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Internal Arc inside Switchboard
Arc Proof metal-clad switchgear and controlgear is designed
and manufactured to prevent the occurrence of internal faults.
If the switchgear and controlgear is installed, operated and
maintained following the instructions of the manufacturer, there
should be little probability that an internal arc occurs during its
entire service life, but it cannot be completely disregarded for
hazard risk assessment.
An internal arc fault, which constitutes a hazard, if operators
are present, though extremely rare might occur due to reasons
such as failure of insulation, contacts due to ageing, overvoltages
in system because of switching or lightning surges, pollution
due to environmental conditions, mal-operation or insufficient
maintenance.
Engineers and site managers have a legislated “duty of care” to
make proper equipment selection, operating procedures and
service conditions. The effectiveness of the selection, at
providing the prescribed level of protection of operators in case
of an internal arc, can be verified by type testing.
Designs which have been successfully type tested qualify as
IAC classified. This classification is intended to offer a tested
level of protection to operators in the vicinity of the equipment
in normal operating conditions and with the switchgear and
controlgear in normal service position, in the event of internal
arc.
Other enhanced measures may be adopted to provide the
highest possible level of protection to operators in case of an
internal arc. These best practices measures are aimed to limit
the external consequences of internal arc;
a) Rapid fault-clearance times initiated by arc detect sensors or
by a busbar protection.
b) Application of fault current limiting fuses to limit the let-through
current and fault duration.
c) Fast elimination of arc by diverting it to metallic short circuit
by means of fast sensing and fast closing devices (arc terminator).
d) Remote control to allow operators to stay outside arc flash
boundary.
e) Pressure relief device.
f) All operations behind type tested arc proof doors.
Normal operating conditions means the conditions of metalclad
switchgear and controlgear required to carry out operations
such as opening or closing HV switching devices,
connecting and disconnecting withdrawable parts, reading of
measuring instruments and monitoring equipment, etc. Therefore,
if to perform any of such operations any cover has to be
removed and/or any door has to be opened, the test shall be
carried out with the cover and/or door removed.
Removing or replacing active components (for example, HV
fuses or any other removable component) are not considered
to be normal operations, neither those required to carry out
maintenance works.
The Internal Arc Classification IAC makes allowance for internal
overpressure acting on covers, doors, inspection windows,
ventilation openings, etc. It also takes into consideration the
thermal effects of the arc or its roots on the enclosure and of
ejected hot gases and glowing particles, but not damage to
internal partition and shutters not being accessible in normal
operating conditions.
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