Lightning / Surge Arrester
Safety
Diverts lightning and switching surges on power lines to earth. Modern units are gapless metal-oxide (ZnO) stacks: insulators at normal voltage, conductors during a surge — protecting transformers and service entrances.
What it looks like
Lightning / Surge Arrester
A typical safety — exact shape, colour and markings vary between manufacturers.
Types & variants
Key specs
Rated voltage (Ur)
kVChosen above the system's maximum earth-fault voltage rise.
MCOV/Uc
kVContinuous voltage the arrester can sit at forever.
Nominal discharge current
kA10 kA class standard for distribution.
Residual voltage
kVVoltage let through at the discharge current — must undercut equipment BIL.
Markings
Nameplate: Ur, MCOV, class, discharge rating; disconnector flag drops when the unit has failed.
Standard values
9–12 kV rated units on 11 kV distribution; low-voltage variants at the service entrance are 'SPDs'.
How to choose
HV selection is utility engineering (system voltage, earthing, BIL coordination). For buildings, what you choose is the SPD hierarchy plus a code-compliant air-terminal/down-conductor/earth system.
Pinout & package
Line terminal on top, earth terminal at base — earth lead short and straight; many hang from a disconnector that visibly drops on failure.
Example circuits
- Arresters on the HV bushings of a pole transformer
- Arrester + earth electrode at a service drop
- Coordinated: arrester → Type 1 SPD → Type 2 SPD
Common failures
Thermal runaway of aged ZnO blocks (ends shorted, disconnector fires), cracked porcelain letting moisture in, and earth leads too long/looped to clamp effectively.
How to test
Utility crews test insulation resistance and leakage current trends; a dropped disconnector flag or visible damage = replace. Nothing here is DIY — HV clearances apply.
Substitutes
Same class and ratings per the coordination study; at the building level, layered SPDs are the accessible piece of the puzzle.