Monday, April 7, 2014

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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