An Impulse is defined as a unidirectional time varying voltage that rapidly rises to a peak value and drops considerably slowly to ‘0’ magnitude with respect to time. It simulates an over voltage. Overvoltage is by definition a time varying voltage whose peak is more than the peak of the system highest rated voltage. The main difference between an Overvoltage and an Impulse is that, the former is an outcome of a natural phenomenon and the latter is a man made pulse usually for simulating a natural event.

Lightning is a natural phenomenon which is one of the major contributors for faults/disturbances on transmission lines. A lightning stroke on the transmission line produces travelling waves at near the speed of light, which superimpose on the already travelling line voltage on the transmission line. Thus, the insulation of the equipments connected to the line are vulnerable for damage. Lightning impulse simulates a lightning overvoltage. A standard notation for specifying a lightning impulse is a wave characterized by a 1.2/50 µs pulse. This simulates Lightning or Atmospheric overvoltages.
Note:
1.2 µs denotes the time taken by the wave to reach its peak (0.9 p.u)
50 µs denotes the time taken by the wave to reach to 50 % of its peak value (0.5 p.u)

A lightning stroke is followed by one or more travelling waves, whose amplitude is often limited by the maximum insulation strength of the concerned over-head transmission line. The rate of voltage rise due to these strokes is directly proportional to the transmission line circuit parameters. Travelling waves with steep wave front may stress the insulation of the high voltage equipments. If the lightning overvoltage exceeds the critical withstand over voltage of the insulation(s), it may puncture the same.

Basically a lightning pulse is characterized by a µs pulse. So, the foremost consideration should be a protection system which operates in the nanosecond (ns) scale of time or faster.
Metal Oxide Varistor (MOV) operates on a few hundred nano second scale. Its approximate response time is close to 50-100 ns.

A standard Switching Impulse is characterized by a 250/2500 µs pulse. The notations for the front time and tail time are the same as mentioned in the case of a lightning impulse. It simulates a switching overvoltage.

A switching overvoltage is initiated by means of changing system topologies. The best example can be a sudden switching of a charged capacitor in and out of a transmission line which is charged. This gives rise to a switching pulse as the impedance of the network changes in a fairly less amount of time span. Switching overvoltage is not a natural phenomenon. The peak voltage of the disturbance is dependent on the system voltage. A Switching surge is initiated from within the system. It does not depend on atmospheric conditions.
                                  
On the other hand, a lightning overvoltage is a natural phenomenon and it depends on atmospheric conditions. The probability of lightning at a specified point on the ground cannot be predicted. The magnitudes of current and the voltage that are induced into the system after a lightning strike are considerably very high ( voltages can be as high as several million volts , and the currents have peak amplitudes exceeding several kilo amperes, typically probable peak current is 40 kA). The magnitude of peak voltage or the peak current does not depend upon the system voltage or the current. The disturbance is initiated by an external source.

Above all, both the impulse wave discussed above superimpose a high energy, high frequency pulse on the already propagating travelling wave through the transmission/distribution system.

Transmission and distribution lines stretch to several thousand kms, housing some very costly equipments like power transformers, relays, circuit breakers. All these equipments are under a constant threat of being struck by natural forces of nature like thunderstorms, lightning etc. Foremost consideration is to provide continuous power supply for industries/homes and protection of several costly equipments. Impulse voltage generation is very essential to simulate a lightning pulse to use it for testing of equipments for the same.

When a self restoring insulation breaks down through its weakest line of force, it is called a flashover. In other words, a spontaneous short circuit of a conducting air gap is called a flashover. It is due to the simultaneous transfer of all the valence band electrons to the conduction band. In view of an impulse voltage test, a flashover is a phenomenon which discharges all the energy acquired by means of a short circuit between the 2 conducting test spheres.

Usually, a self restoring insulation, recovers its insulating properties after a brief period of time, then its termed as a flashover or a temporary discharge.

On the other hand, a puncture is when a solid insulation fails to recover back to its original insulating properties. This is because; the magnitude of the disturbance might have been high enough to cause a permanent damage through the insulation.

The impulse breakdown of air does not result in a single value for a particular temperature and pressure. This fluctuation in breakdown strengths can be better expresses in terms of a statistical distribution. In simple terms, the breakdown of an air gap is purely probabilistic in nature. The best fit to this can be obtained using a statistical (Gaussian) distribution.

Critical Flashover voltage (CFO), in view of physics, is defined as the minimum voltage above which all the electrons in the enclosed gap acquire enough energy to cross the valence band into the conduction band. Gaussian distribution is characterized by mean and standard deviation. Mean value of 50 % value is termed Critical Flashover Voltage. Determination of both mean and standard deviation is termed as determination of withstand strength.

Surge impedance or characteristic impedance, generally expressed as ZL. It is the ratio of voltage and current propagating along the line in the absence of electrical reflections. In other words, when the transmission line is terminated by a resistor equal to the surge impedance of the line, it appears as an infinitely long transmission line to the source. Essentially, the surge impedance resistance physically brings down the reflective coefficients magnitude to near ‘0’. Its SI unit is ohms.

The line is terminated with a resistance whose value is equal to the value of the surge impedance of the transmission line, so as to get the exact Impulse voltage wave tapped into the CRO. If, for instance, the line is not terminated, a part of the wave which propagated to the load will be lost due to reflections because of the difference in coefficients of reflective indices.

Impulse voltage generation finds its place in a variety of fields. Some of the many applications are stated below :

1. Generation of high power micro wave using virtual cathode oscillator devices
2. Lightning testing on cables and insulators at 1.2/50 µs and 8/20 µs.
3. Material and dielectric testing.
4. Breaking of raw diamonds in mineralogy.
5. Flash x-ray generation.
6. Kilo amp linear accelerators.
7. Generation of plasma focusing.
8. To simulate Nuclear Explosions.