Isolator for a second consumer unit

[RegistrarNick]

My house is very long and narrow - 60ft from front to back. The supply comes in at the front of the house, into the meter in the cellar (PME earth), then into an MEM 100A ASN100R isolator installed by the electricity supplier and then into a service connector block. There are two sets of tails from the connector, one straight into a split load 11 way (6 used) consumer unit which services the main part of the house. The second set of tails goes into a 60A DP isolator (installed 20 years ago) from which 20mm (external size) twin & earth -about 40ft long - goes to a separate consumer unit feeding the kitchen at the back of the house. I have just replaced the old kitchen consumer unit with an MK 8 way split load (only the ring sockets are on the protected side). The new MK has the standard 100A isolator. Question - yes, there is one - do I need to replace the 60A DP isolator in the cellar and, if so, which make would be suitable. Any advice will be much appreciated, thanks.

 

[ban-all-sheds]

My immediate concern would be the fact that you've got 40' of T&E cable (what does 20mm external size mean? - 16mm conductors?) running through the house with only the main fuse to protect it if something should go wrong.

Having an isolation switch is optional - at the moment you lose the kitchen circuits if you need to isolate the main CU, so being able to isolate the kitchen but not the rest of the house is probably of dubious importance. But I would certainly install something with a fuse to protect that 40' of cable.

 

[RegistrarNick]

Many thanks for the prompt reply. There is nothing physically wrong with the present isolating switch (even though it is only rated to 60A). I have not added any loads to the kitchen circuit, simply replaced an older CU which had push-button MCB's and no RCD. Will it be in order to leave things as they are?
The cable is rated Pirelli BASEC 300/500V; I am not sure the actual conductor size.

 

[ban-all-sheds]

The problem is not that you could overload the cable via the circuits from the kitchen CU (unless it really is too small), but that it is almost certainly too small to be properly protected by the main service fuse, which is all it's got, and which could well be a 100A one.

It is an unlikely event, but if a fault were to develop on the cable you could get enough current flowing to damage it, or even start a fire, but not enough to blow the main fuse.

You can get 60/63A MCBs, and you've got spare ways in your main CU - I'd suggest running it from that.