What Causes Voltage Surges?

The three main causes of surges are:

Lightning

Load Transients

Faults

Lightning strikes are a common source of external surge transients and carry far more current at far higher voltages than most electronic systems are rated for. These voltage surges are typically large enough to cause the immediate failure of electronic devices if the appropriate level of protection has not been applied.

Surges on the AC supply line can also be caused when other devices on an electrical circuit are switched on or off. Reactive loads, such as motors or capacitor banks resemble short circuits prior to establishing their electric and magnetic fields. When turning off, the energy stored in these fields can also quickly be dumped back into the system. In both cases, the large and fast current transients can induce voltage spikes and lead to failure of unprotected devices.

Faults can also be the source of surges and lead to excessive voltages being applied to the power supply input. The failure of system components and devices can lead to transient voltages and currents in other parts of the system caused by unintentional shorting or opening of circuits.

The intensity and magnitude of the surge voltage at the power supply input depends on many factors including location, cabling, and the level of surge protection that has been applied to the power supply input, whether internally or externally.

International Standards Define Protection Levels

Standards have been developed to classify and provide guidance on the necessary level of protection. The most common for power supplies is IEC 61000-4-5, from the International Electrotechnical Commission. It is referenced in many national immunity standards, such as EN 55035 which sets immunity requirements for multimedia equipment.

The IEC 61000-4-5 standard defines a standardized test method and different levels of protection based on installation class and coupling methods. Dc power supplies are typically concerned with installation classes 3-5, which have test requirements from 1kV to 4kV (Table 1).