What Causes a Fuse to 'Blow'?

[OscarFoxtrot]

Out of interest what can cause a fuse to blow in a domestic appliance?

I appreciate there may be many different things but I was just wondering if some faults were more common than others.

Reason I ask is that I was called to a property that I let out tonight as the boiler wasn't working. I was fully expecting it to have been accidentally switched off (new tenant) or possibly the pressure having dropped and the boiler shutting down.
Turns out the 3A fuse in the adjacent FCU had blown. I replaced it and the boiler fired up.

Can fuses just go over time or is it more likely that a recurring fault in the boiler caused it?

 

[ban-all-sheds]

Out of interest what can cause a fuse to blow in a domestic appliance?

Too much current.

is it more likely that a recurring fault in the boiler caused it?

Yes.

 

[Jackrae]

Or a poor quality fuse of unknown manufacture

How long is it since someone last worked on your plumbing system. If more than several months you can't blame them but if only the day before then the system electrics may have been mucked up and when programer or valve is operated, fuse blows.

If boiler operates for more than a complete cycle, say 2 days, and the fuse doesn't blow again then it's not a recurring problem but possibly an intermittent one which will be a real sod to trace down.

 

[OscarFoxtrot]

Or a poor quality fuse of unknown manufacture

How long is it since someone last worked on your plumbing system. If more than several months you can't blame them but if only the day before then the system electrics may have been mucked up and when programer or valve is operated, fuse blows.

If boiler operates for more than a complete cycle, say 2 days, and the fuse doesn't blow again then it's not a recurring problem but possibly an intermittent one which will be a real sod to trace down.

As far as I know no one has worked on anything since the boiler was serviced and gas safety checks done in February. We have a homecare plan and the previous tenant could in theory have arranged a repair without us knowing but I think thats unlikely.

Every chance that 3A fuse had been there since the boiler was installed 9 years ago.

 

[PBC_1966]

If boiler operates for more than a complete cycle, say 2 days, and the fuse doesn't blow again then it's not a recurring problem but possibly an intermittent one which will be a real sod to trace down.

And possibly a situation which occurs only very occasionally due to incorrect wiring. I think I saw something on this very forum a couple of weeks ago when browsing where somebody had incorrectly wired a frost-stat so that when it closed it put a short on the boiler supply.

 

[JohnD]

Appliances which contain water sometimes have leaks that can make electrical components wet, causing a short circuit.

 

[bernardgreen]

Most fuses age in that their charactoristics change over time. Almost all reduce the rupture current at which they will "blow" by a few per cent. Mainly due to changes in the material of the fuse wire, oxidisation being one cause. Running a fuse close to its operating point ( ie the wire is hot ) for prolonged periods will also change the rupture current.

Vibration will cause necking of the fuse wire and thus reduce the rupture current.

A fuse's rated rupture current is given at a certain ambient temperature. If the ambient temperature increases the the fuse will rupture at a lower current.

Boilers have both high ambient and vibrations so a fuse in a boiler is going to age much faster than the same fuse in another appliance.

Appliances which contain water sometimes have leaks that can make electrical components wet, causing a short circuit.

The amount of current that will flow through water is seldom enough to take the current above the rupture current.

 

[ericmark]

A fuse can be selected to protect motors so that a stalled motor will rupture a fuse before the motor burns out. However it is unlikely the main supply fuse will protect motors. I would agree it is worrying when a fuse ruptures as one wonders if some valve or motor has caused the fuse to rupture.

However the 3A fuse fitted is rarely selected taking account of all it supplies, it is normal to fit a 3A fuse in central heating without much thought, so it could be too close to the power being used and could have aged enough to cause it to blow. If all pumps and valves worked then the first time one would just replace the fuse. However one would check everything is working including any zone valves if fitted.

If it ruptures a second time then more careful checks are called for including checking recommended fuse size. One would hope the boiler has internal fuses to protect any sensitive stuff, but manufacturers can stipulate the main supply fuse.

