Voltage spike

Fast, short duration electrical transients (overvoltages) in the electric potential of a circuit are typically caused by[2] In the design of critical infrastructure and military hardware, one concern is of pulses produced by nuclear explosions, whose nuclear electromagnetic pulses distribute large energies in frequencies from 1 kHz into the gigahertz range through the atmosphere.

In semiconductor junctions, excessive electric current may destroy or severely weaken that device.

[3] Voltage spikes, also known as surges, may be created by a rapid buildup or decay of a magnetic field, which may induce energy into the associated circuit.

However voltage spikes can also have more mundane causes such as a fault in a transformer or higher-voltage (primary circuit) power wires falling onto lower-voltage (secondary circuit) power wires as a result of accident or storm damage.

Power increases or decreases which last multiple cycles are called swells or sags, respectively.

Sine wave with two sharp excursions labeled as transients
Voltage spikes labeled as transients