Surge Protection Device / GDT
Surge
Surge protection devices clamp lightning and switching surges at the panel before they reach your electronics. MOVs absorb energy, gas discharge tubes (GDTs) crowbar big strikes — real installs combine types in stages.
What it looks like
Surge Protection Device / GDT
A typical surge — exact shape, colour and markings vary between manufacturers.
Types & variants
Key specs
Uc
VMaximum continuous operating voltage — above your nominal mains.
Up
kVVoltage let through downstream during the surge — lower is better.
In / Imax
kANominal and maximum discharge current (8/20 µs) it survives.
Status indication
Window/flag showing the MOV is worn out.
Markings
Type class, Uc, Up, In/Imax on the module face; green/red status window.
Standard values
Type 2, Uc 275 V, In 20 kA modules are the standard distribution-board fit in 230 V countries.
How to choose
Type 2 at the board for most buildings, Type 1 where there's a lightning protection system or overhead service, Type 3 strips near sensitive gear. Short, straight connecting leads matter as much as the device.
Pinout & package
DIN-rail module: L(s) and N in, earth out — through a dedicated fuse/MCB per the datasheet.
Example circuits
- Type 2 SPD across the incoming supply at the consumer unit
- GDT + MOV + TVS three-stage protection on a telecom line
- Surge-protected socket strip
Common failures
MOVs degrade with every surge and eventually fail shorted (that's what the thermal disconnect and status flag are for). GDTs can fail open silently after a massive strike.
How to test
Check the status indicator; electrically, a worn MOV shows falling insulation resistance. Proper verification needs a surge tester — replace modules showing red rather than testing them.
Substitutes
Same type class with equal/better Uc, In and lower Up. A power strip's built-in MOV is a Type 3 supplement, not a replacement for board-level protection.