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Impulse ratio of insulator and lightning arresters should be
Both low
High and low respectively
Low and high respectively
Both high
High and low respectively
The impulse ratio is defined as the ratio of the breakdown voltage under a surge (impulse) to the power frequency breakdown voltage. For the coordination of insulation, the lightning arrester must discharge the surge before the insulator sparks over, hence the arrester must have a lower impulse ratio than the insulator.
The impulse ratio is defined as the ratio of the breakdown voltage under a surge (impulse) to the power frequency breakdown voltage. For the coordination of insulation, the lightning arrester must discharge the surge before the insulator sparks over, hence the arrester must have a lower impulse ratio than the insulator.
RimpulseтАЛ=VpowerтАЛVimpulseтАЛтАЛ тАФ Definition of impulse ratio
The principle of insulation coordination relies on the 'protection margin'. By keeping the impulse ratio of the lightning arrester (surge diverter) lower than that of the protected equipment (insulator), we ensure that the arrester path becomes conductive at a lower voltage level during a transient, effectively clamping the surge voltage below the withstand level of the insulator.
High impulse ratio for insulators prevents flashover during minor surges.
Low impulse ratio for lightning arresters ensures early operation during high-voltage transients.
Essential for successful insulation coordination in power grids.
Lightning arresters act as the 'weakest link' for surges but 'strongest' for operating voltage.
Prevents catastrophic failure of expensive equipment (transformers/insulators).
Ensures system reliability during lightning strikes.
Requires precise maintenance of gap settings in arresters.
Subject to degradation due to aging and pollution.
High voltage transmission lines
Substation transformer protection
Switchgear protection
| Feature | Insulator | Lightning Arrester |
|---|---|---|
Requirement | High Ratio | Low Ratio |
Typical values for impulse ratio for standard insulators are around 1.4 to 1.5.
If the arrester had a higher impulse ratio, it would not protect the insulator as the insulator would flash over first.
B is correct тАФ Insulators must withstand transient voltages longer than the lightning arrester, which must trigger early to divert surges.
Always remember that the 'protective device' must act faster and at a lower voltage threshold than the 'protected equipment' in all transient coordination scenarios.