Appliance fuses

[Iggifer]

Here's BBC bitesize explaining it for you...

http://www.bbc.co.uk/bitesize/standard/physics/using_electricity/behind_the_wall/revision/1/

 

[Ray Tay]

bhm1712, 'If something is generating heat due to an Electrical fault then it's not necessarily drawing the same current as what the fuse is rated to! Sorry but your logic is seriously flawed/misguided.'

I think my logic is sound. I'm am aware a fire situation can be evident and the amps drawn is below the fuse rating.
Common sense says put the fuse rated to the draw of the appliance. Having a 13A fuse in an appliance that draws 4A is way over sized, irrespective of protecting cables or whatever. If the appliance draws 6A the fuse will not blow. If it is drawing 6A something is wrong and it is best the appliance cut off by a fuse ASAP. My logic 100% sound.

 

[Ray Tay]

Thanks Iggifer.

http://www.bbc.co.uk/bitesize/standard/physics/using_electricity/behind_the_wall/revision/1/[/QUOTE]

Fuses and circuit breakers
Fuses are used in 3-pin plugs and in the fixed wiring in a house.

Fuses

• Fuses in plugs protect the flex of the appliance, and in doing so, the appliance itself.
• Fuses in the fixed wiring of a house protect the mains wiring.
• If a fault develops and the current in the wire becomes dangerously large, the fuse melts and breaks the circuit.
• Plug fuses come in different values. Although other values are available, the following rule is used to decide on the correct value of fuse to use:
  • for an appliance with a power rating of up to 700 watts, use a 3 ampere fuse
  • for an appliance with a power rating of 700 watts or greater, use a 13 ampere fuse

Look at the first....
Fuses in plugs protect the flex of the appliance, and in doing so, the appliance itself.

That is clear that an appliance that draws 4A does not need a fuse of 13A.

The 4th is a by rule of thumb, but it covers itself by stating 'Although other values are available'.

When I was kid the electrical shops had a whole range of fuse ratings from 3A, 5A, 6A 10A, etc. People found the amps the appliance drew and put the fuse rating in above the nearest match. If the appliance drew 5A you put in 6A as rule of thumb. The rule of thumb above has a massive 10A gap.

 

[Ray Tay]

Well I now know the rule of thumb for today. Which I will ignore and rate the fuse accordingly, as I was always taught as it will not do any harm and if anything more good.
BTW, thanks for the input from all, appreciated. I am not being deliberately contentious.

 

[bhm1712]

Good luck with that pointless exercise then.

 

[bhm1712]

Or of interest what electrical qualifications do you have?

 

[Ray Tay]

I am not an electrician. We were given electrical course in relating to the work we did. BTW, an appliance serviceman is not electrician either.
The exercise is not pointless at all. Not long ago we all engaged in this pointless exercise didn't we, because we put in the right sized fuse for the appliance.

The current rule of thumb has a massive 10A gap, from 3A to 13A fuses. Unacceptable to me. If a 4A rated appliance draws 6A the 13A fuse will not blow. If it is drawing 6A something is wrong and it is best the appliance cut off by a fuse ASAP. My logic 100% sound.
Not pointless at all. Too much slack these days.

 

[wundaboy]

The average electric kettle draws 1800 watts, that is around 8A. A 10A fuse in the plug would do for that, yet 13A is in the plug.

The vast bulk of kettles on sale have ratings between 2200 and 3000w. Can't recall ever seeing a mainstream 1800w kettle, not in recent years anyway.

 

[bhm1712]

How long will it take for a 4A fuse to blow if its got 6A flowing through it?

 

[endecotp]

When I was kid the electrical shops had a whole range of fuse ratings from 3A, 5A, 6A 10A, etc. People found the amps the appliance drew and put the fuse rating in above the nearest match.

No they didn't, they just fitted whatever they found at the back of a drawer.

For many years now, it has been simplified to just two sizes.

Appliance designers ensure that their products are safe with the supplied, standard-size, fuses.

The power mentioned on the rating plate does not normally allow for inrush currents. You should instead look for what fuse the manufacturer says should be used. (Which will be 13A.)

 

[bhm1712]

an appliance serviceman should not therefore be messing around with fuse ratings which differ from that specified by the manufacturer.

 

[Taylortwocities]

You should instead look for what fuse the manufacturer says should be used.

And that is the point

Every electrical course I have ever taken has always stated that appliance fuses must be rated to the appliance amps.

When we are designing a circuit we select the MCB size to provide protection for the cable that the circuit is wired in. The size of the cable is derived from what the total maximum load is calculated to be.

For appliances cannected to the circuit, electrical regulations and every electrical cource I hav ever taken says that you follow the manufacturer's instructions.

And if they will say the fridge freezer needs a 13amp fuse then that is the fuse size. Now you go ahead and put a 4 amp fuse in (if you can find one).
You'll know the advice we have given you is correct the first time you come back from holiday to find that all your freezer food has defrosted and the floor is soggy.

 

[Ray Tay]

wundaboy, my Philips kettle is 2kW. 3kW? They went out with the chrome Swans didn't they?

 

[bernardgreen]

Which I will ignore and rate the fuse accordingly, as I was always taught as it will not do any harm and if anything more good.

It may do harm. An example is a built in oven with cooling fans that keep the external surfaces cool. If the fuse blows while the oven is hot then the cooling will not occur and the heat from the oven may raise the temperature of the door to a dangerously high ( to touch ) temperature and side panels may be come hot enough to adversely affect the MDF sheets that the cabinet is made from.

Also running a fuse continuously at its rated current fatiques the fuse and this shortens the life of the fuse, When a fuse becomes tired it's current rating may drop by 10% or more. Hence selecting a fuse with a rating equal to the normal load current will result in the fuse getting "tired" and after a few months it is likely to blow when the normal current consumption of the appliance is flowing through it.

 

[Ray Tay]

bernardgreen, I do not say if an appliance is rated at 4A put in a 4A fuse. The next size up, so that in that case a 6A. Fuses are supposed to be made for short current spikes and not blow. Your e.g., with the oven is off target. You are saying that if the appliances is drawing too much current it should keep running to cool its surface down somehow?