Surge Protection Devices (SPDs): How They Work

Surge protection devices (SPDs) help safeguard electrical equipment by limiting transient overvoltages that can damage or destroy sensitive components. They divert surge currents away from critical loads to prevent interruptions and shutdowns. This article provides an overview of how SPDs work to protect power distribution systems and connected equipment.

Surge Sources and Effects  

Power surges, or transient overvoltages, can originate from natural and artificial sources. Lightning strikes are a common natural cause of surges. Switching operations, utility grid fluctuations, and EMI/RFI disturbances from neighboring equipment are typical artificial surge sources.  

Surges introduce energy that can overload circuits and degrade equipment over time. They often have high enough magnitude to cause immediate damage like contact arcing, insulation puncturing, and chip failures. The effects tend to be cumulative, reducing overall service life through gradual deterioration even if catastrophic damage does not immediately occur.

 SPD Operating Principles

The basic elements of an SPD include:

•Spark gaps - Two spaced electrodes that create an air gap. During normal operation, the gap prevents current flow. When the voltage increases enough to ionize the air gap, a sparkover occurs that redirects the surge current to ground. Spark gaps are often used for high-energy surge handling.

•Varistors - Ceramic metal-oxide resistors that exhibit variable resistance based on the applied voltage. At normal voltage, resistance is high. As voltage increases, resistance substantially decreases to create a low-impedance path to ground. Varistors are effective but can degrade over time with surge energy exposure.

•Avalanche diodes - Silicon diodes that reach avalanche breakdown at a specified voltage threshold, effectively clamping the surge and passing excess current to ground. They offer fast response but limited energy handling.

•Gas discharge tubes - Contain an inert gas that ionizes at a selected voltage level to redirect surge currents. They have high surge current capability but slower response than varistors or avalanche diodes.

The components are arranged to provide a path from each phase to ground. SPDs monitor the voltage across these paths and activate when a dangerous surge is detected to divert energy away from connected equipment. They automatically reset once voltage returns to a normal level.

SPD Classifications and Applications

SPDs are rated by maximum surge current capacity and voltage protection level (VPL) to suit different applications. Type 1 SPDs are installed at service entrances to handle high-energy surges. Type 2 SPDs are often used farther downstream to protect distribution equipment and critical branch circuits. Type 3 SPDs protect sensitive electronics and have the lowest VPL and surge ratings.

Properly selected and installed SPDs help shield entire electrical systems and connected loads from the damaging effects of power surges and transients. They improve power quality and reliability while reducing life cycle costs through maximized equipment lifetime and minimized downtime. SPDs should be matched carefully to the operating voltages and surge levels in a given application to ensure optimal protection.