What does the main fuse protect?

[rewireIT]

From another thread:

The 60A fuse could be sized to protect the size of cable feeding the property, so having it changed to 100, or even 80, the middle size, might be impossible.

I'm thinking about this and it's got me all confuzzled.

Don't fuses protect anything downstream ie my laptop's charger has a fuse that protects the charger cable, the socket has a breaker that protects its wiring, the boards wiring has a main fuse that protects it from overload..... wouldn't protection for the supply cable be at the suppliers gear away from the house somewhere ie upstream?

 

[James]

No, at the transformer they my have say a 300A fuse per phase
this will feed a cable that runs down the street
from the cable in the street there will be multiple tap offs feeding each house.
I think about 50 or 60 homes fed from each cable.

 

[mainline]

The main fuse in the cut-out is to protect the supply cable.

 

[snowhead]

wouldn't protection for the supply cable be at the suppliers gear away from the house somewhere ie upstream?

Yes,,BUT,,

As James has said, the local distribution cable leaving the substation has protection of 300a or whatever is appropriate for the cable.

Here's the BUT,,
When the cable to the property / house is taken off that cable it's CSA is dramatically reduced at 16mm² or 25mm² or whatever, so the 300a would not be suitable protected and in the U.K that joint is not in a position where it could be.

It would be the same as having a 20a protected radial wired in 2.5mm² with 0.5mm² cables directly joined onto it along the way, you'd need to protect those.

 

[pc1966]

As @mainline says the DNO fuse is there to protect their cables from overload by anything down-stream of it (i.e. fault or excess demands by the installation). It need not meet the disconnection times that the regs demand for sub-m,ain, etc, as its purpose is not ADS for shock protection.

 

[mainline]

It would be the same as having a 20a protected radial wired in 2.5mm² with 0.5mm² cables directly joined onto it along the way, you'd need to protect those.

Would that apply to say an oven fan that has its supply cable in 0.5mm being fed from a 32amp supply ?
Or an alarm panel ?

 

[snowhead]

Would that apply to say an oven fan that has its supply cable in 0.5mm being fed from a 32amp supply ?
Or an alarm panel ?

No response required.

 

[R-fur]

There are two types of fault, short circuit ph-ph or ph-E and overload.
The fuse at the substation protects the distributor from a short circuit and from overload, it also protects the service from short circuits. The cutout fuse protects the service from overload only.

 

[rewireIT]

So on a kettle, is the fuse there to protect the appliance lead?

Cause i still don't understand how this is working.

There's an appliance load drawing on a cable, if load draws too much, fuse at the start of cable breaks, stopping flow, protecting appliances cable.

Load draws on double socket, draws too much, breaker in CU breaks to protect 2.5mm wiring into socket.

Entire load of house pulls too much on 25mm tails, main fuse goes.

That's how it's always been explained to me.

So it should then stand to reason that cut out is there to protect the installation wiring not the main incomer?

Again, i'm in the corner with the dunce's hat and a probably overthinking it.

 

[nicebutdim]

Entire load of house pulls too much on 25mm tails, main fuse goes.

That's how it's always been explained to me.

So it should then stand to reason that cut out is there to protect the installation wiring not the main incomer?

Again, i'm in the corner with the dunce's hat and a probably overthinking it.

Tails will be sized according to cut out fuse and will indeed be protected by it, but that same fuse also protects reduced CSA DNO cable from overload by consumer.