I'll answer your questions to the best of my ability!
I've never been totally clear about these! The supply comes underground (1960's build), and up to the main fuse in what looks like SWA cable. L & N tails go from the main fuse to the consumer unit. There is an earth tapping on the side of the main fuse block, from which an earth feed goes to the consumer unit. I assume that the earth is derived from the sheath of the SWA rather than from the neutral conductor, but I cannot say for sure since I can't access the internal connections of the fuse block.
I propose to use a 40A MCB in the consumer unit. [There is currently a much lower rated one, since it only feeds a shower pump]. I have in mind to use a garage consumer unit the other end, such as Screwfix 63138. This has overall RCD (40A 30mA) protection, plus MCB protection on the individual power and lighting circuits.
On Thu, 22 Jun 2006 10:43:03 +0100 someone who may be "Roger Mills" wrote this:-
The supply cable probably has a central energised conductor, surrounded by a protective earth and neutral conductor. The latter is connected to the neutral tail and the earth block inside the unit that contains the fuse. TN-C-S
Split concentric (for TN-S) was certainly in use by the late 60's. I remember the MEB, as was, replacing the service to my parents' house around that time. I filched an offcut of the cable for examination - phase & neutral were 7/0.064 (neutral being seven individually insulated
0.064 strands) and there were four somewhat larger uninsulated strands for earth.
AFAIK, SWA was and is seldom, if ever, used for service cables.
The supply comes underground (1960's build), and up to the main fuse
Give the date, I suspect TN-S. TN-C-S/PME didn't come into use until the 70s (AFAIK). I've assumed TN-S (Ze max = 0.8 ohm, in what follows).
With a 40 A MCB, the 2.5 mm^2 CPC in the T&E leg is looking a bit puny. With a Type B MCB you should just about be OK, provided those circuit lengths are correct. With a Type C it certainly wouldn't comply. Since cables always tend to end up a bit longer than you expect, I strongly suggest that you either upgrade the T&E section to 10 mm^2, or run SWA all the way, or reduce the MCB at the feeding end to 32 A, Type B.
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