There is no single “best” spot that fits every site. Where you mount a lightning arrester comes down to your system layout, the equipment you are protecting, and how bad the lightning exposure is in your area. That said, the rules below are what most engineers actually follow in the field.
Install as close to the protected equipment as you can An arrester’s whole job is to clamp overvoltage down to a level the equipment can survive. The closer it sits to the transformer, switchgear, or busbar, the less lead inductance you introduce between the two. Keep the straight-line distance to roughly 5–10 m or less — every extra meter of lead adds voltage rise you do not want.
Shortest electrical distance wins Do not just aim for short physical distance. What really matters is the electrical distance between the arrester and the protected device. Keep it as short as possible so the lightning wave does not build up extra voltage drop along the line before it reaches the gear.
Use multi-stage arrangements on medium-voltage or lightning-heavy sites One arrester is often not enough. A common setup is:
- · Level 1 at the incoming feeder — this takes the brunt of an incoming lightning wave.
- · Level 2 near the busbar or capacitor bank — this holds the voltage steady inside the system.
Where to actually put them
- · Transformer HV side. Wire the arrester in parallel with the transformer — top end to the line, bottom end to ground. This protects the HV winding first, and it is a must in lightning-prone regions.
- · Transformer LV side. When the HV arrester discharges, the LV arrester limits the potential rise between the LV winding, the neutral, and the tank. It keeps downstream low-voltage gear — distribution panels, 0.4 kV lines — safe.
- · On a solidly grounded neutral or a system with an arc-suppression coil. If the transformer can run with a single-phase-to-ground fault, put an arrester on the neutral. Otherwise you end up with isolated-system overvoltage that damages the winding.
- · Inside the distribution panel, LV side. Fit the LV arrester between the contactor and the outgoing feeder fuse. Run the earthing conductor through the zero-sequence CT of the RCD so the residual current device still works as intended.
- · On other lightning-exposed equipment. Motors and kWh meters that connect straight to overhead lines should get their own small LV arrester.
Installation rules that actually matter
- · Mount it vertical. Keep the tilt within 5°, or water pools in the housing and you start inviting creepage faults.
- · Fix it properly. M12 galvanized bolts, tightened to roughly 45 N·m. Loose mounting means a bad connection and poor performance under surge.
- · Use the right conductor. Copper cross-section of at least 25 mm², and keep the bend radius above 10× the wire diameter. Cramped, sharp bends hurt the earthing path.
- · Ground it right. Earthing resistance should stay at or below 10 Ω, and the ground electrode buried at least 0.6 m deep. A poor ground makes the whole arrester useless.
- · Leave safe clearance. When mounting next to insulators, keep at least 1.2 m between them, and stay clear of spots where vehicles or machines can bump into it.
- · Check it periodically. Wipe down the zinc-oxide block and check the surface for dust and contamination — a dirty surface can trigger abnormal discharge.
Bottom line Pick the location by thinking about the system structure first, then the lightning environment, then how critical the equipment is. The HV arrester guards the transformer’s HV winding, the LV arrester protects the low-voltage side, and together they close the loop. None of it works without solid grounding, short electrical distance, correct fixing, and regular maintenance.
