Need some help please. I've got an overhead supply with no earth. When I moved
in about 20 years ago there was an old cast iron fusebox with wired ceramic
fuses. All the earth conductors for the (radial) circuits were twisted together
and clamped to the fusebox casing, but there was no connection to earth. Also
the meter tails were old VIR cable about 6mm² and were heating up.
The first thing I did was to put in a new CU and an earth rod, connected by
16mm² sheathed earth cable, and new 16mm² double sheathed tails. The CU has
MCB's and a 30mA RCD main switch which controls all the circuits. Since then
I've rewired all the circuits one by one and put in equipotential bonding, now
altered to comply with 16th Edition, which I'm pretty sure would pass
inspection. The problem is the OSG says I should have a 100mA RCD for
non-ground floor sockets - I don't understand the reason for this. Do I need to
change the CU again? I would have thought my 30mA RCD was even safer.
Also, I've never tested the impedance/resistance of the earth rod and I don't
understand what I need to do here. The rod is driven into the earth oversite
under the floorboards in the understairs cupboard. I drove it in about 1m but
it would not go any further - either I hit the chalk or a big flint - so I cut
off the top 200mm or so.
What do I need to do to bring the installation up to modern standards please?
I've got the latest OSG but it doesn't explain in words I understand.
The 100mA RCD is for fire protection, not direct contact protection. I
have a TT system and have tow earth rods. One at hte front of the house
near the consumer unit and the other at the rear near the water mains
pipe. I don't have an the 100 mA RCD. This requirement may just be for
new installtion TT systems?
I have a TT system with a split board, 30ma rcd protection on 45A cooker
circuit and 2 x 32 A ring ccts. The rest is protected by a 100ma MCB main
switch. If you have a TT system all sockets should be RCD protected.
You need to test the impendence of your earth rod as the rod and the ground or
earth are the fault path back to the transformer where you get your supply. You
will find it on page 148 of OSG and earth resistance of your spike when tested
against any of your final ring ccts must be below 200 ohms ( yes i know that
sounds high ) when tested against any ring cct. My spike was 51 ohms and had a
disconnection speed less than 0.4 s which is in spec
If your bonding meter tails and earthing are up to 16th Edition standards
why not get the supply converted to PME (usually possible but you would have
to check with your local electricity supplier)
Then you could do away with the earth stake and the problem of testing its
Modern practice for TT circuits is to have a time delayed 100mA RCD for the
entire installation. Then, you either use a split load consumer unit with
30mA RCD for sockets, or use a standard consumer unit with 30mA RCBOs for
The regs don't require 30mA RCDs on upstairs socket circuits, although they
probably should for additional direct protection.
The rationale behind it is that in the event of an earth fault on the socket
circuits (particularly those used outside), it won't cut the lights, as the
30mA instant RCDs are guaranteed to trip before the 100mA time delay.
This can be measured with the correct equipment. (See "TT Earthing" thread).
I'd replace the 30mA RCD with a 100mA Type S. Then replace the socket
circuit MCBs with Type B 30mA RCBOs of the same rating. Any circuits running
fixed equipment in the bathrooms should also be off RCBOs. You can probably
keep the old consumer unit, if it is DIN rail and you can find single width
RCBOs for it.
The alternative is to replace the consumer unit with a split load. The
isolator switch is then replaced by the 100mA Type S (or you can have it in
a separate box and keep the isolator). You then load your RCD requiring
circuits to the RCD side and the non-RCD requiring circuits to the
isolator/Type S side.
The final alternative is to upgrade your electricity supply to TN-C-S. This
may or may not be available and you may or may not have to pay for it. You'd
still need to do the split load/RCBO thing, and you might have to upgrade
your main equipotential bonding.
However, you current installation would be regarded as "not to current
specification" rather than dangerous and there is no pressing need to do
anything, apart from test your earth rod.
Excellent advice Christian, thank you very much. I've saved your message and
will be looking to change the CU soon. As for testing the earth rod, unless I
can borrow or hire an instrument locally I think I will be asking a pro to come
and do it.
Thanks also to Neil Bob & Sparky. I enquired about PME a while ago and they
said it wasn't possible, even though the substation is at the back of my garden!
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