er... part 3:
The old practice of protecting a whole distribution board by
a single high-sensitivity RCD can, in many cases, lead to unwanted tripping, particularly in
industrial environments where inductive loads will cause greater transient overvoltages and
where longer cable runs will result in larger values of capacitance to earth. There have been
instances of a contractor supplying a 12-way TP and N distribution board, via a 30mA RCD,
to control banks of fluorescent fittings and then wondering why the RCD kept tripping out!
Apart from initial current surges on switch-on and possible flashover due to a faulty fitting,
an isolated fluorescent fitting by itself does not appear to be a major source of unwanted
tripping. Banks of fluorescent fittings used in conjunction with other appliances will,
however, result in an accumulation of standing protective conductor current and will
present a low impedance path to earth for transient overvoltage currents through the
capacitance to earth.
9.3 Double-Pole Switching
Double-pole switching within the fixed wiring is known to produce a strange phenomenon
whereby switching OFF a double-pole switch supplied through an RCD can cause the RCD
to trip. Single-pole switching does not produce this effect, and it is known that changing
over from double-pole to single-pole switching can overcome the problem, where such
replacement is permissible and safe.
The phenomenon is explained by the fact that while capacitance between neutral and earth
will exist in all installations, the earth leakage through this capacitance will be negligible due
to the low (almost zero) potential between neutral and earth. When the neutral pole of a
double-pole switch is opened, the voltage across this capacitance will suddenly increase,
with a subsequent increase in neutral-to-earth capacitive earth leakage currents. This
increase will be at a maximum if the neutral opens first and will be aggravated by arcing at
both switch contacts which will cause high frequency voltage spikes to cause even higher
neutral-to-earth leakage currents.The effect may be aggravated further by the slow-break
feature of the switch often found in domestic a.c. switchgear.
Closing a double-pole switch may also trip an RCD (as may closing a single-pole switch),
but in this case any tripping is unlikely to be caused by neutral-to-earth capacitance since
the fast-make characteristic of the switch reduces both arcing and the time that any
significant voltage might exist between the load-side neutral and earth. Any circuit, which
incorporates only single-pole switching, will never experience this type of problem since
(under normal operating conditions) the neutral-to-earth voltage is so low that the neutral-to-earth capacitive earth leakage is negligible.
The problem appears to occur most frequently in installations where a separate consumer
unit and RCD are installed. The opening of the consumer unit switch will allow a higher
voltage (possibly 230V) to appear suddenly across the neutral-to-earth capacitance of the
whole installation. This can result in sufficient earth fault current to trip the RCD.
9.4 Cables and Overhead Lines
There are indications that the problem of unwanted tripping occurs more frequently in
installations supplied by overhead lines than by those supplied by underground concentric
cable. This can be explained by examining the capacitance and inductance of these two
types of conductor as shown in Annex 10.2. The analysis shows that: