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The impulse ratio of rod gap is
Unity
Between 1.2 and 1.5
Between 2 and 2.2
None of above
Between 1.2 and 1.5
The impulse ratio is defined as the ratio of the impulse spark-over voltage of a rod gap to the power-frequency spark-over voltage (crest value). For a rod gap, this ratio typically falls between 1.2 and 1.5, indicating that the gap breaks down at a higher voltage when subjected to a short-duration impulse compared to a 50 Hz power frequency voltage.
The impulse ratio is defined as the ratio of the impulse spark-over voltage of a rod gap to the power-frequency spark-over voltage (crest value). For a rod gap, this ratio typically falls between 1.2 and 1.5, indicating that the gap breaks down at a higher voltage when subjected to a short-duration impulse compared to a 50 Hz power frequency voltage.
IR=VpowerтИТfrequencyтАЛVimpulseтАЛтАЛ тАФ Ratio of peak voltages
The impulse ratio reflects the time-dependent nature of gas breakdown in a non-uniform field. Because an impulse wave has a very steep wavefront, the air gap requires a higher voltage to initiate the ionization process and leader formation within the short time duration compared to a slowly varying power frequency voltage where the cumulative ionization process has more time to occur.
It is a dimensionless quantity indicating the sensitivity of a gap to impulse waves.
A value of 1 would imply that the gap is independent of the wave shape.
The impulse ratio depends on the geometry of the electrodes and the polarity of the impulse.
Rod gaps are often used as surge protective devices due to this specific breakdown characteristic.
Simple and inexpensive construction
Provides surge protection against lightning and switching surges
Inconsistent breakdown voltage due to atmospheric conditions (humidity, pressure)
High sensitivity to polarity and electrode geometry
Used in rod gap arresters for transformer protection
High-voltage laboratory testing and calibration
The impulse ratio is generally higher for longer gaps compared to shorter gaps.
Option A is incorrect because the impulse ratio is greater than unity for rod gaps.
Option C is generally considered too high for standard rod gap configurations.
B is correct тАФ The impulse ratio for a rod gap is typically between 1.2 and 1.5, representing its response to fast-rising surge voltages.
Always remember that the impulse ratio is always greater than 1 for rod gaps because impulse voltages have much shorter time durations than power frequency cycles, necessitating higher intensities for breakdown.