Conductors exposed to lightning strikes can be elevated to several million volts by direct strike, and possibly one-half million volts by induction alone, all in a time interval of a few micro-seconds. Voltage surges travel almost instantly along the line of conductors away from the stricken point. Line flash-over may occur which will short circuit the conductor to a ground path. Most circuits cannot contain this voltage surge and will flash over to ground when the high potential is reached. In many cases, the voltage of the power system itself is sufficient to maintain the arc once it is struck. The circuit breaker must operate to stop that follow-through current. Reclosers on a distribution system do this.
Lightning arresters are used to prevent this situation by giving the lightning strike an easier path to ground. They are installed on the lines over which lightning surges may reach the local system, and limit the peak voltage seen as a result of a strike. The reduced voltage surge still travels into the industrial distribution circuits, with further reduction occurring as a result of dispersion and the surge impedance of the circuit. However, voltage waves that reach a point where the impedance increases (such as an open switch or the winding of a transformer) are reflected back at up to twice their value. Therefore, it is important to install lightning arrestors as close as possible to open switches, transformers and underground cable terminations.
This surge is also communicated to the secondary windings and emerges as a crest wave on the distribution bus. Even though it is divided among a number of feeder circuits and is considerably reduced upon reaching the end of a feeder, multiple reflections can occur to increase the surge value above its initial voltage at certain locations. This can cause equipment damage. Rotating machines and dry-type transformers are particularly susceptible to damage by these strong steep front voltage waves. Surge capacitors are commonly used to provide protection.
The ability of a piece of apparatus (motor, transformer, etc.) to withstand these voltage surges is described by its Basic Impulse Level (BIL) number. These high voltage spikes can actually puncture insulation and thereby open a conductive path for current flow. That is why it is so important to protect motors exposed to these surges as mentioned above. Pole-top transformers and most pad-mounted units usually have adequate BIL ability built into their design. This BIL is a number which can be specified before purchase.
Remember that the current in a lightning stroke is really not affected by what it sees near the earth. It was "on its way!" However, Ohm's law still applies, so the only effective way to limit the voltage is to give the current an easy path to ground. For example, a lightning strike of 10,000 Amperes (merely a moderate strike) flowing through a ground circuit of 3 Ohms would develop 30,000 Volts! A wooden pole, already wet with rain, might have a resistance of, say, 300 Ohms. This would cause a voltage impulse of about 3 million Volts. The current flow through the pole would probably boil the moisture so quickly that it would splinter the pole. That is why properly applied, well grounded lightning arrestors limit these voltage pulses.
The newest generation of gapless lightning arrestors have voltage breakdown levels (the voltage difference at which they spill the current from a strike to ground) which are nearly independent of the current. These are quite effective in protecting electrical systems from insulation damage caused by lightning.
