How lightning damages a blade

A lightning current that enters the laminate rather than the conductor does its damage in three ways.

Heating. Current through a resistive path generates heat. Inside a composite, that vaporises moisture and resin, and the resulting pressure rise is what does most of the structural damage.

Pressure. The vapour has nowhere to go inside a closed blade cavity. It vents through the weakest path, which is why lightning damage so often appears as a split leading or trailing edge near the tip, a blown-off tip cap, or shells separated along a bond line. The signature is a burst from inside, not an impact from outside.

Arcing and conduction. Where the current jumps between a conductor and the structure, or runs along a carbon spar cap, it leaves local burning, punctures and delamination along its path. Carbon is conductive, which makes blades with carbon spar caps a different design problem from all-glass blades, and their protection systems are correspondingly more involved.

The most damaging cases are the ones where the attachment happens away from a receptor, because then the whole of the above happens inside the laminate.

What the protection system is meant to do

A blade lightning protection system has three parts: receptors or a conductive surface arrangement at the tip and sometimes along the blade, a down conductor running inside the blade to the root, and the bonding and transition at the root that carries the current into the hub and on to earth.

IEC 61400-24:2019 defines the lightning environment for wind turbines, the risk assessment for that environment, and the requirements for protecting blades, structure and electrical systems against direct and indirect effects, with test methods to validate compliance. It is the second edition, replacing the 2010 first edition, and it is the reference against which a system as built is judged (IEC 61400-24:2019).

The two questions in practice are whether the system captures the strike at the intended point, and whether the path from there to earth is continuous and of low enough impedance. Most failures in service are failures of the second.

Diagnosing a strike after the event

Establish that there was one. Lightning detection network records and the turbine's own alarm and SCADA record give a time. The strike log is often incomplete, and a blade attachment does not always produce an alarm, so the absence of a record is weak evidence.

Inspect the receptors. Melting, pitting, deep erosion of the receptor surface and burning of the surrounding laminate all indicate the receptor did its job, which is the good outcome. A receptor that is loose, missing or has lost its connection is the finding that explains damage elsewhere.

Look for attachment away from receptors. Puncture holes, soot, scorching, local delamination and radiating cracks anywhere other than at a receptor mean the system did not capture the strike. This is the critical distinction, because it converts a repair into a protection system question, and potentially a design or installation question across the fleet.

Measure down conductor continuity. Resistance from the receptor to the root, and across the root transition into the hub, measured rather than assumed. Values are compared against the manufacturer's stated limits. A high or open reading means the current had no path, and explains internal damage.

Inspect internally at the tip. Much lightning damage is inside. Delamination around the down conductor, burning at connections, separated bond lines and debris in the cavity are found with a crawler or by entry, not from outside.

Separate the strike from what was already there. A strike frequently opens a weakness that existed before it: a bond void, water in the laminate, a previous repair. Water is a particular factor, because a wet laminate conducts and flashes to steam. Establishing which came first is done from the fracture and burn surfaces, and it is usually the contested point when the claim is written.

Why lightning claims are disputed

Lightning is generally an insured peril, and a manufacturing or maintenance defect generally is not. That places the dispute exactly at the boundary this diagnosis has to establish.

The recurring arguments are whether the protection system was built and maintained as designed, whether the down conductor was continuous before the event, whether water ingress from an unrepaired defect created the conductive path, and whether repeated strikes on the same blade reflect a protection system that does not work rather than bad luck. A blade that has been struck three times in four years at the same site, where neighbours have not, is telling you something about the blade, not about the weather.

Continuity records from previous maintenance campaigns are the most valuable evidence in these disputes, and are the records least often kept. A fleet that measures and records down conductor resistance at every campaign has an answer available. A fleet that does not is arguing from inference.

What owners should be doing routinely

  • measure and record down conductor continuity at every scheduled campaign, and keep the series so that a trend exists
  • inspect receptors and record their condition, rather than noting only whether they are present
  • treat a logged strike as a trigger for inspection, including internally at the tip, not as an event to be noted
  • keep the strike log, the inspection record and the repair record in a form that can be read together against a blade serial number
  • check that repairs to the tip have restored the protection system and not just the laminate, which is a common omission

Frequently asked questions

How often are wind turbine blades struck by lightning?

Frequently enough that it is a design case rather than an exception, and the rate varies strongly with site, terrain, height and season. Coastal and elevated sites in Northern Europe see winter lightning with characteristics different from summer storms, which is one reason a site-specific risk assessment is part of the standard rather than a generic figure.

Can a blade be repaired after a lightning strike, or must it be replaced?

Most strikes produce damage that is repairable. Replacement becomes the answer where the spar cap or root is involved, where the structure has burst over a long length, or where repeated damage has accumulated. The decision follows a structural assessment, not the dramatic appearance of the damage.

Does a carbon spar cap make lightning damage worse?

It makes the protection problem different and more demanding, because carbon conducts and therefore has to be integrated into the protection concept rather than shielded from it. A blade with carbon spar caps is not inherently worse protected, but errors in that integration have more consequence.

Is the manufacturer's assessment enough after a strike?

It is necessary and rarely sufficient where a claim or a warranty position turns on it. The manufacturer holds the design data and has an interest in the outcome. Where the question is whether the protection system performed as designed, the owner or insurer should have their own expert present for the inspection and the testing.

How Apex Wind can help

We are an independent blade engineering consultancy in Denmark. We investigate lightning damage and establish the sequence of events through blade failure investigation and root cause analysis, including whether the protection system captured the strike and whether a pre-existing defect contributed. We act in disputes and claims as expert witnesses, including review of the other party's assessment. We sell no lightning protection products, repairs or monitoring systems and take no referral fees. If a strike has damaged a blade and the cause is contested, contact us.