Page 49 - Energize June 2021

TECHNICAL

Arc flash detection incident energy is related to the level of current flowing in the
Arc flash detection minimises the time taken to trip the circuit fault and clearing time, the circuit breaker operating time limits
breaker and interrupt the fault. Arc detection in the protective relay the ability to reduce incident energy.
minimizes trip time, cost, and complexity. Enabling arc detection in
a protective relay makes use of current monitoring and protection Arc-quenching switchgear
already available in the device. One method of detecting an arc is In arc-quenching switchgear, arc detection and quenching devices
through the use of optical sensors because the arc emits a very are built into the switch, enabling it to detect and contain an arc
high-intensity light. The optical sensor sends a signal to the AFP fault in less than 4 ms. It also allows the switch to discriminate
which trips the upstream breaker (Figure 4). between an arc fault and arcing which occurs during normal breaker
There are two types of optical sensors used: Continuous fibre operation. This is over ten times faster than systems which rely on
loop and point sensors. The continuous fibre loop employs an a circuit breaker alone to clear a fault and yields a corresponding
unenclosed glass fibre that is routed though the equipment which reduction in incident energy.
is to be protected. Any light which is incident to the fibre will be The system works by detecting the ignition of an arc inside
routed back to the detector mounted in the AFP. Should the fibre the switchgear, using an arc flash relay and transferring it to the
break, alarms can be activated by the AFP. arc-quenching device where it is extinguished. Arc-quenching
The other method employs an optical sensor mounted switchgear has such a rapid clearing time that it not only
on the wall of a compartment. Again, any light incident to the provides excellent personnel protection, but it also provides
sensor is either passed back through a lens to the AFP or will advanced equipment protection and minimizes downtime due to
activate a light sensitive device in the sensor which passes a arc flash events.
signal to the AFP, generating a trip signal to the appropriate Arc-quenching technology applies a parallel path for the
upstream breaker. The advantage of an optical scheme is that arc current to collapse the voltage and extinguish the arc. The
it does not have to coordinate with downstream devices. The quenching device effectively cuts off the energy supply to
operational time of an optical scheme with a solid-state output the arc, by providing a bolted or low impedance short, either
can be as short as 2 to 2,5 ms. between the phase conductors or between the phase conductors
While the optical detector can be used on its own, security and earth, which diverts current away from the arc. This short
can be improved by combining the optical scheme with a current circuit is activated as close as possible to the infeed, in parallel
detector. The detector, located upstream from the switchgear, to the fault location, via a built-in explosive or other device,
will sense an increase in current for any switchgear fault, and the which takes less than 1 ms to operate. The short circuit current
overcurrent element must be extremely fast so as to minimize delay. then trips the upstream protector in the normal manner. These
Using this combination avoids the possibility of false tripping due devices provide a lower impedance path to earth in parallel with
to short switching arcs. In the event of an arc, the current sensing the arc, thus reducing the voltage across the arc and the energy
unit detects the current while the sensors detect the light radiation. flowing in the arc. There are two types, bolted short or earth, and
When the appropriate combination of current and light is detected, controlled arc.
the system will trigger the opening of the circuit breaker. The arc-
detected signal to the circuit breaker by-passes the normal settings Bolted earth
to activate an instantaneous trip. This form of protection device operates by providing a fast-
operating connection to earth upstream of the arc fault. The
Arc-quenching devices earthing switch is operated by signals from the optical sensor.
In all of the above solutions, the total clearing time of the The operation of several devices depends primarily on mechanical
systems includes the clearing time of the upstream circuit force caused by either explosive material or pressurised gas. In
breaker. Power circuit breakers, often found in switchgear, can one device an explosive charge is used to force a connecting plug
take up to four cycles (80 ms at 50 Hz) to clear a fault. Since into the earthed busbar, in others a contact is provided. Figure 5
shows a typical device.
Figure 6 shows the operation of the device.

Figure 4: Combined high speed light and overcurrent sensing (Schneider) Figure 5: Bolted earth device (ABB)

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