jun 8, 2024

Which ever way you choose to look at it a protective device operated and a number of trips later you decided to investigate why it continually tripped and then found a damaged cable if the fault had developed into a fire may be you would have found it sooner

If you had investigated why the MCB had tripped the first time would you be asking the question

MDJ · Jun 8, 2024

Well I will say it and get slaughtered no doubt, I would pick an MCB any day, this is the 21st century, although cartridge fuses have their uses, MCBs are IMO far more appropriate for this type of circuit.

pc1966 · Jun 8, 2024

clanky said:
The problem is i dont understand why the 13A fuse in the plug didnt blow

People could, and have, written whole books on the topic of protective devices and achieving selectivity between them.

The short version is a 20A MCB will fire in 10ms or so at a current of as little as 60A on its "instant" magnetic trip side, and at 60A a 13A fuse will take between 40ms and 600m to blow, so it is just getting hot (but not molten metal stage) inside and then the MCB opens so it survives.

clanky said:
and why was a 13A fitted anyway when the original flex cored was 0.75mm2, which according to the tables, is only good for 6A.

Probably so it survives the surge when the compressor motor kicks in so by-design it needs this sort of a fuse. The purpose here is not overload protection as you would need for an extension cable that could have any sort of a load connected, probably the motor has some thermal cut-out for that.

The purpose here is fault protection so if there is a short circuit, the fuse will blow before the cable has got to insulation-damaging levels, and in under 400ms so any metalwork is unlikely to be at a dangerous voltage for long enough to cause a fatal shock.

pc1966 · Jun 8, 2024

MDJ said:
Well I will say it and get slaughtered no doubt, I would pick an MCB any day, this is the 21st century, although cartridge fuses have their uses, MCBs are IMO far more appropriate for this type of circuit.

The fuses are part and parcel of the UK domestic plugs so not much choice.

For most situations I agree you want a MCB that anyone (within reason) can reset with minimal knowledge, and not a fuse that takes more skill and has a risk of the wrong replacement being used.

Debates about I2t levels, system-wide selectivity plans, etc, are a bit beyond the OP's problem!

clanky · Jun 8, 2024

Chivers said:
It really depends how much the fault current was as a 13A fuse won't actually blow unless it's actually a little over 20A.

As for the cable supplying the fridge what power rating is the appliance? A short length of cable can carry quite a bit more current than tables suggest but a 5A would indeed be more acceptable, only got to look at DNO cables protected by 100A fuses and see how thin they are!

Google 13A Trip Time curve to see images

Thanks. No idea on power rating but the freezers tend to run at around 0.3A. I'll look into the time curves as you say but for now ive dropped the fuses to 10A. When time permits i might try experimenting with 5A/3A fuses to see if they can stand the start up current. The problem i have here, being in the countryside, is continual dropouts although that does seem to be improving.

clanky · Jun 8, 2024

pc1966 said:
People could, and have, written whole books on the topic of protective devices and achieving selectivity between them.

The short version is a 20A MCB will fire in 10ms or so at a current of as little as 60A on its "instant" magnetic trip side, and at 60A a 13A fuse will take between 40ms and 600m to blow, so it is just getting hot (but not molten metal stage) inside and then the MCB opens so it survives.

Probably so it survives the surge when the compressor motor kicks in so by-design it needs this sort of a fuse. The purpose here is not overload protection as you would need for an extension cable that could have any sort of a load connected, probably the motor has some thermal cut-out for that.

The purpose here is fault protection so if there is a short circuit, the fuse will blow before the cable has got to insulation-damaging levels, and in under 400ms so any metalwork is unlikely to be at a dangerous voltage for long enough to cause a fatal shock.

Thanks and food for thought. Its been a while since i took my 18th and selectivity was definitely one of the hardest things to grasp.

clanky · Jun 8, 2024

UNG said:
Which ever way you choose to look at it a protective device operated and a number of trips later you decided to investigate why it continually tripped and then found a damaged cable if the fault had developed into a fire may be you would have found it sooner

If you had investigated why the MCB had tripped the first time would you be asking the question

A bit harsh especially as theres more to this story (history of failing freezers) so yes, i assumed certain things based on past experience. The methodology i used was to determine which freezer was failing before i had the pump changed out. But lesson learnt i suppose.

UNG · Jun 8, 2024

clanky said:
A bit harsh

When you hide your profile how does anyone know who you are or what you are so what do you expect

clanky said:
especially as theres more to this story (history of failing freezers)

I didn't see that mentioned in the original post

clanky said:
so yes, i assumed certain things based on past experience. The methodology i used was to determine which freezer was failing before i had the pump changed out. But lesson learnt i suppose.