In general when testing portable appliances we only use two fuse sizes. If the supply cable is over 1.25 mm² we can fit a 13A fuse, and we would look at the appliance rated wattage below 750W in general a 3A is fitted above 750W a 13A is fitted. Only when the supply cable is under 1.25 mm² and the appliance is over 750W would one need to fit any of the other sizes. So although the fuses are made 1, 2, 3, 5, 7, 10 and 13 amp there is a tendency to only buy 3A one gets enough 13A fuses from changing when wrong size fitted.

You say a new occupant so an EICR should have been done before they moved in. It is possible that the electrician doing the report has found a 13A fuse fitted and has automatic swapped it for a 3A fuse. Motorised valves are rated about 5W so it is unlikely using zone controls would push a boiler over 750W but boilers do vary and I as an example looked at a Vaillant boiler as an example which stated no power requirement but says maximum of 3A BS 1363 (standard fuse for 13A plug) in supply and has an internal 2A slow blow, but also 3.15A fuse for the fan I assume a flue fan which it stated should be supplied from same supply as boiler. So it would seem even the manufacturers don't really think about size of fuse used.

So it is possibly the electrician doing the EICR has changed either a 13A or 5A fuse to 3A. You need to read the technical data on your boiler to find correct fuse size I just selected one at random. Unless it blows a second time I would not worry.

 

[OscarFoxtrot]

Thanks for all the replies.

Whilst I don't always (nor am I allowed in some cases) carry out repairs myself I always try to learn a bit when something goes wrong, so thanks for all the info.

The new fuse I installed in the FCU blew again this morning so I called out BG under the homecare plan. The engineer fitted a new fuse of his own and it has blown also. After a bit of testing he has diagnosed a faulty motorised valve and is currently replacing it.

Cheers

 

[stillp]

Some cheap imported fuses have been found to have poor solder joints between the end caps and the fuse element. Over time these become crystalline and their resistance increases, so they get hotter until the fuse blows.

 

[PBC_1966]

Some cheap imported fuses have been found to have poor solder joints between the end caps and the fuse element.

I've come across a few which appeared to have no solder joints between fuse element and endcaps at all, just the fuse wrapped around the end of the ceramic tube and the caps pushed on.

 

[Iggifer]

Some cheap imported fuses have been found to have poor solder joints between the end caps and the fuse element.

I've come across a few which appeared to have no solder joints between fuse element and endcaps at all, just the fuse wrapped around the end of the ceramic tube and the caps pushed on.

I've come across a few of them, and they're not to BS1362, they're cheap knock offs, usually there's no silica in them

 

[stillp]

Yes, I think Electrical Safety First did a study on those.

 

[PBC_1966]

I've come across a few of them, and they're not to BS1362, they're cheap knock offs, usually there's no silica in them

Yep, more counterfeit Chinese garbage. I'm generally not at all in favor of excessive regulation and believe in free and open trade across borders, but when it comes to China I'd happily see a complete ban on the importation of everything from that country until it actually manages to do something about (a) the general quality problem, (b) the dangerous products which have emerged from time to time, and (c) the amount of counterfeits. Unfortunately, as our governments (both British & American) seem to have sold out to keep the masses happy with ridiculously cheap goods, it's unlikely to happen.

 

[OscarFoxtrot]

I've come across a few of them, and they're not to BS1362, they're cheap knock offs, usually there's no silica in them

Yep, more counterfeit Chinese garbage. I'm generally not at all in favor of excessive regulation and believe in free and open trade across borders, but when it comes to China I'd happily see a complete ban on the importation of everything from that country until it actually manages to do something about (a) the general quality problem, (b) the dangerous products which have emerged from time to time, and (c) the amount of counterfeits. Unfortunately, as our governments (both British & American) seem to have sold out to keep the masses happy with ridiculously cheap goods, it's unlikely to happen.

A few years back, when downlighters were the new craze, we had a spark working with us. At that time GU10's were fairly pricey but we would run into customers who would query the sparks prices and insist on sourcing cheaper alternatives themselves. His response was always "no I don't fit cheap chinese sh1te"