Where surges come from
Lightning striking near an overhead line, or heavy switching on the grid, can push a spike of thousands of volts into your wiring for a fraction of a millisecond. Electronics die quietly; insulation can be punctured.
What an SPD does
An SPD (surge protective device) sits between the phases and earth. When the voltage jumps above its threshold it becomes conductive, shorts the spike harmlessly to earth, then returns to standby.
When you need one
- Overhead service line into a private house: strongly required — the line acts as an antenna for lightning.
- Underground supply in a city apartment: the risk is lower, so an SPD is a recommendation rather than a must.
An SPD only works with a proper earthing connection — a licensed electrician should verify it.