The subject of electrical insulation level design and surge performance of high voltage equipment, transmission lines and  sub stations is customarily referred as 'insulation co-ordination'. The insulation provided to electrical installations in the power system is subjected not only to the normal operating voltage which varies within quite narrow limits, but it has to withstand a variety of over voltages having their shapes, magnitudes and durations in a wide range. Thus one can distinguish between steady state and transient over-voltages with which the power system is stressed some times.

The steady-state over voltages, also known as temporary over voltages, are generated within the system due to the connection or disconnection of circuit elements or the initiation or interruption of faults. By circuit elements is meant here the bulk loads  connected over lines.

The transient over voltages have external as well as internal sources in the power system. The lightning strike (an external source) on the power system gives rise to lightning over voltages. The circuit breaker operation within the system gives rise to "switching over voltages".  It is nothing but the restriking voltage impressed across the circuit breaker electrodes as it opens.

Both these types of transient over voltages proceed in the form of 'traveling waves' from the point they are generated and stress the insulation above the system voltage as they travel . In the process, they attenuate and ultimately they end up at the loads.

The intensity of lightning over voltage depends upon the magnitude of lightning impulse current injected into the system by the lightning strike which in turn depends upon the nature. The magnitude of current injected into the line, multiplied by its surge impedance determines the over voltage magnitude. The lightning over-voltage wave shape is standardized as ≈ 1/50 µs duration to be generated in the laboratory by the Impulse Generator for insulation testing. The magnitude of lightning current could be as high as 200 kA recorded by researchers. The probability of higher order of current is very low. However, the average magnitude of impulse current accompanied  with li strike is estimated to be between 10-15 kA. After the lightning strike, the injected charge (the current) always tries to find the least resistance path to the ground. Only when it does not get passage to the ground, it can create havocs.

The magnitudes of switching over voltages, their waveforms (much slower than the lightning) depends upon the factors such as the speed of switching operation of the circuit breakers, their arc quenching characteristics/properties, the instant at which the arc across the electrodes is extinguished and the energy stored in that part of the power system inductance. The modern SF₆ gas and vacuum circuit breakers give rise to "Very Fast Transient Over Voltages", (VFTO). These restrike on the system repeatedly very fast before, the arc is extinguished, hence pose a big problem. The magnitude of switching over voltages could rise to even 3.0 p.U. Every time it occurs, it has a different shape/waveform. Hence it is difficult to standardize the switching waveshape for its production in the HV laboratory. A popular standard waveshape is 250/2500 µs . As compared to lightning, it is a much slower waveshape. It is because of this, the breakdown strength of dielectrics is highest for lightning impulse voltage and it is minimum for a particular shape of switching impulse. Further, the magnitude of switching impulse voltage keeps rising as the rated voltage of operation of the system is raised over the time.

The insulation level provided to various installations in the power system by design must withstand the expected maximum voltage of lightning and also switching it is going to face as transient over voltages in its life. The Basic Insulation Level (BIL)  provided for lightning and switching are defined separately in the following:

BIL for lightning impulse insulation level is the electrical withstand voltage of insulation expressed in terms of the crest value of the 'standard lightning impulse', as per IEC-71. In case of switching transients, it is defined as the switching impulse withstand voltage of specified magnitude and the shape depending upon the rated system voltage.

The BIL level is actually determined by the transient over voltage protection techniques provided by the  horn gaps and surge diverters or the lightning arrestors (LA). ZnO₂ gapless surge diverters are used at the sub-stations and in particular for the transformers being the costliest equipment. It is the residual transient voltage across the LA which is impressed upon the transformer. The transformer must be designed, developed and tested to withstand the residual voltage of the specified LA in the network.