I propose to do the following:

Re-wire upstairs power circuit
Re-wire downstair power circuit
(because they're both totally inadequate!)

Leave kitchen power circuit because it's perfectly adequate (the
kitchen is a 1970s extension)
Leave the lighting circuits alone (again, the wiring is fine, and the
circuits are adequate)
Leave the cooker circuit (no electric cooker, cable seems very recent)

Completely re-wire the garage


Replace the 6-fuse consumer unit with a split load 13 or 20 way
consumer unit. RCD everything except lighting and garage (which will
have it's own CU with RCD).

My question is: is it OK to connect the old wiring to the new consumer
unit?

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?), and I'm happy that the wiring is in good
condition, but I can't be sure that the installation of all this
wiring meets new regs. (e.g. The earth bonding in the bathroom is
non-existant, but I will fix that).

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?

Cheers,
David.

"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.

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 may be able to borrow or hire one.

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.

It is actually quite difficult to make a 6A lighting circuit run in 1mm (or
1.5mm) cable non compliant. The cable is so overrated and carrying so little
current, that there are rarely issues with cable lengths. However, you
should check that the circuits have an earth and are run in PVC insulated
cable. Also, you should check inside all the accessories and pendants for
any bodging gems.

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

As you don't know the length of the installed cable, it will be easier to
measure the cable resistance directly and calculate what the maximum earth
loop impedence and voltage drop will be. Then just compare with what is
allowed. You must have an earth loop impedence that allows 160A to flow to
be sure of tripping a B32A MCB.

Does this give me better protection than one RCD overall?

Not as such. It reduces both the likelihood and consequences of nuisance
trips.

Christian.

"David Robinson" wrote
| 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.

Perhaps they only put new faceplaces on the old wiring? One house I lived in
had lovely lead-covered cable and iron-clad DP fuseboards but the brown
bakelite light switches were the modern square style.

Owain

"Owain" wrote in message ...
"David Robinson" wrote
| 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.

Perhaps they only put new faceplaces on the old wiring? One house I lived in
had lovely lead-covered cable and iron-clad DP fuseboards but the brown
bakelite light switches were the modern square style.

Thanks for the warning Owain. Luckily I've already changed two light
fittings, and been in the loft, and all the wiring is fine. If it was
an old house, I'd rather change the wire and keep the nice old
fittings, but I guess that's not advisable, and it's not applicable
here anyway.

The only thing I could fault is that the earth posts in the metal
boxes behind the sockets are not used - the earthing just loops into
the earth on the socket, with no lead to the metal wall box (inset
into the plaster) itself. Should I add a lead, or is it enough that
the sockets themselves connect the earth to the 2 screw holes? I was
going to go around and add a wire to them all.

Cheers,
David.

If it was an old house, I'd rather change the wire and keep the nice
old fittings, but I guess that's not advisable,

You're right. The problem with old fittings is that the insulation in them
will normally have broken down by now and be unsafe, just like the the
rubber insulated cable originally used to connect them. I intend to get some
reproduction ones for my house with modern internals. In particular, some
round Bakelite light switches for the hallways and a couple of dome brass
ones for the principal rooms.

Christian.