Electrical Contracting News (ECN) September 2017 | Page 53

TEST & MEASUREMENT
SPECIAL
FEATURE
CLAMP DOWN ON
EARTH LEAKAGE
Thanks to changes in the IET Wiring Regulations over recent years, earth leakage protection is now part
of almost every new low voltage electrical installation. Sometimes, howev er, this protection can cause
problems of its own as Peter Wade of Megger explains.
M
ost electrical
contractors have
experienced a
circuit breaker
that keeps
tripping for no
apparent reason.
Likewise, most
contractors will know that it’s much more
likely to be a residual current device (RCD)
or residual current circuit breaker with
overload protection (RCBO) that’s tripping,
rather than an ordinary miniature circuit
breaker (MCB). In short, it’s a typical case
of ‘nuisance tripping.’ So, what’s to be
done? Let’s start to answer this by looking
at a few earth leakage basics.
What is earth leakage?
Surprisingly perhaps, the IET Wiring
Regulations (BS7671) don’t include a
definition of earth leakage current, but there
are two definitions that are related. Leakage
current is defined as ‘Electric current in an
unwanted conductive path under normal
operating conditions,’ and protective
conductor current is defined as ‘Electric
current appearing in a protective conductor,
such as leakage current or electric current
results from an insulation fault.’ From these
definitions, it’s clear that earth leakage
current is simply current that finds its way to
earth via some unintended path.
But where does this leakage current
come from? The first possibility is
unintentional earth leakage. This usually
results from an insulation fault that allows
current to leak from the live circuits to earth,
hopefully via the protective conductor. The
second possibility, sometimes referred to
as intentional leakage current, results from
essential design features in equipment
connected to the supply.
It might seem strange to ‘design in’ earth
leakage, but sometimes this is unavoidable.
The operation of filter circuits in computer or
television power supplies relies on capacitors
connected to earth, and although the
impedance of these capacitors is high they
still allow a small ‘leakage’ current to flow.
Because excessive earth leakage
current is potentially hazardous if it
were to flow to earth through someone’s
body rather than through the protective
conductor, the IET Wiring Regulations now
insist that protection is provided on almost
An earth leakage
clamp meter
can help resolve
nuisance tripping.
all circuits. In most cases, this takes the
form of an RCD or an RCBO.
Both devices work by monitoring the
current flowing in the line conductor, or line
conductors in a three-phase system, and
comparing this with the current flowing in the
neutral conductor. The difference between
these currents is the earth leakage current
and, if it exceeds the sensitivity of the device,
the device will trip and open the circuit. RCDs
and RCBOs used in domestic installations
typically have a sensitivity of 30mA.
Nuisance tripping
Earth leakage protection usually works well
but where it trips repeatedly, apparently for
no good reason, nuisance tripping can be
very annoying.
Resolving nuisance tripping can be
difficult, but an earth leakage clamp meter
can make it easier. When clamped around
the line and neutral conductors in a circuit,
but not the protective conductor, it will
show the difference between the currents
September 2017 | 53