Assumption in a lot of cases will come back to bite you on the **** at some point. If freezers are continually failing then may be the real fault is with the fixed installation or an external DNO issue

As for MCB's and fuses and methodology when fuses are involved you can use the old school method, keep on using bigger and bigger nails till it blows the fault clear

EricMark · Jun 9, 2024

This is the power used by my chest freezer, on average

so even a 1 amp fuse should be OK, but you can see the start amps, and one has to question what the peak really is, and not all freezers are the same, my upright freezers with inverter drives have no where near that start amps, and since you say drip tray I would assume auto defrost, and upright and my upright highest load is the auto de-frost element at around 160 watt.

The mouse did not hit the earth wire so RCD would not likely trip, best way to protect against this type of problem is

They have been known to bring mice and birds into the house, but normally not in a state where they can chew cables. He's called Binky after deaths horse.

UNG · Jun 9, 2024

From past experience mice and rats seem to be able to survive 240v but not 415v

clanky · Jun 9, 2024

UNG said:
When you hide your profile how does anyone know who you are or what you are so what do you expect

I didn't see that mentioned in the original post

Assumption in a lot of cases will come back to bite you on the **** at some point. If freezers are continually failing then may be the real fault is with the fixed installation or an external DNO issue

As for MCB's and fuses and methodology when fuses are involved you can use the old school method, keep on using bigger and bigger nails till it blows the fault clear

Oh dear, I cant even think of where to start so i wont bother. Engaging with a random little man on the internet isn’t my favourite pastime so hard pass from me. Just a pity you couldn’t put that much thought into your original post instead of defaulting to the obvious narcissism.

clanky · Jun 9, 2024

EricMark said:
View attachment 116822 This is the power used by my chest freezer, on average View attachment 116823 so even a 1 amp fuse should be OK, but you can see the start amps, and one has to question what the peak really is, and not all freezers are the same, my upright freezers with inverter drives have no where near that start amps, and since you say drip tray I would assume auto defrost, and upright and my upright highest load is the auto de-frost element at around 160 watt.

The mouse did not hit the earth wire so RCD would not likely trip, best way to protect against this type of problem is View attachment 116824 They have been known to bring mice and birds into the house, but normally not in a state where they can chew cables. He's called Binky after deaths horse.

Hah, i wish we could. But after lockdown we started a dog daycare business which means at any one time we can have upto 10 dogs running around. A cat might not enjoy that environment! Fwiw, i think the last count was 32 mouse traps in and around our property. Sadly we just have to accept that we live in a wooden shed in the middle of a large field. A necessary evil in my book.

Im intrigued with your monitoring software though, what is that?

Teddy Lop · Jun 9, 2024

pc1966 said:
People could, and have, written whole books on the topic of protective devices and achieving selectivity between them.

The short version is a 20A MCB will fire in 10ms or so at a current of as little as 60A on its "instant" magnetic trip side, and at 60A a 13A fuse will take between 40ms and 600m to blow, so it is just getting hot (but not molten metal stage) inside and then the MCB opens so it survives.

Probably so it survives the surge when the compressor motor kicks in so by-design it needs this sort of a fuse. The purpose here is not overload protection as you would need for an extension cable that could have any sort of a load connected, probably the motor has some thermal cut-out for that.

The purpose here is fault protection so if there is a short circuit, the fuse will blow before the cable has got to insulation-damaging levels, and in under 400ms so any metalwork is unlikely to be at a dangerous voltage for long enough to cause a fatal shock.

The fuse is required to be rated to protect the flex though; I suspect it's more a case of 5a being perfectly adequate to handle the compressor but someone just bunged in whatever didn't blow/was easy, many people assume that operating protective devices must be themselves faulty. As did the OP. I get a regular request to change a "faulty" RCD!

Anyway if you really want to flip the lid on the worms look up extractor fans 3/5 amp fuses Vs 6amp MCBs

R-fur · Jun 9, 2024

Its important to remember that a plug top fuse is just a rewireable fuse that's convenient to change and shouldn't be confused with a HRC fuse.
When I started out with this nonsense in the 70's the 14th edition was in vouge, protective devices were classed as course or close. The definition of close protection was that at twice the rated current it would operate within four hours! Plug top fuses are course protection.

UK MCB's vs Cartridge fuses

clanky

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Chivers

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UNG

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MDJ

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pc1966

pc1966

clanky

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clanky

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clanky

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UNG

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EricMark

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UNG

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clanky

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clanky

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Teddy Lop

R-fur

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