Lightning protection: IS/IEC 62305, LA types, down conductors, separation

Every pre-monsoon storm season brings calls about dead inverters and burnt SPDs, and customers ask for a "lightning arrester" (LA) on every quote. This lesson explains what a lightning protection system (LPS) is, how IS/IEC 62305 designs one, and why the usual "rod on a pole on the module structure" does less than people think.

What lightning does to a PV system

There are two kinds of damage:

1. Direct strike on the array or building: tens of kA looking for a path to earth through modules, structure, cables and walls.
2. Indirect effects from a nearby strike: surges induced in cable loops, or arriving along the DISCOM line.

Indirect effects cause most of the damage seen in the field. They are handled by SPDs, small cable loops and good bonding (lessons 2, 3 and 5). An external LPS handles direct strikes.

IS/IEC 62305 in brief

The Indian standard IS/IEC 62305 (adopted from IEC 62305) has four parts:

• Part 1: general principles.
• Part 2: risk assessment: whether a structure needs an LPS, and which class.
• Part 3: physical damage and life hazard: air terminals, down conductors, earthing, separation.
• Part 4: electrical and electronic systems: SPDs and shielding.

There are four LPS classes, I (most stringent) to IV. For each class Part 3 sets the design parameters:

Homes and small commercial roofs often end up at class III or IV, but the risk assessment decides, not habit.

Placing air terminals: the rolling sphere

Imagine a sphere of radius R (45 m for class III) rolled over the building: anything it touches can be struck; anything sheltered beneath it by a rod is protected.

For a single vertical rod of height h above a flat roof, the protected horizontal radius at height y above the roof is:

d = √[h × (2R − h)] − √[y × (2R − y)]

Worked example (class III, R = 45 m)

The modules' top edge is 1.2 m above the roof slab.

• Rod tip 3 m above the roof: √(3 × 87) − √(1.2 × 88.8) = 16.2 − 10.3 = 5.8 m
• Rod tip 5 m above the roof: √(5 × 85) − 10.3 = 20.6 − 10.3 = 10.3 m

So a 3 m rod protects only modules within about 6 m of it. A 10 m long array with the rod at one end is not fully covered. Use a taller rod, or two rods, and check every corner of the array.

Rod types: conventional and ESE

• Conventional (Franklin) rods, meshes and catenary wires are what IS/IEC 62305 recognises. Coverage comes from geometry: rolling sphere, protection angle or mesh.
• Early Streamer Emission (ESE) heads are sold with claims of a much larger protection radius, often 50–100 m, based on the French standard NF C 17-102. IS/IEC 62305 does not recognise any enhanced radius for ESE devices. Many Indian tenders and DISCOM specs still name NF C 17-102, so you will be asked to supply ESE heads. If you do, design the coverage as if the ESE head were an ordinary rod of the same height. Never sell the brochure radius as protection.

Down conductors

The down conductor carries the lightning current from the air terminal to the earth termination. IS/IEC 62305-3 sets minimum cross-sections (Table 6 of Part 3):

A footnote lets 50 mm² drop to 25 mm² where mechanical strength is not essential. It never comes down to 16 mm².

Other rules: at least two down conductors where practical, spaced around the building; routed as straight and short as possible, with no sharp bends or loops; a test joint on each, near ground level; each terminating on the earth-termination system, which is bonded to the building's other earthing.

The market reality

The usual Indian rooftop "LA" is one ESE or spike head on a 3–6 m GI pipe bolted to the module structure, with 16 mm² aluminium PVC-insulated cable run down through the same conduit as the DC cables to a separate, unbonded "LA pit".

Compared with the standard: - 16 mm² aluminium is about a third of the smallest aluminium down conductor allowed (50 mm² round; 70 mm² for tape), and it is often taken down into the pit where aluminium corrodes. - Ordinary PVC insulation is not lightning insulation. A strike can flash over from the cable to DC cables beside it, straight into the inverter. - A rod on the module structure makes the array part of the lightning path, and the PM Surya Ghar specification forbids it (Annexure 3, cl. 8.2.1). - An unbonded LA pit raises the risk of side flashes and of large voltages between the LA earth and the system earth. - The mast shades the modules for part of every day.

Where does 16 mm² come from? The PM Surya Ghar technical specification (Annexure 3, cl. 8.2.4) names 6 mm² copper, 16 mm² aluminium or 25 × 3 mm GI strip as the minimum for an LPS of class III. Those figures match the IS/IEC 62305-3 sizes for bonding conductors, not down conductors, so meeting the scheme minimum does not make a down conductor compliant with IS/IEC 62305-3. Where you can, use the IS/IEC 62305-3 sizes; they also satisfy the scheme.

The honest summary for a customer: "A rod on a pole is not the same as lightning protection. Most of the damage we see comes from surges, which the SPDs and bonding handle. If your building needs an LPS, we design it properly."

Separation distance

If the array is not bonded to the LPS, it must be kept at least a separation distance s from the rod and down conductors, so lightning cannot flash across. IS/IEC 62305-3 gives:

s = ki × (kc ÷ km) × l

• ki: 0.08 (class I), 0.06 (class II), 0.04 (classes III and IV)
• kc: share of current in the conductor: 1 for a single down conductor, about 0.66 for two, lower for more
• km: 1 for air, 0.5 for solid material such as brick or concrete
• l: length along the LPS conductor from the point considered to the nearest equipotential bonding point, in metres

Worked example

A class III rod and one down conductor; the nearest point of the array is 13 m along the conductor from the bonding point at ground level, through air:

s = 0.04 × (1 ÷ 1) × 13 = 0.52 m

With two down conductors: s = 0.04 × 0.66 × 13 = 0.34 m.

So keep the rod, its mast and its down conductors at least about half a metre from any module, structure member or DC cable. If you cannot, bond the structure to the LPS with 16 mm² copper (or equivalent) and fit Type 1 SPDs on the DC and AC sides, because now part of the lightning current will flow into the system.

Key points

• Most PV damage comes from indirect surges; SPDs, bonding and small loops are the first defence.
• IS/IEC 62305-2 risk assessment decides whether an external LPS is needed and its class.
• Check rod coverage with the rolling sphere: a 3 m rod covers only about 6 m at module height for class III.
• Down conductors need at least 50 mm² copper or 50–70 mm² aluminium (round or tape); 16 mm² aluminium "LA cable" does not meet IS/IEC 62305-3.
• Keep the separation distance s = ki × kc/km × l, or bond the array to the LPS and use Type 1 SPDs.
• IS/IEC 62305 gives ESE heads no enhanced radius: design them as ordinary rods.

Check yourself

1. For class III, what is the protected radius at 1 m height from a 4 m rod?
2. Why does PVC-insulated cable not provide separation from a down conductor?
3. A class II LPS has one down conductor; the nearest array point is 10 m along it from the bonding point, in air. What is s?