Down Conductor
A down conductor is a dedicated, low-impedance, electrically conductive path—often copper or aluminum cable/tape or suitably thick structural steel—that runs vertically down a structure, connecting the roof air-termination/strike collection system to the grounding electrode system, to safely conduct lightning current to earth without hazardous arcing, sideflashes, or thermal damage, as defined by CSA B72:2020 and Ontario Reg. 712.
On a shop floor, down conductors are installed from roof air-termination devices down the exterior or interior structural steel to ground electrodes, with at least two per structure placed at exposed corners to share lightning current. They must provide a low-impedance path, using materials like round braided copper cable or tinned copper tape, and be continuous with secure clamps to ground rods. In industrial facilities, down conductors should be protected from physical damage in pedestrian or forklift traffic zones, bonded to plant grounding systems for equipotential bonding, and routed directly to minimize impedance. Structural steel or rebar can serve as natural down conductors if thickness is ≥4.8 mm and electrical continuity is assured at joints.
- Insufficient number and poor placement of down conductors, such as having only one or placing them on one side, leading to high thermal stress and risk of sideflashes.
- Compromised continuity and poor connections to ground electrodes, including splices with non-rated fittings, loose clamps, or corrosion, increasing impedance and flashover risk.
- Physical damage and unprotected routing in worker or equipment traffic zones, such as dents, cuts, or use as makeshift tie-offs, compromising the lightning path and causing arcing.
How do CSA B72 and IEC/EN 62305 determine the required number and spacing of down conductors?
CSA B72:2020 aligns with IEC/EN 62305, requiring at least two down conductors per structure, distributed around the perimeter, preferably at each exposed corner. Spacing depends on LPS class, typically 10–20 m between conductors, with additional down conductors for longer buildings per Ontario Reg. 712.
Can structural steel or rebar be used as a down conductor in a BC manufacturing facility, and what must be verified?
Yes, under CSA B72, structural metal components can be used if thickness is at least 4.8 mm (3/16 in) and electrical continuity is ensured at joints and splice points. Bonding to the LPS at top and bottom is recommended, and engineering verification may include continuity testing and drawing review.
How does the presence of a lightning down conductor interact with the plant’s electrical grounding and WorkSafeBC Part 19 requirements?
A lightning down conductor is a specialized grounding conductor for high-energy impulse currents, requiring integration with the facility’s grounding system via equipotential bonding to avoid sideflashes. WorkSafeBC Part 19 expects safe grounding practices, so down conductors must be properly grounded, mechanically secured, and documented in site electrical safety plans.