Selecting Surge Protective Devices (SPD Type 1, 2, and 3) for Industrial MDBs

1. Transient Overvoltages and the Role of SPDs

Transient overvoltages arise from direct lightning strikes to the building's lightning protection system, nearby strikes inducing surges into power and signal lines, and internal switching of large loads such as capacitor banks and motors. These transients last only microseconds yet peak at several kilovolts — enough to puncture electronic insulation, degrade control boards cumulatively, and cause hard-to-diagnose failures of PLCs and instrumentation.

A Surge Protective Device (SPD) provides a momentary low-impedance path that diverts surge energy to the earthing system instead of through equipment, using components such as metal-oxide varistors (MOVs) or spark gaps that switch from very high to very low impedance faster than the surge front rises. Effective protection relies not on one device but on cascaded protection staged from the main board down to the terminals of critical equipment.

2. SPD Classification under IEC 61643-11

IEC 61643-11 classifies low-voltage SPDs into three types by test waveform. Type 1 is tested with the 10/350 microsecond impulse current representing partial lightning current entering the installation on a direct strike, and belongs at the main board of buildings with air-termination systems or overhead supply. Type 2, tested with the 8/20 microsecond wave, handles induced and switching surges at sub-distribution boards. Type 3, tested with a combination wave, is installed close to sensitive terminal equipment.

Key selection parameters are the impulse current Iimp for Type 1; nominal and maximum discharge currents In and Imax for Type 2; the voltage protection level Up, which must be below the impulse withstand of the protected equipment; the maximum continuous operating voltage Uc, matched to the building's earthing system; and the SPD's short-circuit withstand with its dedicated overcurrent protection. Coordination between Type 1 and Type 2 stages requires sufficient cable length or decoupling inductance so each stage absorbs energy in the intended order.

• Type 1 (10/350 µs wave): main boards of buildings with LPS or overhead supply
• Type 2 (8/20 µs wave): sub-distribution boards and typical MDBs
• Type 3 (combination wave): close to sensitive terminal equipment
• Up must be below the protected equipment's impulse withstand
• Uc must suit the building's earthing system

3. Installation Practices That Determine Real Performance

SPD effectiveness depends on installation as much as on the device. The cardinal rule is that the total lead length from phase conductor through the SPD to the earth bar must be as short as possible — common practice targets roughly 0.5 metres or less — because every metre of conductor adds inductance that generates additional voltage while surge current flows, making the voltage seen by equipment far higher than the nameplate Up. Where short leads are impossible, use a V-connection so load current passes directly through the SPD's terminals.

A dedicated overcurrent device (fuse or breaker per the manufacturer's table) must back up the SPD so a degraded, short-circuited unit is disconnected without blacking out the whole board. Route the SPD's earth lead to the same earth bar the protected system references, avoid running the SPD's line and earth leads bundled in parallel over long distances where surges couple between them, and apply equipotential bonding of metallic services and structure to the earth system per lightning protection principles so no dangerous potential differences arise during a surge event.

4. SPD Condition Monitoring and Common Mistakes

SPDs are consumable by nature: varistors degrade cumulatively with every surge absorbed and under prolonged power-frequency overvoltage. Most units therefore carry a status window or remote signalling contact that operates when the internal disconnector isolates a failed module. The most common field problem is an SPD sitting in failed state for months unnoticed — the system is unprotected while everyone assumes otherwise. Next come excessive lead lengths, missing dedicated backup overcurrent devices, and unprotected signal lines entering the same equipment.

Good practice includes the SPD status check in the monthly panel round, wires the signalling contact into the BMS or SCADA for immediate failure notification, re-torques and thermally scans connections during routine panel maintenance, logs major local lightning or surge events against SPD status, and plans protection across every path — power, telephone, LAN, and instrumentation lines entering the same critical equipment — because a surge always finds the one line left unprotected.

• Check SPD status windows in the monthly panel round
• Wire signalling contacts into the BMS/SCADA
• Keep total lead length as short as possible (~0.5 m)
• Protect every path — power and signal lines alike