ARC-FLASH
DETECTION SYSTEMS FROM SCHWEITZER ENGINEERING LABORATORIES
Understanding arc-flash and why you
need to address personal safety
Personnel safety is paramount in industry, and arc
flash is a work place hazard where technology
has played a large part in minimising the hazard,
with many utilities and large industrials installing
arc-flash protection systems as a matter of course.
Arc-flash is a dangerous condition associated
with the release of energy caused by an electrical
arc, typically from faults in electrical equipment
and switchboards. Arc-flash events can cause
dangerous and potentially fatal levels of heat,
ultraviolet radiation, blast pressure, flying shrapnel,
and deafening sound waves.
Thankfully, a range of SEL arc protection is
available that detects arc-flash hazards and sends
a trip signal in as fast as two milliseconds.
The range includes SEL-710-5, SEL-751 and
SEL-849 protection relays that significantly reduces
are-flash incident energy, improves safety and
minimises the damage caused by arc-flash events.
An arc-flash occurs when electrical current flows
through an air gap between conductors, i.e. short
circuits. This could be driven by, direct or indirect
contact with live equipment, breaks or gaps in
insulation and equipment failure due to normal
wear and tear or impurities on conductor surfaces.
The amount of energy in the arc (defined as
incident energy in joules/cm2) is directly proportional
to the arcing current and the duration of
the fault. It is however inversely proportional to
the distance from the fault point, with the incident
energy decreasing as the distance from the fault
point increases. IEEE has developed guidelines
on the calculation of incident energy levels, for
the determination of operating distances (arc
flash boundaries) as well as PPE requirements for
different operating environments.
Based on this, safe work practices, personal
protective equipment (PPE), approach boundaries
(Figure 1) and warning labels (Figure 2) are
developed.
A key aspect of determining the arc-flash
boundaries is the speed of operation of the
arc-flash protection system employed. A lack of
sensitivity or slower than specified performance
can dramatically increase the available incident
energy thereby exposing personnel and equipment
to greater risks.
Types of arc-flash detection systems
The most common schemes use either light or
current detection, or a combination of both. Other
systems use; sound, pressure and heat detection.
For the purposes of this article, only light and
current detection techniques have been explored
as they have been proven to be the fastest and
most effective methods.
Current-only detection systems
Current-only detection schemes were the first
arc-flash detection systems employed. These
systems were originally intended to protect
equipment and are set according. The current
elements of some relays as well can take up-to
50ms to operate, thereby exposing personnel to
greater arc flash hazards.
Light-only detection systems
During an arc-flash event, light is produced and
using light-sensitive equipment, the flash can be
detected using an arc-flash detector (AFD).These
systems are very fast (1-3ms) but pose a risk of
false tripping from a light source not related to an
electrical arc-flash.
Light with current supervision systems
To overcome the sensitivity issues of current-only
systems, and the risk of false tripping by light-only
systems, arc-flash protection schemes supervise
the light detection element with a fast-acting
over-current element. The current detection
techniques used in these cases uses high-speed
sampling and logic to achieve operating times
close to or matching light-only systems, such as
the SEL-710 – motor protection with AFD and
SEL-751(A) - feeder protection with AFD.
The last word in arc-flash safety
The safety of personnel while working near
energised electrical equipment is of paramount
importance and the proper performance of the
arc-flash protection system is critical. The speed
of operation of equipment within the system significantly
reduces arc-flash hazards. For greater
reliability, sensitivity and speed, systems employing
both light and current detection are recommended.
Learn more about arc-flash solutions at
www.selinc.com/solutions/arc-flash-solutions/
Figure 1: Arc-flash protection boundaries
(image supplied)
Figure 2: Typical arc-flash warning label
SHOW PREVIEW
42 April 2018
/