Lightning and Surge Protection for a Rooftop Solar Plant: LA, SPDs and Earthing

A rooftop solar plant needs two kinds of protection: air terminals and an earthing system that take a direct strike away from the array, and surge protective devices on the DC and AC sides that clip the voltage surge that still gets through. Both are designed from an IEC 62305 risk assessment, and both must tie into the factory's existing earthing.

Why does a solar array change the lightning risk of a roof?

A solar plant adds metal, height and long cable loops to a roof. Module frames and structures are conductive. String cables run for tens of metres between modules and inverter, forming loops that pick up induced voltage when lightning strikes nearby, even without a direct hit. Inverters sit at the end of those loops, and they contain the most expensive electronics on the site. The building's own lightning protection, if it exists, was designed without any of this.

So there are two separate hazards to deal with. A direct strike can damage modules, structure and the roof, and may start a fire. A nearby strike induces a surge in the cabling that damages inverters, monitoring and the AC side of the factory. The first is handled by air termination and earthing, the second by surge protection. Mixing them up is the commonest design error: arrestors on the roof do little for the inverter's DC input.

How is the protection level decided?

The IEC 62305 series is the reference. Part 2 sets out a risk assessment: the structure's size and use, how often lightning is expected to strike the area, the type of services entering the building, and the consequences of a loss. The result tells you whether a lightning protection system is needed and to which class, from I to IV. Part 3 covers physical damage to the structure and Part 4 covers electrical and electronic systems inside it. Indian practice follows IS/IEC 62305, and BIS lists at least Part 4 as an Indian Standard; confirm which parts and editions your tender calls up.

MNRE's draft Quality Control Manual for grid-connected rooftop solar, circulated for comments in February 2023, asks for the whole array area to be protected by a sufficient number of lightning arrestors, designed to NFC 17-102:2011 or IEC 62305. It says the arrestor must not be installed on the mounting structure, and it sizes the down-conductor for a class III system as at least 6 mm² copper, 16 mm² aluminium or a 25×3 mm GI strip, run in a separate pipe. The manual is a draft and we could not confirm a final version, so the specification in your contract governs. A plant at a chemical or fuel-handling site, or one with a high-value process below it, will usually justify a higher class than a warehouse roof.

Where do arrestors, SPDs and earthing go?

On SPD selection, the MNRE draft manual points to IEC 63227 and IEC 61643-11 for the devices, and to IEC 62561 for lightning-protection components such as connectors and earth electrodes. A device on the DC side is typically a Type 2 unit; a Type 1 or combined device is used where part of the direct-strike current can reach the circuit. State the choice in the design basis, not just "SPD provided".

How should the plant earth tie into the factory earth?

Here the sources do not say quite the same thing. The MNRE draft manual calls for separate earth pits for DC-side earthing, AC-side earthing and lightning-arrestor earthing, with earth resistance as low as possible and never above 5 ohms, and it refers to IS 3043 for earthing. As we read IEC 62305-3 and IS 3043, the preference for an installation with lightning protection is to bond the earthing systems together so that potentials rise and fall together during a strike. Confirm the wording of the edition your tender cites. Our note on lightning protection and earthing compliance explains why individually good pits can still produce dangerous potential differences when they are not bonded.

The usual reconciliation is separate electrodes that can each be tested, joined through a bonding conductor into one equipotential system, with the factory's main earth. Whichever way your contract words it, ask the designer to show the earthing layout, the bonding points, and the test value at each pit. Resistance moves with soil moisture, so a reading taken in the monsoon is not the number the plant will see in May. See also the glossary entry on earthing to IS 3043.

What goes wrong most often on site?

• Arrestors that shade the array. A tall air terminal placed late casts shadow on modules. Place terminals in the shadow study, not after it.
• DC and AC SPDs sized to a catalogue, not to the string. The DC device's continuous operating voltage must sit above the array's open-circuit voltage at the coldest module temperature, otherwise it ages early.
• Pits tested once. The commissioning value is filed and never repeated, so degradation goes unnoticed.
• Bonding by assumption. The structure, the inverter earth and the lightning system are each connected, but not to each other.
• Unlabelled SPD status windows. Many SPDs show when they have worn out. If nobody looks, the plant runs unprotected for years.
• Cables loose on the roof. Mechanical damage turns into an earth fault long before lightning arrives. This is also how DC fires start, as the MNRE draft manual notes.

What should you ask an EPC to show before award?

• The IEC 62305 risk assessment, or a written reason for not doing one.
• The air-termination layout drawn on the same roof plan as the array, with shadow impact noted.
• The SPD schedule: location, type, voltage rating, and the standard each device is tested to.
• The earthing layout, bonding points to the factory earth, and test values at commissioning.
• A maintenance plan that includes SPD checks and earth re-testing once a year at the least favourable season.

The Electrical Inspector's inspection, where the state requires it for your installation, also looks at earthing. Our guide to Electrical Inspector approval covers when that applies.

How Secured Engineers helps

• We design the plant in-house, including the earthing, lightning and surge-protection scheme, on the same drawings as the array layout.
• Fire NOC, CEIG and DISCOM net-metering filings are prepared in-house.
• MEPF and solar can sit under one contract, so the plant earth is tied to the factory earth by the same team.
• We work from offices in Noida (Delhi NCR) and Ludhiana, with 535+ projects across 18+ states and 89.77% repeat clients.
• For an existing plant, we offer rectification and design review of earthing and surge protection. We do not sell audits.

For earthing and lightning protection work, see our earthing and lightning protection service and our solar EPC service. To get a plant designed with protection built in, message us on WhatsApp, call +91 70099 85817 or ask for a free proposal.