Theoretical question regarding ratings..

[Mrdjc]

Say you have a supply rated at 63A coming into a property, directly from the overhead wires.
Through the meter - and into the consumer unit.
The consumer unit has a 100A rated isolator switch.
The load consists of 4 circuits each protected by 16A MCB's, and one circuit protected by a 32A MCB.

What happens if each of the 4 circuits draw say, 14A each (56A) and at the same time the 32A rated circuit draws 20A.

That's a grand total of 76A, which exceeds the supply rating.
What is there to protect the supply cable coming into the house from overcurrent if there is no master fuse fitted before the consumer unit?

None of the circuits are exceeding their maximum rated value.

 

[bernardgreen]

The big fuse at the electricity sub station is one protection.

Though in some countries there may be a fuse for the supply cable where the cable leaves the distribution pole.

 

[Taylortwocities]

And there's usually a fuse in the house that protects the incoming supply.

Also, a cartridge fuse (eg BS 1361 or 88 ) will take an overload for a long, long time before it eventually blows. A 63amp BS88 fuse will live for hours with 76 amps going through it.

In fact BS 88 Part 2 specifies that a fuse rated at 63 A or less must NOT operate within one hour when carrying a current 20% greater than its rating. For higher rated fuses, operation must not be within four hours at the same percentage overload. The latter are only required to operate within four hours when carrying 60% more current than their rated value.

See the time graphs in the Wiring Regulations. But expect you do not have a copy? There's a sub-set here http://www.tlc-direct.co.uk/Book/3.6.3.htm

 

[plugwash]

There should be a fuse or other protective device somewhere between the distribution network and your meter. If there isn't then the electrical system was designed and installed by dangerous cowboys.

Depending on local practices the location of that fuse may vary. In the UK it is usually but not always located between the supply cable and the meter.

If you are concerned that there is no fuse protecting your supply I would suggest speaking to your supplier as a matter of urgency.

 

[westie101]

A lot of supply networks don't protect the network against overload really, we rarely do to any extent.

As an example a 500kVA 11kV/LV transformer will often be protected on the HV side by 3x56A fuses. (which allows for a load of over 1MW)
On the LV side it could have 6 or more circuits each protected by 400A fuses.
(6 @ 400A again is over 1MW). The maximum LV fuse size is dictated by the need to maintain discrimination with the HV fuses, and to operate for a fault on the LV cable. (though it is acceptable to leave lengths of LV cable unprotected at remote ends of the network)

It is quite common when there is a major gas incident for Transco to issue electric heaters & cooking rings to their customers.
We've had a few transformers destroyed with the subsequent load as have other DNO's. Fortunately we've not had any cables burnt out (it has happened in other circumstances)