"Christian McArdle" wrote in message et...
I know I need to test it to check that it won't trip the RCD (is
20Mohm+ from the earth to phase and neutral, measured on a cheap
multimeter, good enough?)

You need to use a megger on it. A cheap multimeter (that probably applies 1V
DC) can't really cut the mustard on this one. A megger will apply 500V DC to
the cable to be sure. Many insulation breakdowns only occur above a certain
voltage.

I can understand that. We're talking about £200, aren't we? That might
make it cheaper to get someone else to do the job.

You should test it with a proper continuity tester that supplies 200mA
(usually on the same meter as the insulation resistance) and check that it
has sufficiently low resistance. On a ring main, the live and neutrals
should have a similar resistance. The earth should be somewhat higher. 2.5mm
cable has only a 1.5mm earth. Older 2.5mm cable had only a 1mm earth. This
test will tell you if this is the case, as the resistance readings should be
in proportion to the cable cross sectional areas. If it is only 1mm CPC, you
really need to do careful calculations to ensure that earth loop impedence
is OK, as much shorter ring distances will be allowed where limited by earth
loop impedence.

I'll check all that on the kitchen ring main.

Does all the wiring connected to a new consumer unit have to meet
current regs, or is it OK to transfer older wiring (late 1970s,
possibly not quite meeting current regs in all respects) to the
consumer unit?

It is unlikely that late 1970s wiring won't comply.

I know the kitchen was only built in 1978, but I'm not certain when
the lighting in the rest of the house was wired. (The power circuits
in the rest of the house don't matter, because I'm replacing them).

From the way almost all the wiring and sockets/switches match
throughout, I think the whole house was re-wired when the extension
went up, with the electric cooker circuit upgraded (and shower circuit
added) later. But I could be wrong. If it was re-wired all at once,
they chose sensible routes and positions for the sockets in the
extension, but stuck with the existing stupid positions for the
sockets in the rest of the house.

Whatever - if I test it, I'll know if it's good enough. Dread to think
of the mess I'll cause if it isn't!

If it doesn't, it
shouldn't be difficult to bring it up to standard, as it is more likely to
be a fault than by design. Minor technicalities can be ignored, but as you
are now using Type B MCBs instead of fuses, it is important that you ensure
the earth loop impedence is enough to drive 5x the nominal current through
every circuit. It is dangerous not to do this, as earth faults would take
over 10 seconds to clear, except on RCD protected circuits which you aren't
really supposed to rely on for primary protection anyway (except TT
systems).

Which bit of the on-site guide deals with this? The tables of allowed
cable sizes, currents, and MCBs?

On your circuit design, consider replacing with a non-RCD consumer unit and
using RCBOs for circuits requiring RCD protection. It will be more
expensive, but a better design.

Another mid way alternative is to use a split load, but have socket circuits
on RCBOs on the non-RCD side, with other circuits as you previously
suggested.

Does this give me better protection than one RCD overall?

You may want to install a non RCD freezer circuit to prevent RCD
cutouts ruining your stocks.

Yes, that'll be in the garage. That all needs re-doing because there's
a single FCU running everything.

Christian.

Thanks for your help.

Cheers,
David.