in this Q and A video we're going to answer the question are split
load RCD boards compatible with the 18th edition of BS 7671 the wiring regulations
now this question relates to the video we made on the Verso consumer unit from Premspec which has
some brilliant features so if you haven't seen it already please go and check that video out
now before we say anything on this subject we want to make it clear that the very best solution for
a consumer unit is to have an individual RCBO feeding each circuit that requires
RCD protection however manufacturers continue to make and sell consumer units that have one
or more likely two RCBO's feeding several circuits and the electricians continue to
install them so is this compliant with BS 7671 and why might people not think so well it's to
do with that term found in the Regs selectivity or discrimination in previous regulations
the idea that a fault on one circuit shouldn't affect other circuits but is that what selectivity
means well let's see what the Regs say in the definitions section selectivity coordination
of the operating characteristics of two or more protective devices such that on the incidence of
an overcurrent or residual current within stated limits the device intended to operate within these
limits does so while the other does not so that's interesting on the incidence of residual currents
the device intended to operate within these limits does so so looking at a split load board with
one RCCB feeding several circuits which device is intended to operate when there's 30 milliamps
of imbalance in the circuit well this one no other device would operate in that circumstance
so technically we haven't caused an issue with selectivity with this installation
obviously there's a little more to this so which of the Regs apply the following are often quoted
in the context of this subject 314.1 states that every installation shall be divided into circuits
as necessary to reduce the possibility of unwanted tripping of RCD's due to excessive protective
conductor currents not due to a fault so that's interesting this division of circuits is to avoid
nuisance tripping when several loads are running current to earth as part of their normal operation
not due to a fault in the load this can be achieved in two ways according to 531.3.2
either by subdivision of circuits with individual associated RCD's or by arranging the circuits so
that no more than 30 of the rated residual operating current of the rcd is present in
the protective conductors of the circuits it's supplying so for a 30 milliamp RCD that would be
9 milliamps again a hint that a split load RCD board is acceptable going back to regulation
314.2 it continues separate circuits shall be provided for parts of the installation which need
to be separately controlled in such a way that those circuits are not affected by the failure
of other circuits and due account shall be taken of the consequences of the operation of any single
protective device so again this could be taken to mean that a fault on one circuit shouldn't affect
another but it actually refers to different parts of an installation being controlled not
individual circuits having a fault and encourages to think about the consequence of a single device
operating as in if all the lights in a property are on the same RCD then a fault on one of those
circuits would cause loss of lights to the whole property then there's this little overlooked
beauty this is in the section of the Regs that deals with RCD's for fault protection and in the
note to 531.3.5.2 it states that except where particular restriction for selectivity applies
several circuits may be protected by the same device so how do we reconcile all these
seemingly contradictory Regs so they harmonize with each other well think about the principles
that we've got here unwanted tripping or nuisance tripping is referring to devices operating when
there's no fault or no need for them to trip not to a fault on one circuit affecting another
where 314.2 is referring to the failure of a circuit not affecting other circuits it means
that we wouldn't want to lose the whole floor of a building because of a fault on another floor that
area of the building is separately controlled you could take that principle to extremes and
say that each room in a house should have its own individual lighting circuit so that a fault in one
room won't cause the loss of lighting to the room next door we need to apply some good engineering
judgment it also encourages us to think about the consequences of a single device operating
so yes a fault on a circuit fed by an RCCB will disconnect other circuits
so to avoid the consequence of losing say all the lighting to a property from a fault on one circuit
split the lighting across two RCCB's so there's always a lighting circuit left on do the same
with the rings so there's a chance that table or floor lamps can still be turned on and why
not put the upstairs lighting on the same RCCB as the downstairs sockets and vice versa so there's
always a live circuit on one floor after all that note in 531.3.5.2 tells us that unless there's a
particular restriction for selectivity we can protect more than one circuit with one device
so looking at the principles of the regulations that we've considered we can see that while a full
RCBO solution is undoubtedly the best one with a little engineering judgment and sound application
of the regulations we can make our split load RCD consumer unit installations comply as well
unless amendment 2 comes up and changes everything of course but as always we want to hear from you
on this issue do you prefer full RCBO installs or are you still happy with split load boards
do you think we'll see increased stringency in this area of protection either way please leave
your thoughts and questions in the comments below and as always thank you very much for
watching