that only the required power levels reach critical components. Such protection also helps prevent costly downtime and equipment replacement. This article explores the fundamentals of surge protection devices and design challenges, followed by examples from Littelfuse and Phoenix Contact.
How does surge protection work?
Surge protection devices (SPDs) operate in a high impedance state, functioning as an open circuit. In this state, they maintain electrical isolation between the active conductors and ground, ensuring no connected equipment is affected. However, during transient overvoltage, SPDs switch within nanoseconds to a low-impedance state. This closed-circuit condition allows them to divert excess current to ground, thereby limiting the surge voltage and discharging the associated surge current. Surge voltages can occur between active conductors (normal mode) or between active and protective conductors (common mode). To protect electrical components, SPDs are typically placed in parallel—either between phase conductors or between phase and ground potential—depending on the surge path, as shown in Figure 1.
Figure 1: Parallel installation of SPDs for both normal and common mode surge protection. (Image source: Phoenix Contact)
Nonlinear components inside an SPD This dynamic behavior of an SPD is enabled by the presence of at least one nonlinear component within its design. These components conduct electricity only when the voltage across them exceeds a defined threshold. Common types include metal oxide varistors (MOVs), avalanche breakdown diodes (ABDs), and gas discharge tubes (GDTs).
Among these, MOVs are the most widely used in AC power circuits. Their surge current rating depends on their cross-sectional area and composition. The larger the cross- sectional area, the higher the surge current rating of the device. MOVs are made of zinc oxide grains mixed with other additives. These grains form a network of semiconductor junctions at their boundaries, which act as diodes, allowing current to pass only during overvoltage events.
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