Christian McArdle wrote:

There are 3 basic methods (with variations, of course)

1. Whole system 30mA RCD
2. Split load 100mA time delay+30mA RCD
3. 100mA time delay + RCBOs

Only 1 or 3 would be viable for me...

The problem with system 1 is that it kills the lights in the event of an
earth fault, which could be very dangerous, either due to rotating machinery
or fire escape. Some would say that system type 1 is no longer allowed by
the regulations, although it isn't very explicit about it, if I recall.

In my case, most rotating machinery is in my workshop which is on the
end of an external feed that is already connected via the 30mA RCD - so
I have lost that one before starting. While not an ideal situation, I
can't think of any situation where it would be a major problem should
all power get cut - just a case of stand still, stop pushing whatever
you are pushing and wait for the noise to stop ;-)

(all non hand tools have NVR switches)

The advantages of system 3 over system 2 are twofold.

Firstly, it reduces the chance of a nuisance trip as there are fewer
appliances protected by one device, leading to a lower quiescent residual
current that isn't so close to the trip point.

Secondly, it reduces the effect of a nuisance trip, limiting it to the
circuit the trip occured on.

The main nuisance would be loss of lights - I expect that in many case
some emergency lighting could be provided for less than the cost of
option 3...

Obviously, system 3 can be very expensive if many circuits must be 30mA
protected. You would probably err on the side of protecting more circuits on
a TT system, as the consequences of a nuisance trip on the non-RCD protected
circuits are greater, as they would trip the time delay RCD and kill the
lights.

I would as a minimum need to protect the downstairs, and kitchen ring
circuits. I also plan to install a heated towel rail in my new second
story that will have a 100W backup heater in it for summer use. Hence
that would really require another 30mA RCBO for that ring.

Personally, on a TT system, I'd be tempted to abandon the whole installation
RCD and use RCBOs for every circuit without exception and an insulated
consumer unit. This provides maximum discrimination, a single box solution
and the required RCD protection for every circuit. However, it is very much
more expensive than the cheapest permitted solution. I can't remember if the
consumer unit internals require RCD protection. If they do, you'd still need
the 100mA time delay incomer.

That or option 3 would certainly be the "nicest" solution... to be
honest I am leaning toward option 1 at the moment based on a suck it and
see approach. If it runs like the current setup (i.e. no unwanted trips
at all) then it can stay that way, if it causes problems, then I can
"upgrade" to option 3 at a later date without too much wasted expense
(i.e. just the cost of the unused incomer RCD that would get replaced by
the time delayed one).

As an aside, anyone know of a good source of time delayed 100mA RCDs
(60A rating would probably be enough). The only ones I can find are in
RS with their usual prices!

--
Cheers,

John.

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