suppliers main fuse (100a)

[j.capes]

Hi, Im sure this has probablly already been posted on this forum elsewhere, however i have a question i would just like clearing up if any one can help. Looking at most domestic electrical supplies the main suppliers fuse is 100a and the main switch in the consumer unit is 63a (or less than 100a). From my limited understanding this dosen't seem to make sense beause the installation load from the house would possibly exceed the main breaker size regardless of diversity and it would take up to 100a for the supply fuse to blow even though the main switch's contacts can only take 63a ? Wouldn't this damage the main switch in the consumer unit? If this makes sense to you what im trying to get accross, any help would be appreachited. Im not doing any work like this i have just started to get an intrest in some electrical work and would like this explaining. Thank you

 

[malcolmsanford]

I think your thinking of a 63amp RCD and normally in that case there is 2 of them 1 X 80amp and 1 X 63 amp so you are spliting the load.

The 63amp is as you say a rating that the RCD can take without destroying, but as that is protecting a certain amount of MCBs, the total loading on that side of the circuit will very rarely exceed the 63amps. As for a fault current, well it takes 30mA to trip a 30mA RCD, in fact it is less and can be below 20mA in certain makes, so there is very littel chance of a fault current being so high as to damage the RCD.

A main switch will be different, almost all the double pole switches you find in a CU will be 100amp rated.

Also in many many installations the main fuse will be 80amp or as low as 60amp, so there is very little chance of the 63amp RCD being overloaded.

 

[Electricalserv]

Hi, Im sure this has probablly already been posted on this forum elsewhere, however i have a question i would just like clearing up if any one can help. Looking at most domestic electrical supplies the main suppliers fuse is 100a and the main switch in the consumer unit is 63a (or less than 100a). From my limited understanding this dosen't seem to make sense beause the installation load from the house would possibly exceed the main breaker size regardless of diversity and it would take up to 100a for the supply fuse to blow even though the main switch's contacts can only take 63a ? Wouldn't this damage the main switch in the consumer unit? If this makes sense to you what im trying to get accross, any help would be appreachited. Im not doing any work like this i have just started to get an intrest in some electrical work and would like this explaining. Thank you

The main switch can withstand more than its rating for a certain period of time.

 

[mike2790]

is the supply a confirmed 100A?
the 100A label on the DNO cutout is probably just what the cutout is rated to.
check actual size of main fuse and upgrade main switch if necessary.