SPD Coordination: Why One Surge Device at the Panel Is Never Enough
8 March 2026 · 5 min read
Type 1, Type 2 and Type 3 devices do different jobs. Here is how to stage them across an Indian LT distribution.
Three stages, three duties
Type 1 SPDs at the main incomer handle partial lightning current (Iimp, 10/350 µs). Type 2 devices at sub-distribution clamp the residual to equipment-tolerable levels. Type 3 devices sit close to sensitive loads to control the last few hundred volts of let-through.
Coordination rules that get missed
Maintain the manufacturer's minimum cable length or decoupling inductance between stages, keep connecting leads under 0.5 m total, and bond every SPD earth to the same equipotential bar as the LPS down conductors. An uncoordinated SPD set can be worse than none at all.
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