Electrical Contracting News (ECN) September 2017 | Page 54

SPECIAL
FEATURE
TEST & MEASUREMENT
out a ramp test using a multifunction
installation tester. A 30mA RCD or RCBO
should operate somewhere between 24 and
27mA. If it operates at a lower current, then
it may be the source of the problem and
should be replaced.
Where does the
leakage current
come from? The
electrical contractor
must establish
whether it’s from
unintentional
earth leakage or
intentional leakage
current.
fl owing in these conductors, which is the
earth leakage current. Because no direct
connections are needed to the circuit under
test, this form of testing is fast, easy and
safe to carry out.
Earth leakage measurements can
be made with a clamp meter in almost
any location where the line and neutral
conductors are separate from the
protective conductor. Typically, locations
are at the mains incoming connections,
at each of the outgoing circuits from a
distribution board, including the sub-main
circuits, at intermediate points on radial
circuits and at the connected equipment.
A useful technique for locating
troublesome circuits in domestic and small
commercial installations is to turn off all
the MCBs and RCBOs in the consumer
unit, then position the clamp meter around
the incoming mains cable. As each circuit
is switched on in turn, it will often be found
that one causes an unexpected increase
in the earth leakage current. This circuit
is likely to be the source of the nuisance-
tripping problem.
Sometimes, however, life is not quite so
easy, such as when nuisance tripping is the
result of an intermittent fault. In this case,
a clamp meter with a peak hold function
can be helpful; this can be left in place to
monitor a suspect circuit until a trip occurs
and will show the maximum earth leakage
current that has fl owed in that circuit.
In challenging cases, it’s also worth
remembering that nuisance tripping can
be caused by an overly sensitive RCD or
RCBO. This can be checked by carrying
‘Resolving
nuisance
tripping can
be diffi cult,
but an earth
leakage
clamp meter
can make it
easier.’
Intended or unintended?
Assuming the fault is not with the RCD
or RCBO and that the problem circuit has
been located, the next step is to decide
whether it results from unintended earth
leakage – from a fault in the circuit or
the equipment connected to it – or from
intended earth leakage. If it is unintended,
the fault must be found and repaired.
Typically, this will involve safely isolating
the faulty circuit and carrying out insulation
tests on the circuit itself and the equipment
connected to it.
If it results from high intentional
leakage current, it may be possible to
employ circuit splitting. Here some of the
devices with high leakage currents are
transferred to different circuits so the earth
leakage in each circuit is kept to a level
where it won’t cause problems.
Correctly used, RCDs and RCBOs do
an excellent job