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Author Topic: Fuses  (Read 1271 times)

Weaver

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Fuses
« on: November 23, 2020, 01:01:35 PM »

Consider a traditional old-fashioned 13 A fuse, or a 100A house main fuse. When they are functioning normally and there is a current flowing that is within limits, how warm does the fuse get? I presume it must have some modest resistance as it might be a slight current bottleneck. I have no idea how MCBs work, I need to look it up.
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sevenlayermuddle

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Re: Fuses
« Reply #1 on: November 23, 2020, 02:49:37 PM »

Complex subject.  Others may explain it better, but I'll chip in anyway, at risk of being upstaged by somebody better qualified.

A 13A plug fuse might indeed get warm within rated current, and the reaction to further exceeding rated current may just be getting hotter and hotter, without blowing.

Thing is, 13A is the current it is designed to tolerate, rather than any implication it might magically blow at 13.1 Amps.  If you were to gradually increase the load, beyond 13A, the fuse would simply get hotter and hotter.  Eventually, if the current exceeds 20A or so,  and is sustained for long enough the fuse eventually will blow, but by that time it may be very hot indeed.

I think it's why people sometimes get into trouble with extension leads, as it allows them to plug in (say) two electric kettles pulling 10A or so each, with the combined current of 20A flowing through the common plug fuse.   They think they are protected by the common fuse, but it probably won't blow in such circumstances, and it might get extremely hot.

From a quick search for the right terms, this link explains it well...

https://www.pat-testing-training.net/articles/fuse-operation-characteristics.php

I think MCBs have response curves that are more closely defined, but are still designed to trip not at the rated current, but at some multiple of the rated current.   

Real life example A dishwasher in our house once melted its power input socket, creating a fairly solid live to neutral short.  We could smell something melting but couldn't trace it, being hidden down the back of a built in cupboard.  After quite some time (hours) the 32A MCB in the consumer unit tripped, but the dishwasher's 13A plug fuse remained intact.  The fact that the fuse remained intact worried me at first, but when I studied the characteristics of both devices, it turned out to be within bounds of expected behaviour.
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licquorice

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Re: Fuses
« Reply #2 on: November 23, 2020, 03:10:56 PM »

There are different ratings of MCB with different trip curves. I only know this as I have just had a sewage pump installed and it specifies using a type C MCB because of the high inductive start up load of the pump.
https://info.e-t-a.com/ES-2017-02-IndustrialAutomation-DCurve_LP-Destination.html
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Weaver

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Re: Fuses
« Reply #3 on: November 23, 2020, 03:58:12 PM »

I usually moan about using really low rated fuses in plugs so they will blow if an evil current flows. A Rasperry Pi for example drawing 13A should never happen as it will be a three bar fire. York Universityís Chemistry department once caught fire and blew up because a calculator went evil and caught fire. The optics lab in the Physics department once had a some wiring burned out because a mains-powered tiny lamp became evil.

I tend to put 1, 2, 3 A fuses in things, 5 A fuses less often and rarely allow 13 A fuses. Have had a washing machine catch fire in the kitchen when we were in London and in Skye we had a disastrous house fire in 2003 which was possibly because a central heating pump melted.
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Chunkers

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Re: Fuses
« Reply #4 on: November 23, 2020, 09:54:59 PM »

On your mains supply most people have circuit breakers and RCD, from memory circuit breakers use either an electromagnet or a bimetallic strip to break the circuit and shouldn't really get very hot i.e. they are not melting a wire.

I had a (very) old fuse box when I first moved to my current house with fuses which used wire and needed replacing quickly as we had small children with small curious pokey fingers .....

As sevenlayermuddle says its worth bearing in mind fuses can pass higher currents than their rating briefly before blowing, some are rated to do so ... basically if you want protection from a shock then a fuse is not really your best choice, use an RCD

The thing that worries me often is when I see people overload extension leads which are often sold with lower (than 13A) rated cable especially those one which coil up (which warms up even more!).  Also, Chinese stuff which is made so cheaply they skimp on the wire gauge to save money!  :o
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Weaver

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Re: Fuses
« Reply #5 on: November 23, 2020, 11:36:46 PM »

Thatís why I replace fuses with ones with a really low rating.
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burakkucat

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Re: Fuses
« Reply #6 on: November 24, 2020, 12:06:32 AM »

I will routinely consider the operating load of the device and whether or not it is an inductive load with a high switch-on current. Then an appropriate fuse is selected, from 3, 5, 10 and 13 amperes.

[deviation]
Earlier this year, I was reading about the UK's early warning system during the Cold War. (I'm sure the name "Handel" will be familiar to those employed by GPO/PO/BT during that era.)  Significant use was made of the air-raid sirens from WWII. What surprised me was the switch-on current drawn by those sirens. They need a three-phase supply and, when connected as a delta, drew over 30 amperes on start-up. See "Air Raid Sirens and their Control Gear".
[/deviation]
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sevenlayermuddle

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Re: Fuses
« Reply #7 on: November 24, 2020, 12:14:40 AM »

Thatís why I replace fuses with ones with a really low rating.
Iím not actually convinced thatís always a good thing.

