Surge due to internal switching

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Not all surges are caused by lightning. Many surges occur inside buildings, when large currents are interrupted in the power system. These surges occur when switching large loads, such as elevators, or when fault currents are interrupted. When a fault occurs, a short-circuit current flows through the power supply, and when the short-circuit current is suddenly interrupted by a fuse or circuit breaker, an inductive transient voltage is induced in the line.

At this time, the waveform is usually in the form of a damped oscillation, and the peak voltage varies depending on the inductance of the power line and the rate of change of the current. The reason why surges and noises occur a lot when working with an arc welder is because a high voltage is induced momentarily whenever the arc is cut off during the work.

V = L * di/dt

The process by which a surge occurs in a power line when the fault current is interrupted

References

Surge protection for Local Area Networks, MTL Surge Technologies