When a surge occurs during a thunderstorm, it is usually caused by direct strike, nearby lightning strike or distanced lightning strike. The high energy generated by impulse current and voltage are extremely dangerous and it is imperative to protect these electrical/electronic systems and equipment from the danger they are exposed to.

Taking into consideration that there is an earth impedance, a direct or nearby lightning strike will cause a high voltage drop from the earth impedance from a building to the surrounding earth distance. This voltage drop from the earth impedance from the building to the remote earth or potential difference will create an electromagnetic wave which will cause a surge into the electrical/electronic systems and equipment.

Distanced lightning strike can happen at a faraway distance and propagated to the point of protected devices. Lightning strikes at distanced overheard power lines, discharging at cloud to cloud are also causes of surge current to be induced into the electrical/electronic systems and equipment.

Surge can also be caused by switching operations.

Making and breaking of electrical circuits with switchgear in power system having large inductance and capacitance. (The operations of circuit breakers)

Power transmission for opening and closing of electrical circuits on inductive loads
(e.g. Transformers, motors etc.)