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45
BY
DAVID
HERRES
them is probably faulting. Electrical
equipment, especially motors, tend
to draw more current as they age,
due to internal insulation gradually
breaking down. This can also happen if a
transformer is connected.
A hair dryer or laptop computer,
when the internal battery is being
charged, may be the culprit. Unplugging
connected loads one at a time, with
a clamp-on ammeter at the entrance
panel, will quickly locate the fault. The
Amprobe ‘hold’ function is useful in this
procedure.
Nuisance tripping can also be a
problem with residual current devices
(RCDs). In the United States and Canada
these are known as ground-fault circuit
interrupters (GFCIs). The purpose and
inner workings of both are similar, but
the circuit deployment varies due to
different phasing arrangements.
NEC and CEC, as well as the Australian
and New Zealand Wiring Rules, require
such devices in sensitive locations –
wherever the proximity of electrical
power and water may lead to a shock.
Since their introduction in the 1960s,
and subsequent widespread use, RCDs
have greatly reduced the number of
non-utility electrocutions, especially
in homes.
The equipment-grounding conductor
usually completes the circuit to the
entrance panel, then the over-current
device trips. However, in some instances
that conductor breaks or has not been
properly hooked up. The result can be an
electric shock – sometimes fatal.
The RCD takes the form of a breaker
in the entrance panel or load centre,
a receptacle at the wall outlet, or a
moulded-case device built into a power
cord and located adjacent to the plug.
The hot wire and neutral return
conductor of a 240V circuit both
pass through the device. Normally,
the current in the supply and return
conductors is equal (conforming to
Kirchhoff’s Current Law that electrical
current is everywhere the same in a
non-branching circuit).
In a two-fault situation (involving
connectivity of the hot wire and loss
of connectivity of the equipment-
grounding conductor), when someone
touches the energised metal, some of
the current goes to earth.
The RCD monitors both current paths,
performing a continuous differential
measurement. If the difference exceeds
a specified limit, typically 6mA, the
device interrupts the circuit. The amount
and duration of the brief electric shock
is not enough to constitute a hazard.
Many of these devices combine the
function of an over-current device, and
they can also be used as a switch by
pressing the ‘test’ button.
The newer RCDs have an LED that
indicates a tripped condition, so it is
possible to know at a glance where
the problem lies. Moreover, if the RCD
is not getting power, it will not reset,
providing immediate information on the
status of the upstream portion of the
branch circuit.
Unfortunately, RCDs are prone to
nuisance tripping, which is often
caused by a defective load. The first
step is to unplug all downstream loads,
one at a time.
(Remember: nothing that happens
upstream of the RCD, including
conductor imbalance, can cause tripping.
The device monitors only downstream
wiring, devices and loads.)
Some loads are incompatible with
RCDs, and refrigeration equipment is
one example.
In a hermetically sealed compressor,
motor windings are submerged in
the grounded refrigerant, and any
imperfection in the light coating of
insulation will allow current leakage to
ground.
Moreover, an undetected nuisance-
tripping incident can lead to the loss of
freezer contents.
A dedicated non-RCD circuit should be
run for a refrigerator, even in a kitchen.
RCDs should not be used in circuits
for fire alarms, life support and other
essential services.
If the cause of nuisance tripping is not
apparent, and substitution has ruled
out the device, take a good look at the
wiring. Cable rubbing on grounded
metal pipe or an errant nail can create
a problem, or there may be moisture in
the wall.
Does the nuisance tripping occur
after heavy rain? Perhaps an outdoor
receptacle has been bugged off a
kitchen or bathroom receptacle to
obtain the required RCD protection.
If cable connectors have been over-
tightened in older concrete block or
masonry constructions with metal
wall boxes, there may be enough
current leakage to cause RCD tripping.
If so, it may be possible to loosen the
connectors and slide in a slit piece of
cable jacket.
If wiring runs are suspected, the
diagnostic procedure is to progressively
isolate segments of the branch circuit
by temporarily unhooking the hot
wires, one at a time, until the fault has
been located.
However, in residential construction
most of the wiring is behind finished
walls, so some hard work may be
in order.
Since their introduction in the 1960s, RCDs
have greatly reduced the number of non-utility
electrocutions, especially in homes.