Rooftop Solar Safety: Earthing and Lightning Protection

A power plant on your roof

Rooftop solar looks like a set of glass panels quietly producing free power, but it is a small electrical power plant. Solar panels generate direct current at voltages that can reach several hundred volts on a string, and unlike a mains circuit you cannot simply turn them off, because they produce power whenever light falls on them. Faults, loose connections, storms and poor installation can cause electric shock, fire or damage to expensive equipment.

A properly designed and installed system is safe, and the protective measures are well understood and cost little compared with the total price. The danger comes from cutting corners: thin earthing wire, no surge protection, cheap connectors and no testing. A low quotation tends to omit these first, and a buyer can hardly see them once the work is finished.

This article is general information for owners. It does not replace the design of a qualified electrical engineer or the requirements of the Indian standards and your DISCOM. Ask your installer which standards the design follows and ask for the documents.

What earthing does

Earthing, also called grounding, connects the metal parts of your solar system to the ground through a conductor and an earth electrode. If an insulation fault makes a panel frame or a metal structure live, the earth connection gives the fault current an easy path into the ground, which makes the protective devices operate and keeps the metal at a safe voltage. Without it, a person who touches the structure could receive a dangerous shock.

A good design earths the panel frames, the mounting structure, the inverter body and the metal enclosures, and connects them together so that all are at the same potential. This is called equipotential bonding. The earth electrodes are usually pits with a rod or plate, backfilled with suitable material and kept moist, and a typical rooftop system may have separate pits for equipment earthing and for lightning protection, connected as the design requires.

Soil affects earthing. Dry, sandy soil has high resistance, while moist clay has lower resistance, and resistance changes between seasons. Measure resistance after construction, and expect to water or maintain the pits in the dry months.

• Panel frames and structure are bonded together and connected to the earth system.
• The inverter body and electrical panel are earthed with correctly sized conductors.
• Earth pits are accessible, marked and protected with a cover.
• Earth resistance is measured with a proper tester and the result recorded.

Surge protection devices

A surge is a short spike of voltage. It can come from lightning nearby, from switching on the grid or from faults. Inverters and other electronics are sensitive to such spikes, and a strong surge can destroy an inverter, which is the most expensive electronic part of a system. A surge protection device (SPD) diverts the excess energy to earth before it reaches the equipment.

A well-protected system usually has SPDs on both sides of the inverter. On the DC side, an SPD protects against surges that come in through the panel cables. On the AC side, another protects against surges from the grid or the building wiring. Quality matters: SPDs are rated for the system voltage and a certain amount of surge current, and they wear out after a big event, so they should be checked and replaced when indicator windows show a fault.

Lightning protection in Uttar Pradesh

Uttar Pradesh sees frequent thunderstorms in the pre-monsoon and monsoon seasons, and lightning strikes cause injuries and damage every year. A rooftop array is a large metal and glass object in an open place, so owners often ask whether it attracts lightning. The array does not by itself make a strike more likely, but a strike on the building or nearby can damage the system or travel through wiring into the house, so a protection plan is sensible.

A lightning protection system usually has air terminals, which are rods placed so that they capture a strike, down conductors that carry the current to earth, and an earth termination. It must be designed so that the panels are within the protected zone, and so that the lightning current does not flow through the solar cables. Standards on lightning protection exist for this, and a qualified designer should decide what is needed for your building, based on its height, location and use.

If your building already has a lightning arrestor, tell the installer so that the solar earthing is connected properly to it. A mistake in connecting the two systems can create dangers instead of removing them.

Wiring, connectors and fire safety

Many solar fires start at poor connections, not at panels. Connectors that are badly crimped, mixed between brands or exposed to water can heat up and arc. Use cables rated for solar use, with UV resistance and the correct size for the current, and run them in conduit or trays that protect them from sunlight, sharp edges and animals. Keep cables off the roof surface where water collects.

Place the DC isolator and the inverter where they can be reached safely, labelled clearly. Keep paths clear around the array so that people can move across the roof, and so that firefighters can reach the area. Some building codes and fire authorities have rules for rooftop systems, especially for larger buildings, hospitals and institutions. Ask your installer what applies to you.

Never store anything flammable near the inverter or the electrical panel, and keep the area dry and ventilated.

• Use solar-grade cables and matching, properly crimped connectors.
• Protect cables with conduit and keep them clear of standing water.
• Label isolators, breakers and the earth connections.
• Keep walkways clear and do not store combustible items near the inverter.

Working safely on and around the system

Panels produce power whenever there is light, even if the grid is off and the AC switch is open. Switching off the inverter does not make the DC cables dead. Only qualified people should open junction boxes, connectors or the inverter. Cleaning the panels is a job you can do, but do it safely: from a stable position, in early morning or evening, away from the roof edge, and without touching cables.

Roof work carries a fall risk. Make sure the parapet is high enough, or use edge protection. Do not clean in rain, wind or when the panels are wet and the roof is slippery. Keep children away from the roof and from the inverter. Our guide on cleaning panels in dusty regions covers a safe method.

What to ask for in the quotation

Ask for the protective parts in writing before you sign. A good quotation lists the earthing scheme, the SPDs, the isolators and breakers, the cable type and size, the lightning protection decision and the tests that will be done at commissioning. If these are missing, ask why, and compare with other quotations. Our article on common mistakes when buying rooftop solar lists similar warning signs.

At handover, ask for the test results, such as earth resistance values and insulation tests, along with the single-line diagram and the commissioning report. Keep them with your warranty papers. They show that the system was checked, and they help in any later claim or inspection.

• How many earth pits, what materials, and what resistance was measured?
• Which SPDs are fitted on the DC and AC sides, and what are their ratings?
• Is a lightning arrestor included, and was the need assessed for my building?
• Which standards does the design follow?
• Will I receive a single-line diagram and test report at handover?

Inspection and maintenance

Protection can degrade unnoticed. Earth connections corrode, pits dry out, SPDs absorb a surge and stop protecting, and connectors loosen with heat and vibration. A yearly inspection by a qualified technician, ideally after the monsoon, should check the earth resistance, SPD indicators, connector tightness, cable condition, the structure bolts and the inverter. Keep a dated record of each visit.

After a major storm or a nearby lightning strike, have the system checked even if it seems to work. Damage can be hidden, and a weakened SPD or a burnt connector can fail later. If the inverter shows an earth fault or an insulation fault, do not ignore it. Switch off as per the manual and call a qualified person.