If an appliance, by its nature consumes  (say) 10 Amps, and the plug is fitted with a 13 Amp fuse, then the appliance and the fuse will be within their comfort zones.

But if you replace the plug fuse with a 5 Amp fuse, and the appliance consumes 10 Amps, then the fuse is stressed...  it probably wonít blow any time soon but it may become rather hot.   

Is that dangerously  hot?  Iím really not sure. ???

The main purpose of the plug fuse is to protect against a fault in the cable leading from the mains plug  to the appliance.   If the appliance itself needs more stringent protection then it should implement that itself, maybe as a lower rated fuse within.

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Weaver

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Re: Fuses
« Reply #8 on: November 24, 2020, 12:31:51 AM »

I donít replace fuses with randomly chosen ones. I look at the current consumption of the devices to be fitted. Itís just to stop stupidity such as plugging kettles or electric fires into 3A extension blocks and so forth. I keep advising my beloved on what kit has a high current consumption and what does not - the simplified advice is : Ďbasically heat producers and electric motors are high, and all other things are tinyí.
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sevenlayermuddle

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Re: Fuses
« Reply #9 on: November 24, 2020, 12:47:46 AM »

Iíd still argue that fuses are a compromise and the person who knows the most appropriate fuse is the person who designed the appliance.   

Itís possible the designer made a mistake, and a lower rated fuse would be a better compromise, but we should not jump to that conclusion lightly.

Then again, there is an awful lot of E-marked, but dangerous, tat on sale on Amazon and elsewhere.   I have heard of UK mains plug mouldings fitted to Amazon-sold appliances, that contain no fuse at all. :o

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Weaver

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Re: Fuses
« Reply #10 on: November 24, 2020, 01:02:57 AM »

Quite true but Iím thinking of extension blocks and that kind of thing and then devices that draw very very low current not having 13A or 5A fuses in their plugs. Some people (certain relatives) were quite happy to take out say a 3 A or 5 A fuse and put in a 13 A one because thatís all they have at hand.
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tubaman

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Re: Fuses
« Reply #11 on: November 24, 2020, 07:32:57 AM »

...

The main purpose of the plug fuse is to protect against a fault in the cable leading from the mains plug  to the appliance.   If the appliance itself needs more stringent protection then it should implement that itself, maybe as a lower rated fuse within.

Fully agree. @burakkucat example of the sirens is why some appliances with motors, eg fridges and freezers, need a 13 amp fuse even though the running current draw is small.
 :)
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Weaver

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Re: Fuses
« Reply #12 on: November 24, 2020, 09:53:03 AM »

Agree with tubaman.

Might shout at me for this, but I would possibly at least try a lower rating fuse and if it blows on startup, then so be it, back to plan A. After my experience with the washing machine and possibly central heating pump Iím paranoid about electric motors turning into a dead short so want to stop any abnormal current level and a fridge actually drawing 13A for an extended period really really isnít something you want as it will already have set fire to your kitchen. I hope you follow me. Feel free to disagree.
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Alex Atkin UK

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Re: Fuses
« Reply #13 on: November 25, 2020, 04:57:55 AM »

The problem with that is those loads require a high startup current so the chance of the fuse blowing at random is probably quite high.

Might not be a big deal with the washing machine but you don't really want say a fridge to just randomly blow the fuse.  Even the central heating I'd imagine problematic in your situation if you wouldn't be able to change it yourself.
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Weaver

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Re: Fuses
« Reply #14 on: November 25, 2020, 06:06:13 AM »

Indeed, I would need to test the suitability of the fuse with a few test cycles. But after the central heating pump (albeit unproven) set fire to the kitchen and room above back in 2003 and killed four of my animals, with smoke damage throughout the whole house, Iím very paranoid about overheating. In fact though the cause is very unclear and we were not in the house at the time, because I had just had a serious operation in Fort William on the mainland so Janet was staying with me at the Belford hospital.

When our washing machine decided to start melting, when we were in London, the fuse never blew; the thing just started pouring out toxic smoke. Luckily Janet was at home but unfortunately I was not, and she was a hero manhandling the thing out of the house into the garden. In doing this she got a whiff of poison gas (cyanide?) and had bad lungs for a week or two. The fire brigade arrived and stuck a crowbar under the top of the washing machine, flipping it off and flames shot way up high, as the oxygen got in to it, so Janet told me. We said very harsh words to the manufacturer and got a better washing machine, further up the range, as a replacement, after iirc mentioning a popular BBC consumer affairs programme.

The point occurs to me that thatís one advantage of a ground floor flat: you have a garden (yay ;D) and can shift fiery kit out into it; in an upstairs flat you canít reasonably hoik a washing machine out of a window.

My dear friend Alistair had a similar experience, white-goods catching fire in his house in West Yorkshire, smoke damage ruining the whole house as happened to me in Skye. It took a long time for him to get all the remedial work done and to sort out things with the insurance company.

Luckily for me, my insurance company was superb. Also all my computers were not in the house at the time because we were doing a lot of work on the old place, which had been a ruin, basically. So it just so happened that the computers were in another, temporary office, in Broadford five miles away, to keep them away from dust and to ensure they were not in the way of work going on. So they were completely safe, thank goodness.

Iím going to check our fire extinguishers and smoke alarms today after writing this!

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