why don't two kettles blow a 13amp fuse?

[arc]

if you've got two kettles, both around 2500w - plug them into a cube and plug that in. switch on both kettles. why doesnt the fuse blow. it should. two kettles is around 20amp.. so why doesnt it blow the 13amp fuse in the cube?


and yeah, i should go to bed.

 

[Bushboy]

Magic duh...

 

[serin]

i think you should stop plugging your appliancies into cubes and into electrical sockets that way they get power

 

[steve1219]

if you've got two kettles, both around 2500w - plug them into a cube and plug that in. switch on both kettles. why doesnt the fuse blow. it should. two kettles is around 20amp.. so why doesnt it blow the 13amp fuse in the cube?


and yeah, i should go to bed.

Are they fused

Steve

 

[The Negotiator]

1) Because they don't take their maximum rated power.
2) Because the fuse will have a tolerance before it melts.

 

[arc]

1) Why not. They;re just heater elements, surely they'll draw what they're rated too.
2) Yeah, they can tolerate 13amps at approx 230v running through them!

 

[Helz]

These cubes are known for not being safe and overheating and sparking anyway - presumably there's some kind oif dodgy circuitry inside that means you can use higher rated appliances than is theoretically safe or possible.

Our works insurers now won't insure our building if we have one on the premises

 

[The Negotiator]

Power draw is proportional to temperature of the coil, the kettle will only ever draw maximum with a full capacity of water at maximum temperature.

I meant tolerance of around +/- 20 % of a fuse, fuses work on sustained draw and are pretty slow to melt.

 

[JonnyBoy]

Fuses are designed to blow when a BIG current flows through them - i.e. if a short-circuit occurs the instantaneous fault current is in the thousands of amps. The fuse is rated so that it blows very quickly if this happens - so quickly that there isn't time for the wiring to heat up and become damaged, and so that higher rated fuses further back in the supply (e.g. the 100A electricity board fuse before your electricity meter) don't blow.

The 13A rating is the maximum current that the fuse can carry consistently over a very long period of time (i.e. years) with many on/off cycles (i.e. thousands). If you keep using your "cube" with 2 kettles for a while the fuse will eventually blow - whereas with 1 kettle it should last indefinitely unless there is a short-circuit.

Also, running 2 kettles on a single socket will result in neither kettle running at it's full power rating - this is due to the excessive curent draw causing a larger than usual voltage drop across the resistance of the all the supply cables. The high continuous current will eventually damage the wiring as it causes the cables to heat up and melt the insulation - IMHO you should stop using 2 kettles at once!!!

 

[Stu DemonD]

Or buy a bigger kettle

 

[Ransoman]

Not all cubes have fuses Be careful.

 

[arc]

These cubes are known for not being safe and overheating and sparking anyway - presumably there's some kind oif dodgy circuitry inside that means you can use higher rated appliances than is theoretically safe or possible.

Our works insurers now won't insure our building if we have one on the premises

aye, i dont actually use one here but just know of this happening before.

Power draw is proportional to temperature of the coil, the kettle will only ever draw maximum with a full capacity of water at maximum temperature.

surely a heating element will draw full current when the coil is cold, not when it is at maximum temperature?

Fuses are designed to blow when a BIG current flows through them - i.e. if a short-circuit occurs the instantaneous fault current is in the thousands of amps. The fuse is rated so that it blows very quickly if this happens - so quickly that there isn't time for the wiring to heat up and become damaged, and so that higher rated fuses further back in the supply (e.g. the 100A electricity board fuse before your electricity meter) don't blow.

see now i'd have thought a 13A fuse should blow if it anything more than 13A is put through it. it's a bit silly really imo.. its not doing what it says on the tin.

