A surge, also known as a transient overvoltage, occurs when there is a spike in voltage on power or signal lines. They are large, sudden and can overload electronic components, leading to failure as well as the potential to cause injury or even death.
Lightning strikes cause the largest surges, with spikes of up to 6,000V within an electronic installation. Buildings often withstand the strike itself, but suffer huge damage to electrical equipment from the surge afterwards – which can cost tens of thousands of pounds.
A more common cause source of surges, which is lower in amplitude, is the electrical switching of transformers, motors and drives, or other inductive loads. They will create a spike that may also affect other devices connected to the same electrical network.