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.

 

[rewireIT]

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.

I guess that goes without saying but it's the notion that it's there to protect the main incomer that's got me confused. I guess it's a kind of upstream fuse that has a dual purpose then

 

[mainline]

I guess that goes without saying but it's the notion that it's there to protect the main incomer that's got me confused. I guess it's a kind of upstream fuse that has a dual purpose then

Imagine a radial circuit with lots of branches off it protected by a 20 amp breaker, the 20 amp breaker is supplying the whole tree each branch has a fuse calculated to not overload that 20 amp breaker.

Each branch has its own fuse calculated to not overload the tree or the branch.

Sounds like a children's story.

 

[Boristheengine]

I'm no electrician, but my here is my take.
The key point here is that the regulations and design calculations work on the basis that cables can have too much current through them either due to
a) too many devices/too high load being applied, or
b) a fault in the cable itself.

Cable faults are assumed to be dead shorts so the current is defined by the loop impedance. It is furthermore assumed that partial faults that overload without short circuit doesn't happen.

And too much load can only can only happen where it is possible for a non electrician to exceed the maximum current of the cable (e.g. by plugging too much into sockets, or turning too many things on at once)

Normally in a house, protection for both a) and b) is the same device. But it technically doesn't have to be.

For the DNO the 300A fuse at the substation will blow for a short circuit on the main feeder, or the smaller branches to houses. This covers b). The cut out fuse blows for overload from the consumer equipment, which covers case a). Therefore the supply cable is protected.

The same logic is used in other places. For example allowing a 2.5mm spur on a 32a ring to supply one double socket. The MCB at the consumer unit protects the spur cable against cable faults, and the sour cable is protected against overload by 2x 13a fuses in whatever is plugged in. (Total 26a)

 

[pc1966]

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

Yes.

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.

Exactly.

Normally the over-current protection occurs at the point where a cable changes size, as you pointed out you go from, say 25mm tails then a 20A MCB for your example socket circuit in 2.5mm cable and finally the 13A fuse for your 1.5mm or similar kettle lead.

But imagine a fuse half way along a 1.5mm cable, if you overload the end appliance the fuse blows and so protects the downstream cable (as before) but the interruption of excess current also protects the upstream section of cable.

What it may not protect is damage upstream of such a fuse, and that is why there are specific regulations on cables not adequately protected by the upstream OCPD. See reg 433 and 434.2 for its specific exceptions.

 

[rewireIT]

Yes.

Exactly.

Normally the over-current protection occurs at the point where a cable changes size, as you pointed out you go from, say 25mm tails then a 20A MCB for your example socket circuit in 2.5mm cable and finally the 13A fuse for your 1.5mm or similar kettle lead.

But imagine a fuse half way along a 1.5mm cable, if you overload the end appliance the fuse blows and so protects the downstream cable (as before) but the interruption of excess current also protects the upstream section of cable.

What it may not protect is damage upstream of such a fuse, and that is why there are specific regulations on cables not adequately protected by the upstream OCPD. See reg 433 and 434.2 for its specific exceptions.

Now it actually makes sense. Thanks!