The 13A rating is the maximum current that the fuse can carry consistently over a very long period of time (i.e. years) with many on/off cycles (i.e. thousands). If you keep using your "cube" with 2 kettles for a while the fuse will eventually blow - whereas with 1 kettle it should last indefinitely unless there is a short-circuit.

thats silly too really isn't it, i accept current will spike when things are switched on.. but whats the point in something that'll allow more current through than it says it does. Like with lighting circuits, you use 5amp cabling and then run it with a 5amp fuse. If the fuse is going to let more than 5amp through then it's not protecting the cabling is it. But then if both the fuse, and the cabling are actually designed to take more current then why not rate them at what they are actually designed to take?!

Also, running 2 kettles on a single socket will result in neither kettle running at it's full power rating - this is due to the excessive curent draw causing a larger than usual voltage drop across the resistance of the all the supply cables. The high continuous current will eventually damage the wiring as it causes the cables to heat up and melt the insulation - IMHO you should stop using 2 kettles at once!!!

pretty much the same result if you run two high power appliances from a double socket then, cos the supply to the socket is the same. that'd mean it'd have a similar effect if you ran two high power appliances from any two points on the ring main. surely it'd be a good idea to have some sort of device to prevent high current draw from causing damage to the cables.. kinda like what i thought a fuse was meant to do!!

Or buy a bigger kettle

shush

 

[The Negotiator]

surely a heating element will draw full current when the coil is cold, not when it is at maximum temperature?

Going by my experience, I have used kettles on power limited circuits, they have been fine until close to boiling at which point they trip out.

 

[Dr_Pepper]

They will blow if much more than 13A goes through them, but they just don't always use full power. If 2 kettles are both genuinely pulling 13A then the fuse will blow, not always immediately though unless it is really high.

As for the design of the cubes, there's no trickery involved at all. The + and - are all connected up to the same wire, just like in a double wall socket. I don't think they're unsafe in the slightest, in fact the ones that have a fuse in are more safe because they have that extra protection layer.

 

[raptor]

a fuse is there to protect the wiring not the appliance. not a lot of people know that.

 

[JonnyBoy]

see now i'd have thought a 13A fuse should blow if it anything more than 13A is put through it. it's a bit silly really imo.. its not doing what it says on the tin.
shush

It will, eventually, but with an inverse time characteristic; i.e. the higher the overload the faster it will blow.

e.g... short circuit, a few thousand amps flow, fuse blows in a matter of milliseconds

constant but not too heavy overload (20-30A), fuse may carry this for several minutes before it blows

very slight overload (14A), fuse may well cope with this for several hours/days/weeks before failing.

The rating of a fuse is NOT the current at which it blows; it it the current that the fuse is designed to carry consistently without failing. You wouldn't want a 3kW heater (which draws nearly 13A constantly) to constantly blow the 13A fuse in it's plug.

Fuses are pretty crude and imprecise devices; they rely on the heating of a thin conductor to melt it. It's impossible to make a fuse that will blow instantaneously at exactly 13A. The idea is that a serious fault will pull enough current to blow the fuse pretty quickly. They're intended as protection against short circuit faults and severe overloading, NOT against relatively small overloads.

If you were trying to run 2 kettles on a 13A MCB (well, 16A really - 13A isn't a standard rating for an MCB) then the MCB would trip very quickly as they operate more predictably and with smaller tolerances than a fuse.

also depends on the rating of the kettles; at 240V, 13A gives a power of 3120W. 2500W@240V requires 10.4A; so your 2 kettles are actually pulling 20.8A. This isn't high enough for the fuse to blow straight away, it'll hang on for a while before the fuse element heats up to melting point. How long that takes is anyone's guess; depends on the design of the fuse itself and its manufacturing tolerances.

 

[Hellcat]

MCB's are great

If only I could understand the different classes more thoroughly.

 

[Phoenix3dfx225]

well in that case arc, the next lan we should had the two kettles to the loop and see what we can get out for a "sustained" period of time, i.e the weekend

My only idea was that the kettles just got the maximum 3200watt but split between the two, so kinda brown out on the appliance.