6.6Limitations of surge protection devices#

SPDs are designed to provide electrical equipment with protection from surges. Different divertors and filters will reduce the surge energy but appropriate devices need to be selected for the application. As with all electrical devices, SPDs have their limitations. Some are included here:

• SPDs do not provide 100% protection for all electrical loads. They protect against surges, one of the most common types of electrical disturbances. Some SPDs also contain filtering to remove high frequency noise (50 kHz to 250 kHz).
• SPDs do not reduce harmonic distortion from the third to fiftieth harmonic (150 to 2500 Hz).
• The SPD cannot prevent damage caused by a direct lightning strike. A direct lightning strike is a very rare occurrence. In most cases, lightning causes induced surges on the power line, which can be reduced by the SPD.
• The SPD cannot stop or limit problems due to excessive or prolonged overvoltage. Overvoltages are rare disturbances caused by a severe fault in the utility network, or a problem with earthing (poor or non-existent N-E bond). Overvoltage occurs when the ac voltage exceeds the nominal voltage for a short duration (millisecond to a few minutes). If the voltage exceeds 25% of the nominal system voltage, the SPD and other loads may become damaged.
• SPDs do not provide protection against power outages and undervoltage (brownouts).
• Once the SPD has failed, it will no longer protect the system. If the SPD is not monitored, failure of the SPD may go unnoticed for some time.
• To operate successfully, SPDs must be connected to an effective earthing and bonding system by short, low impedance leads.