58 / 84
be selected for each individual
application. Circuit breakers can
also provide other functions like
emergency stop and mains switching
via a modular accessory assortment.
Residual or leakage
currents
Residual or leakage currents are
not as large or energetic as short-
circuits, but if a leakage current
as low as 30 mA is allowed to
flow through a human being for
more than a fraction of a second,
it can cause cardiac arrest or
serious harm. Accordingly, power
distribution systems must include
residual current devices (RCDs)
that open when they detect an
imbalance between energised line
and neutral conductor currents. Any
such imbalance normally indicates
a short circuit or other electrical
anomaly. Apart from electric shock
risk, there is also the danger of
fire arising from excessive residual
currents.
However, machine systems often
contain variable speed drives and
these generate operational earth
leakage currents. Therefore it
is essential that the RCD reacts
adequately to fault currents that
are actually dangerous, without
‘nuisance tripping’ in response to
normal drive system earth leakage
currents, or allowing reduced
protection of the operator.
Machine builders should be
concerned with Type B RCDs to
meet the protection requirements
in machinery equipment. The
challenge is to keep a high system
up-time combined with a high
protection level for the equipment
and the operator wherever the
machine is located. Therefore it is
essential to consider the compliance
with all standards and regulations.
Digital RCDs are now available that
offer several advantages to machine
users. With real-time measurement
of the residual current, they can
provide notification both locally via
LEDs and remotely via potential-free
contacts. Faults can be recognized
before tripping occurs, which
reduces the need for unscheduled
maintenance, therefore increasing
system uptime.
Arcing faults
Arcing faults can occur from
insulation faults or loose contacts
on wiring, and is the main cause of
damage in electrical installations. As
well as any such electrical installation
damage, arc faults can easily
ignite fires which may have severe
impacts on operators, machinery
and infrastructure. A typical cause
for such an arc would be damage
to a machine cable by a mechanical
lifter. Insurance companies estimate
that 25% of all fires caused by
electrical failure have at one stage
been an arc. Initially Arc Fault
Detection Devices (AFDDs) were
designed to protect people from fire
hazards in residential buildings, but
since the technology has proven
to be reliable and affordable, they
are now becoming increasingly
attractive to machine builders.
The detection of an arc is handled
by complex electronic circuitry that
senses high frequency signals on
the power line. Arcs have a noise
pattern on a wider bandwidth,
different to other high frequency
noise. Once an arc is detected, a
connected miniature circuit breaker
(MCB) or residual current circuit
breaker (RCBO) will trip and cut the
supply power to the arc.
The most important quality
differentiator for an AFDD is
low nuisance tripping. This is a
challenge for a machine builder as
there are many signals on the line
that might be misinterpreted as
arcs, for example, relay switching.
A quick and safe detection (and
mitigation) of an arc is extensively
tested during the approval period.
Accordingly, arc fault protection
strategies centre on detection.
AFDDs work in partnership with
circuit breakers or RCBOs. The
AFDD should trip on detection of any
arc with the energy of 100 joules
or greater, with the allowable trip
time reducing as the energy of the
arc increases. Overall, protection
success depends critically on fast
response to minimise arc energy.
AFDDs are essential even in systems
that already have over-current
protection. Circuit breakers and
RCDs cannot detect arc faults, which
typically cause neither overcurrent
nor residual currents. AFDDs
combined with miniature circuit
breakers protect from serial arcing
faults as well as phase-neutral or
phase-phase parallel faults. AFDDs
combined with RCDs provide
protection from phase-protective
conductor faults.
Surge Protection
The need for surge protection across
the distribution network has grown
steadily with the ever-increasing
use of electronics in machinery.
Computers, PLCs, displays and
communication components are
becoming increasingly common as
Industry 4.0 is adopted. Surges can
wreak havoc on electronics, causing
catastrophic failures, process
58 l New-Tech Magazine Europe