What does the main fuse protect?

[MJPD29]

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.

10mm concentric in our house and most of the surrounding area. Always found it interesting that the tails are bigger than the incoming cable.

 

[davesparks]

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

The fuse in the plug is there to protect the appliance lead from overload or fault current.

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.

To protect against overload the fuse needs to disconnect the appliance from the supply, so whether it is at the beginning or end of the cable it doesn't technically matter.

This is only true of overload protection, for fault protection the fuse needs to disconnect the fault from the supply, a fault can occur anywhere in the cable so the protection needs to be at the beginning.

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

Yes, but equally if there was a fuse built in to the socket then that would protect the 2.5mm wiring exactly the same.

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

Yes, and because that fuse has operated the overload is disconnected from the cable upstream of the fuse.

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

It is put there to protect the incoming service cable, it is not put there to protect the installation wiring.
It will in reality protect the installation wiring but it is not installed for that purpose and we are not allowed to use it for that (except for meter tails up to 3m long)