Re: [OT] Wiring - RCD's

["David Buckley" <db@xxxxxxxxxxx>]

--- In ukha_d@xxxxxxx, "Paul Gale" <groups@s...> wrote:
> Anyone know if you can fit two RCD's in line?

Yes, you can put multiple RCDs in line.  The way they work is to
measure the difference in current flowing in the phase and neutral
conductors, and if they are different by more than a certain amount,
then to open the circuit.

However, ensuring the right one trips is harder.

If you use different current threshold ratings (your example of 30ma
and 100ma) then for a 50ma leak, the RCDs should discriminate
correctly.  Put a 500ma leak on, and its a crap shoot.  Either could
win.

Under these circumstances, the best approach is time
discrimination.  The idea is that the faster unit will always trip
first.  This approach is widely used in circuit breakers of all
sizes from the home consumer unit to the national grid.

However, i've not seen any RCDs with either adjustable or delayed
trip action.

Also, if your higher RCD is also protecting other stuff of the plug-
in variety, then you would not want either to delay the trip, nor to
have a higher trip current.  You want to live.  It could be argued
that a slower trip would have negligable impact on permanently wired
loads such as water heaters, cookers, lights etc.

Advice:  get a non-RCD feed for your garage, and put the sole RCD
nearby.