AC Surge Protection Device

An AC surge protection device (SPD) is an electrical component installed in low-voltage power systems to limit transient overvoltages caused by lightning strikes or switching operations. It detects voltage spikes above the normal operating level and diverts the excess current safely to earth, preventing damage to connected equipment. AC SPDs are classified under IEC 61643-11 into Type 1, Type 2, and Type 1+2, each suited for a different installation point in the power distribution network.

During normal operation, an AC SPD remains in a high-impedance state and does not interfere with the power supply. When a transient overvoltage occurs — from a lightning strike or switching event — the SPD’s internal Metal Oxide Varistor (MOV) switches to a low-impedance state in under one microsecond, diverting the surge current to the earthing system. Once the surge has passed, the device returns to its standby state. A status window on the front of the unit indicates whether the SPD is functioning correctly (green) or requires replacement.

Type 1 SPDs (Class I) are designed to handle direct lightning strike currents, measured as Iimp (impulse current). They are installed at the main service entrance of a building — between the utility supply and the main distribution board. Type 2 SPDs (Class II) protect against induced surge currents, measured as In (nominal discharge current), and are installed inside distribution boards and sub-panels. Type 1+2 combined units provide both functions in a single DIN-rail device, which is preferred when installation space is limited.

AC SPDs are installed at three main points in a low-voltage electrical system. Type 1 at the building service entrance or main switchboard — the first line of defence against direct lightning currents. Type 2 at sub-distribution boards and electrical cabinets — protecting downstream circuits from induced surges. Type 3 at the point of use, directly upstream of sensitive equipment such as PLCs, inverters, or precision instruments. For most industrial and commercial installations, a coordinated Type 1 + Type 2 scheme is required under IEC 60364-5-53.

Start with four parameters. First, Uc (maximum continuous operating voltage) — select a value at least 10% above your system’s nominal voltage, typically 275V or 320V for 230V systems and 385V or 420V for 400V three-phase systems. Second, Type — choose Type 1 for service entrances, Type 2 for distribution boards. Third, In/Iimp rating — match the discharge capacity to your lightning risk assessment per IEC 62305. Fourth, earthing system — confirm compatibility with your site’s TN-S, TN-C-S, or TT configuration. When in doubt, contact our technical team with your single-line diagram for a recommendation.