Fuses blowing at twice the rated value

[PLW]

I have a feeling I may be horribly confused and need someone to set me straight...

As stated a fuse will blow at twice the rated value so a 5A fuse could potentially blow at 10A. However when using a fuse as a means of protection, you obviously want the fuse to blow to in order to prevent overcurrent. As it is true to say that a fuse will blow at twice the rated value, if we're talking about a dishwasher rated at 1005 watts using 1005watts divided by 230volts we get 4.369amps so a 5amp fuse would be used. As previously stated a 5amp fuse could blow at 10amps. However in order to protect the wiring you'd want the fuse to blow at less than twice the rated value, otherwise wouldn't overcurrent continue meaning the fuse wouldn't blow?


If I'm correct we want the fuse to blow in order to stop current above 5amps flowing through the circuit, but if a fuse can blow at twice the rated value how does this work?

Thanks.

 

[telectrix]

basically the fuse is there to protect the cable, not the appliance. so taking you dishwasher, that will have a 1.5mm flex with a plug on the end fitted with a 13A fuse. don't matter what the current draw is, as long as it's below 13A. now, the 13A fuse will withstand overcurrent of 1.45 x rating for 1 hour before blowing. 1.5mm cable is rated @ 16A and so will happily withstand that 21A for the hour.

 

[PLW]

But the fuse will blow with over current I just lost at where it says fuses can blow at twice the rated value if a 5a fuse is required at it can blow at 10 amps but you need it to blow before over current over 4.6amps how does this work?

 

[Paul.M]

A 5A fuse will be the old type BS3036 semi enclosed rewire-able type. Newer MCB's (BS60898) are 6A, these can hold a current @ 1.45 times there rating for a maximum of up to an hour, they can trip within 1 minute or 60 minutes) so therefore just less than 9A.

If a fault on that circuit happens, Line to earth (live to earth) the fault current would be a lot greater than 9A and trip the MCB within the timescale that circuit was designed for.

Think your getting mixed up with what load can pass through a fuse or MCB before blowing/tripping. Below is an example that you and others may be able to follow because we all have a cooker in the house (unless it is gas lol)

3Kw oven with 4 cooking rings. Ok 3000w = 13A

If this circuit was on a 10A MCB, would it trip if you only put on one of the cooking rings? No, if each cooking ring maximum demand is 2A. If the oven has a maximum demand of 8A, what would happen if all the rings were turn on full at the same time of the oven???? 16A demand on a 10A MCB which will trip @ 14.5A (this is without diversity lads)

 

[telectrix]

it don't work like that. cables will withstand overcurrent for short periods and the fuse characteristics allow for that. and from your example, a 5A fuse passing 10A will blow in 100 secs. but @ 20A it'll blow in 0.7 secs. it's all to do with energy which is dependent on both current and time.

 

[PLW]

I just read that fuses are designed to blow at different speeds. So if a "fast blow fuse" requires twice the rated current to blow in 0.1 seconds, how does it blow below twice the rated value?




What I don't understand is if you need to use the 5 amp fuse to protect the 1005watt dishwasher at 4.6amps, then how does the fuse blow below twice the rated current?

 

[Marvo]

As Tel says the fuse isn't there to limit the current to the value stated on the appliance, it's there to prevent the cable from getting too hot if overloaded. With appliance flex sizes under 1.0mm[sup]2[/sup] and especially when the trailing cable is long it's possible to have a scenario where a short circuit at the load end of the flex, especially on a 32A ring final circuit (rather than a 20A radial for example), won't cause sufficient fault current to flow to achieve safe disconnect times which is a fire hazard. In this case the fuse in the plug will blow open circuit and prevent overheating or fire damage of the flex.