Earthing and Lightning Protection for Factories: A Guide

Earthing and lightning protection are two systems that most factory owners only think about after something has gone wrong: a damaged drive after a storm, a shock from a metal frame, or a burnt PLC card that nobody can explain. Done properly, earthing and lightning protection work together to keep people safe, protect equipment and keep production running through the storm season. This guide explains what each system does, which standards apply, how they are designed and installed, and how to keep them working.

Earthing and lightning protection: two jobs, one system

It helps to separate the two ideas before joining them back together.

• Electrical earthing (with protective bonding) is part of the power installation. Its job is to make sure that if a live conductor touches a metal enclosure, enough fault current flows for the protective device to disconnect quickly, and that exposed metalwork does not rise to a dangerous voltage.
• Lightning protection deals with a direct strike to the building or a strike nearby. The external system intercepts the strike and carries the current safely to earth. The internal measures, including bonding and surge protective devices (SPDs), stop dangerous voltages and sparks inside the building.

The two must be connected. The IET notes that an equipotential bond between the lightning protection system and the building’s main earthing terminal has been required since BS 6651, the predecessor to today’s standard. Separate, unbonded earths can rise to very different voltages during a strike, which is exactly what destroys equipment and endangers people.

The standards: IEC 62305 and the wiring rules

Lightning protection is covered by the IEC 62305 series, published in Britain as BS EN 62305. It has four parts:

Electrical earthing and protective bonding fall under the wiring rules, IEC 60364 or BS 7671 where British practice is followed. In practice the two sets of standards meet at the main earthing terminal and the SPDs at each distribution board.

Does your factory need lightning protection?

There is no simple height or size above which a building must have lightning protection. The IET’s guidance on BS EN 62305 is clear on this: the need for protection is decided by a risk assessment under Part 2, not by a rule of thumb. The risk assessment considers:

• The size, height and location of the building and how often lightning strikes in the area.
• What the building contains: people, flammable materials, explosive dust, critical processes.
• The consequences of loss: injury, fire, loss of service, or loss of production.
• Incoming services such as power and telecom lines that can carry surges into the building.

For many Nigerian factories, the answer will be yes. Plants often have tall structures such as silos, chimneys, boiler houses and steel-framed warehouses, they may store flammable liquids or combustible dust, and they depend on drives and control systems that are sensitive to surges. Thunderstorms are a regular feature of the rainy season across much of the country.

Lightning protection levels and design

The risk assessment produces a required lightning protection level (LPL), from I (most onerous) to IV. Each LPL corresponds to a class of lightning protection system (LPS), which sets the geometry of the design. ABB’s guide to the standard summarises the key values:

Air termination

Air terminations are the parts designed to be struck: rods, horizontal conductors along ridges and parapets, or a mesh across the roof. Designers use the rolling sphere, protective angle or mesh method to check that every part of the structure is covered. Metal roofs and steel structures can sometimes act as natural components of the system if they are thick enough and electrically continuous, which a competent designer will check.

Down conductors

Down conductors carry the current from the roof to the earth termination. They should be spread evenly around the perimeter, take the most direct route, and avoid sharp bends. In steel-framed buildings, the structural steel can often serve as down conductors if the connections are electrically continuous. Each down conductor should have a test joint so the earth can be measured on its own.

Earth termination

The earth termination network disperses the lightning current into the ground. It can be made of rods, buried tape rings or, in new buildings, the reinforcement in the foundations. BS EN 62305 focuses on the overall resistance of the earthing system rather than individual electrodes. The IET explains that the recommended target is 10 ohms or less overall, mandatory only for structures with explosive risk but widely treated as good practice.

Ground conditions matter. Dry, sandy or rocky soil has higher resistivity than moist clay, so an electrode that tests well in the rainy season may test worse in the dry season. Deeper rods, more rods, ring electrodes and foundation earthing are common ways to reach the target in difficult soil.

