Wrong fuse rating?

[Zipperhead]

I need a little stupidity check here, am I being stupid or really stupid.

A few weeks ago the hoover stopped working, the fuse had blown so I replaced it with a fuse of an identical rating - 5A.

A couple of days ago it failed again, so I've just been having a look at the hoover (well, Miele really) to see if there's anything obviously wrong. Nothing obvious, the bag is nearly empty, the filters are clean. I ran it for a while on each of  its power settings, everything seems ok. The exhaust temperature got up to about 55 degrees after running on maximum power. That doesn't seem dangerously high. So time for a field test (well, carpet).

When I unplugged it I noticed that the plug seemed warm. I hoovered the stairs and landings, office and bedroom and it all seemed ok. But when I unplugged it the plug was uncomfortably warm, so I popped the fuse out and burnt myself it was so hot.

Ouch, bugger. Must be a fault in the motor and it's pulling too much power. But when the pain receded I started thinking, checking the rating, the maximum power is 2200W, I started thinking that a 5A fuse is the wrong rating - it is going to require 9A roughly.

Just before I bung in a 13A fuse, have I made a stupid mistake with my calculations?

[spesh]

Going by RoSPA's web page on fuses, you're on the right track - they are suggesting 13A fuses for appliances rated at more than 720W. Also,

Some appliances such as refrigerators and vacuum cleaners need a size larger fuse than the wattage indicates

https://www.rospa.com/home-safety/uk/northern-ireland/electricity/plugs-fuses/choosing-fuses/

HTH

EDITED TO ADD: Just looked at my Dyson vacuum cleaner - it's rated at 1100W nominal, 1200W max, and the plug is fitted with a 13A fuse.

[Canardly]

Not sure which model you have but ours (C1) has 13A fuse.

[mcshroom]

I=P/V so somewhere around 9-9.5A sounds right to me.

[Zipperhead]

Thanks everybody. I wonder how a 5A fuse got in there before.

I'll change it and put my pinny on. Oh wait, that's not for the hoovering.

[Kim]

BS1363 fuses are there to protect the cable, not the appliance.  I'd have no qualms about putting a 13A fuse in a hoover plug.

(Generally, the appliance should have its own arrangements for overload protection.  So a computer, say, might have a 5A fuse protecting the 'kettle' lead, and then a 1.5A fuse internal to the power supply.)

[Feanor]

But when I unplugged it the plug was uncomfortably warm, so I popped the fuse out and burnt myself it was so hot.

That needs investigation and rectifying.

There's a high-resistance contact somewhere.
Either between the plug and socket, or on the fuse clips.

Are the pins of the plug hot when you remove the plug from the socket?
Have you tried it in different sockets?

If the fault is in the plug, then you may be able to clean up the contacts with wire wool or similar, otherwise I'd cut the plug off and replace it.

[Kim]

But when I unplugged it the plug was uncomfortably warm, so I popped the fuse out and burnt myself it was so hot.

That needs investigation and rectifying.

There's a high-resistance contact somewhere.
Either between the plug and socket, or on the fuse clips.

Or on one or more of the screw terminals (probably the live one, if it's heating the fuse) inside the plug.

Or it's just that a 5A fuse being subjected to 9A is going to get quite warm before it actually blows.  These fuses are designed to protect against cable damage rather than overloads, so you could conceivably get several minutes of operation (ie. enough to hoover a room) at merely double the rated current without it blowing...

[Feanor]

But when I unplugged it the plug was uncomfortably warm, so I popped the fuse out and burnt myself it was so hot.

That needs investigation and rectifying.

There's a high-resistance contact somewhere.
Either between the plug and socket, or on the fuse clips.

Or on one or more of the screw terminals (probably the live one, if it's heating the fuse) inside the plug.

Or it's just that a 5A fuse being subjected to 9A is going to get quite warm before it actually blows.  These fuses are designed to protect against cable damage rather than overloads, so you could conceivably get several minutes of operation (ie. enough to hoover a room) at merely double the rated current without it blowing...

Yes, perhaps.
Do moulded-on plugs that you get these days even *have* screw terminals?
If they do, they will not be accessible.
At best you might be able to access the fuse clips.
But it needs investigating anyway.

If simply replacing the fuse with a higher rated one has fixed the overheating issue, then fine.
Otherwise, further investigation is required.

[Jaded]

I could tell a story about when a (fuse box) fuse was replaced with an nail.

But it's not relevant here 

[Zipperhead]

It was the fuse that was getting hot, it must have been close to blowing. With a 13A fuse fitted the plug doesn't get warm at all.

I'm still curious about how a 5A fuse got in there at some point.

[Biggsy]

It's not just obviously wrong ratings with fuses you should look out for, but fake fuses.  Bussmann, for eg, have discovered fuses with their name on that were not made by them and contained the wrong wire and no sand inside the cartridge.

Also don't trust mains plugs on cheap Chinese cables and goods, etc.  The ones with the earth pin half insulated are blatently stupid rubbish, but who knows what's inside the moulded ones that look ok from the outside?

[Kim]

Also don't trust mains plugs on cheap Chinese cables and goods, etc.  The ones with the earth pin half insulated are blatently stupid rubbish, but who knows what's inside the moulded ones that look ok from the outside?

This sounds like a job for Big Clive and the Dremel of Knowledge.

[ScumOfTheRoad]

Also don't trust mains plugs on cheap Chinese cables and goods, etc.  The ones with the earth pin half insulated are blatently stupid rubbish, but who knows what's inside the moulded ones that look ok from the outside?

Indeed. I bought a portabler charger for an electric wheelchair from Ebay. The charger stopped working, and as I am reasonably adept with electronics I took it apart to repair it. Firstly I found that the wispy strands of coppe in the three wire mains cable were so flimsy they you could not get them fastened in to a replacement mains plug with screw terminals.  But the big fault in my eyes was that the earth wire was simply cut off where it entered the plastic case.
OK, as far as I know devices with plastic cases are fine to be powered by a two wire mains cable, if double insulated.
But quite honestly this device smaked of "lets bung on a three wire mains lead, they'll never notice when it is sold"

[Kim]

Also don't trust mains plugs on cheap Chinese cables and goods, etc.  The ones with the earth pin half insulated are blatently stupid rubbish, but who knows what's inside the moulded ones that look ok from the outside?

Indeed. I bought a portabler charger for an electric wheelchair from Ebay. The charger stopped working, and as I am reasonably adept with electronics I took it apart to repair it. Firstly I found that the wispy strands of coppe in the three wire mains cable were so flimsy they you could not get them fastened in to a replacement mains plug with screw terminals.  But the big fault in my eyes was that the earth wire was simply cut off where it entered the plastic case.
OK, as far as I know devices with plastic cases are fine to be powered by a two wire mains cable, if double insulated.
But quite honestly this device smaked of "lets bung on a three wire mains lead, they'll never notice when it is sold"

That sort of thing tends to happen when molishers of cheap devices who come from a country with two-pin mains plugs are confronted with the task of adapting their design for a BS1363 plug.  So they just ignore the earth wire, even when the device has a metal case.

The proper solution is to use an IEC mains inlet and if necessary leave the cable as an exercise for the retailer/end user.