Arc flash accidents are not as rare as is often imagined and the hazards
they create are severe. In wind turbines, these hazards are further
magnified. Stuart Greenwood looks at how they can be minimised
Arc flash accidents are not as rare as is often imagined and the hazards
they create are severe. In wind turbines, these hazards are further
magnified. Stuart Greenwood looks at how they can be minimised
EU statistics show that between
eight and ten arc flash accidents
occur in the UK every week.
Almost everyone who works with
electricity has heard of arc flash
accidents and many know they can be
extremely dangerous. But what exactly
is an arc flash accident and how does it
occur?
An arc flash accident happens when a
large electrical current passes through
ionised air and gasses, for example when
a circuit breaker fails during a switching
operation. The temperature near the arc
increases by around 20,000ºC which
vapourises copper conductors and, since
the volume of the vapour produced is
around 67,000 times that of the metal, a
dramatic and violent explosion results.
When such an accident occurs, there
is a high risk that anyone in the vicinity
will be severely injured or even killed.
The equipment involved is also usually
damaged beyond repair. In a wind
turbine the immediate effects of the arc
flash accident are unchanged but dealing
with the consequences is much more
difficult. For example, getting medical
treatment to an injured person in a
nacelle many metres above ground is not
easy, while replacing damaged
switchgear is difficult and costly.
Reducing the risk To minimise these events, the first step
is to try eliminating the conditions
under which arc flash accidents occur.
Regulation 14 of the Electricity at Work
Regulations makes it clear that live
working should not be the norm and
must only be sanctioned under specific
conditions. However it's worth bearing
in mind that arc flash accidents are not
limited to situations where live working
is being carried out intentionally. A
greater risk is found in situations where
an 'isolated' system is accidentally made
live while it is being worked on.
Whilst there is no certain way of
eliminating arc flash accidents, the risk
can be reduced by specifying switchgear
that features insulated arc-free busbar
assemblies. Unfortunately, such
switchgear is larger than conventional
equipment and is unlikely to find favour
in wind turbine applications. Risks can
be minimised by providing those
working on electrical equipment with
suitable Personal Protective Equipment
(PPE). The Protective Equipment at
Work Regulations 1992 make it
mandatory for employers to provide PPE
if a risk assessment indicates a need. The
same regulations make it clear that the
use of PPE is a last resort because it does
not control the problem at source and
protects only the wearer.
So what else can be done? A popular
option is the use of electrical/electronic
systems that quench the arc produced in
an arc flash accident so that there is no
time for it to cause damage. The time
scales involved are very short: to be
effective, quenching must take place in
5mS or less. No circuit breaker can clear
a fault that fast, so a different approach
is used: a bolted short-circuit is placed
across the supply within milliseconds of
the arc being initiated. This effectively
'robs' the arc of the energy it needs to
develop and become dangerous. The
upstream circuit breaker will
subsequently trip and clear the fault
within about 50mS.
Devices that can place what is, in
effect, a bolted fault across the supply
within as little as 2mS of being triggered
are now available. They are used in
conjunction with a high-speed detector
that reacts to the arc in its very earliest
stages. In the Eaton Arcon system, for
example, which is confirmed by the IPH
testing institute in Berlin as an effective
choice for use in wind turbine
applications, the detector comprises a
photoelectric
system with a
flexible fibre-optic
cable that can be
routed through all
the areas of the
switchgear where
an arc fault could
develop. This cable
detects light over
its entire length
and responds to the
characteristic light
of an arc by
sending a signal to
a logic module that
also monitors
current. Only if the
photodetector
signal is accompanied by a rapid
increase in current is the short-circuiting
device triggered - an arrangement that
ensures reliable response to real faults
with freedom from false operation.
No method of preventing arc flash
accidents can ever be described as totally
effective, but suppression systems of the
type described come much closer than
any alternatives with the benefit of
protecting everyone in the vicinity and
virtually eliminating equipment damage.
These benefits are important in every
application and in wind turbine
applications might well be considered
indispensable. Arc fault suppression is,
an important tool for enhancing wind
turbine reliability and protecting the
safety of those who work on them.
Stuart Greenwood is product marketing
manager, industrial control &
automation, at Eaton
