On Fri, 23 Jan 2004 07:03:40 -0000, "Peter Taylor"
I was just going to say seems fine apart from the tails, could do with
upgrading to 25mm but there you go.
Although the current isn't up to current regs there's nothing that
says you have to change the CU. If you have a 30mA RCD as the main
incomer then you're covered for faults, the only reason to change is
convenience and safety. (See loads of discussions on this type of
thing). If it was me I would change it to a time delayed CU.
Yes, although you may find it cheaper to buy a complete CU with MCBs
and then swap the main isolator for a spearately purchased 100mA time
delay RCD. Make sure you can get the same make, 100mA time delay RCDs
are often not 'off the shelf' items and you may find that it's easiest
to find the one of these that you want and then buy a CU to match.
Those ratings seem entirely normal to me. 6A for lighting, 16A for
immersion heater or similar, 32A for ring circuits, 45A for cooker or
shower. You might want some 20A ones for radial socket circuits.
However MCBs are so cheap now that it may well make sense to get all
noew ones which are certain to be correct for the new CU.
"Peter Taylor" wrote
| Hope you don't mind if I tag another question on this thread.
| I have TT earthing and the CU I fitted in 1984 which has just the one
| 30mA RCD (also acting as the main isolator) and 8 Type B MCB's. I
| want to change the CU to a split load one, with a 100mA time delayed
| RCD and a second 30mA RCD for GF sockets, garage etc.
On a TT system you will need 30mA RCD protection on *all* sockets, not just
ground floor and garage.
Owain, if you've got a copy of the On Site Guide, could you please have a look
at Fig. 3a on pages 21/22. The middle diagram is my one, the one with the
time-delayed 100mA RCD doubling as the main isolator. The 30mA RCD is fed via
the 100mA, and the circuits it serves are labelled "note 2". Circuits fed
directly from the 100mA are labelled "note 1".
Note 1 says "circuits to lights, cooker, water heater, smoke alarms"
Note 2 says "circuits to portable equipment outdoors and socket-outlets that may
reasonable supply portable equipment outdoors (ground floor socket-outlets)"
It doesn't mention upper floor sockets at all. Am I missing something? Where
does it say *all* sockets.
Also Clause 3.6.2 says "Where an RCD is fitted ...... in a TT system, the rated
residual operating current should not be less than 100mA". Doesn't this mean
that 30mA is too low????
I am getting more and more confused here by the minute!
Thanks for your help
"Peter Taylor" wrote
| > On a TT system you will need 30mA RCD protection on *all*
| > sockets, not just ground floor and garage.
| Owain, if you've got a copy of the On Site Guide, could you please
| have a look at Fig. 3a on pages 21/22. The middle diagram is my
| one, the one with the time-delayed 100mA RCD doubling as the main
| isolator. The 30mA RCD is fed via the 100mA, and the circuits it
| serves are labelled "note 2". Circuits fed directly from the
| 100mA are labelled "note 1".
| Note 1 says "circuits to lights, cooker, water heater, smoke alarms"
| Note 2 says "circuits to portable equipment outdoors and socket-outlets
| that may reasonable supply portable equipment outdoors (ground floor
| It doesn't mention upper floor sockets at all. Am I missing something?
| Where does it say *all* sockets.
I think they've missed something, not you.
Reg 471-08-06 requires *every* socket-outlet on a TT system to comply with
413-02-16 requires ZsIdeltan < 50V
413-02-20 for TT systems requires RAIa < 50V
In practice, you use a 30mA RCD unless you can prove your earth
electrode system is and will remain effective. Whitfield simply says that
all TT sockets require RCD.
| Also Clause 3.6.2 says "Where an RCD is fitted ...... in a TT system,
| the rated residual operating current should not be less than 100mA".
| Doesn't this mean that 30mA is too low????
I assume it's talking about whole-house RCDs which should be 100mA Type S
time-delay to allow discrimination against the 30mA Type G socket circuits.
Thats it. I was trying to find the relevant regs. I remember you had
to do it, just not why! Being that it would be very unlikely to get
the Zs down on TT system far enough to not be able to not use 30mA
RCD's then they just get put on all socket circuits. I think it'ws
called 'a rule of thumb'!
That's brilliant, thanks chaps. I wonder how many other DIY-ers like me follow
the OSG not knowing about this. I will feel happier with all sockets coming off
the 30mA - I think I'll put the immersion on it too, in fact everything but
lighting & cooker. I don't have a dedicated freezer supply, but I might put one
in now after I've done the CU. Presumably that will be a 2.5mm² T&E radial
coming directly off the 100mA RCD. Does it need a different type socket or
maybe a permanent flex connection?
If it's going to be off the 100mA side it would have to be through a
I think putting the immersion on the 30mA could be asking for trouble.
There really isn't any need.
I would just have sockets, shower and cooker, if it has a socket
incorporated into the switch, on the 30mA. Everything else on the
No, you need RCD protection, but it doesn't have to be as low as
30mA as it's purpose is not protection against electrocution, but
as protection against high earth fault loop impedance.
Now some sockets are likely to need 30mA RCD protection to protect
against electrocution and you might decide to provide such protection
even on sockets where it isn't required by regulations, but a TT system
does not require all sockets to have 30mA RCD protection.
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