Electrical earthing in the factory

The power system’s earthing arrangement (TN-S, TN-C-S or TT) is chosen at the design stage and affects how earth faults are cleared. Whatever the arrangement, the key elements are the same:

• A main earthing terminal at the main switchboard, connected to the earth electrodes and to the transformer and generator neutrals as the design requires.
• Protective conductors run with every circuit to every piece of equipment.
• Main protective bonding to incoming metal services such as water, gas and fuel pipes.
• Supplementary bonding where needed, for example around process equipment in wet areas.
• Bonding of structural steel, cable trays, pipe racks and tanks so that all metalwork stays at the same potential.

Generators deserve special attention. Each generator’s neutral and frame earthing must be coordinated with the transformer and the changeover arrangement so that earth faults are detected and cleared whichever source is running.

Surge protection: the internal defence

Even with a perfect external system, a strike to the building or a nearby power line sends surges through incoming cables. IEC 62305-4 divides the building into lightning protection zones, from LPZ 0 (outside, exposed to direct strikes) through LPZ 1 (inside, where services enter) to LPZ 2 and beyond (further protected areas such as control rooms). Wherever a cable crosses from one zone to the next, it should pass through an SPD.

• Type 1 SPDs (lightning current) at the service entrance, where a building has an LPS or is fed by overhead lines.
• Type 2 SPDs at sub-distribution boards to limit overvoltages further.
• Type 3 SPDs close to sensitive equipment where needed.

These devices should be coordinated, so that the upstream SPD takes most of the energy and downstream devices clip what remains. Do not forget data, telecom and instrument cables, especially those that run between buildings or out to field equipment such as tank level gauges and borehole pumps.

Installation checklist

• Risk assessment to IEC 62305-2 completed and LPL agreed.
• Design drawings showing air terminations, down conductors, test joints and earth network.
• Materials and conductor sizes as specified in the standard; avoid mixing metals that cause galvanic corrosion.
• Fixings that allow for thermal movement and resist wind.
• Bonds to roof plant, handrails, flues, pipework and tanks.
• Test joints labelled and accessible.
• Earth electrodes installed, tested and recorded before backfilling where possible.
• LPS bonded to the main earthing terminal.
• SPDs installed at each zone boundary, with status indicators visible.
• As-built drawings and test results handed over.

Testing earthing and lightning protection

The usual method for measuring an earth electrode is the fall-of-potential test. Current is passed between the electrode under test and a remote current probe, and voltage is measured with a second probe placed on a straight line between them. Metrel’s application note explains the “62% rule”: in uniform soil, placing the voltage probe at about 62% of the distance to the current probe gives the true resistance of the electrode. The current probe must be far enough away, and the result checked by moving the voltage probe slightly either side.

Other checks include continuity of conductors and bonds, condition of fixings and joints, and confirmation that SPDs are healthy.

Inspection intervals

IEC 62305-3 sets maximum periods between inspections according to the class of LPS:

Structures with explosive risk need more frequent attention: the standard’s notes call for visual inspection every six months and electrical testing every year. In addition, inspect after any known strike and after building work on the roof or structure. Contractors fixing new plant to roofs often disconnect conductors and forget to reconnect them.

FAQ

Can I use the same earth for lightning protection and power?

The systems are designed separately but must be bonded together at the main earthing terminal. Separate, unbonded earths can create dangerous voltage differences during a strike.

Is 10 ohms a legal requirement?

Under BS EN 62305 it is the recommended target for the overall earthing system and is mandatory only for structures with explosive risk. It remains good practice for most industrial sites.

Why do drives and PLCs fail during storms even when the building has lightning rods?

Air terminals protect the structure, not the electronics. Surges enter through power, data and instrument cables. Coordinated SPDs and good bonding are needed to protect equipment.

How often should we test the earth?

Follow the IEC 62305-3 intervals for your LPS class, and test after strikes or building alterations. Testing in different seasons shows how soil moisture affects the result.

Talk to Maven about earthing and lightning protection

Maven Technical Services installs electrical systems, steel structures, silos and tanks for industrial clients, and can help you plan earthing and lightning protection that fits the way your plant is built and run. To discuss a risk assessment, an upgrade or a new installation, speak to Maven’s engineers.