How the protection is meant to work

A blade's lightning protection system, designed to IEC 61400-24, gives the strike a path: receptors on the surface near the tip and along the blade, connected to a down conductor that runs inside the blade to the root, then through the hub and nacelle to earth. When it works, the current follows the conductor and the laminate is untouched. When it does not, the current finds its own path through or along the laminate, and the blade is damaged.

The system fails in three ways: the strike attaches away from a receptor, a connection in the conductor has degraded so the current arcs across it, or the conductor itself is damaged. The first shows on the outside. The second and third show on the inside.

Where damage shows inside the blade

Receptor connections. The joint between the receptor block and the conductor is the most loaded point in the system. Arcing across a loose or corroded connection burns the surrounding laminate and leaves a blackened or discoloured area, sometimes with a smell that lingers for weeks. From the inside, look for discolouration around the receptor block and for any sign the block has moved.

The down conductor. A copper cable or braid running along the inside of the blade, usually fixed to the web or the shell at intervals. Damage shows as melted or broken strands, burn marks at the fixings, and a conductor that has come free of its fixings and is lying against the laminate. A conductor that is loose will chafe against the shell every revolution and can wear through both itself and the laminate.

Laminate around the conductor. Where current has left the conductor and passed through the laminate, the resin is charred or whitened and the plies may be delaminated or blown apart. Damage to the spar cap in this way is structural and urgent.

Puncture paths. A strike that attached away from a receptor punctures the shell, and the puncture is a path for water. Inside, look for the exit point of the puncture and for moisture staining running from it. Carbon spar caps deserve extra attention: carbon is conductive, and a strike that reaches the spar cap can damage it internally with little to see on the surface.

Tip damage and water. The tip takes most strikes. Delamination and cracking at the tip open the blade to water, which collects at the tip and adds mass. Standing water at the tip is one of the most common things an internal recording finds after a lightning season.

Moisture and corrosion at joints. Salt air and moisture corrode conductor connections and raise their resistance, which is how a connection that passed its last continuity test becomes an arcing point at the next strike. Offshore, this is the dominant failure mode. See Offshore wind blade inspections.

Inspecting after a strike

A strike is usually known from a lightning detection system, a strike counter on the turbine, or a fault in the control system. The inspection then has two parts.

Electrical. A continuity or resistance measurement from the receptors to the root tells you whether the conductor path is intact and whether any connection has gone high-resistance. It is a pass or fail on the whole system and says nothing about where the problem is or what else was damaged.

Visual, inside and out. The outside inspection covers the receptors, the attachment point and any puncture or surface damage. The inside inspection covers the conductor route from the root to as far as can be reached, every fixing and connection along it, the laminate around each, and the tip for water. For a technician from the root this ends a short way past the root, and the conductor beyond that is not seen. A crawler follows the conductor along the blade: Detectra records the inside in 8K 360° up to 80 % of the blade length, with the distance from the root on every take, so a burn mark or a loose fixing has a position and can be sent to a repair technician as a location rather than a description.

What to record

Lightning damage is the case where the record matters most, because the same blade may be struck again and because insurance and warranty positions depend on when the damage occurred:

  • The date and, where known, the strike event that triggered the inspection.
  • Every finding along the conductor with its distance from the root.
  • The condition of each receptor connection reachable from the inside.
  • The presence and depth of water at the tip.
  • The continuity test result alongside the visual findings, so the two can be read together.

In Fenestra, each finding is flagged on the frame it was seen on with its distance readout, and the history of the blade shows whether a fixing that was loose last year is the one that arced this year. From inspection findings to asset decisions sets out why that history is what an owner needs.

Frequently asked questions

What does lightning damage look like inside a wind turbine blade?

Burnt or discoloured laminate around receptor blocks and conductor fixings, melted or broken conductor strands, a conductor that has come free and lies against the shell, charred or delaminated laminate where current passed through it, puncture exit points with moisture staining, and standing water at the tip.

Is a continuity test enough after a lightning strike?

No. A continuity or resistance test tells you whether the conductor path is intact and whether a connection has gone high-resistance. It does not tell you where the damage is, whether the laminate around the conductor was affected, or whether the strike opened the shell to water. A visual inspection inside the blade answers those.

How far along the conductor can an inspection reach?

A technician from the root sees a short way past the root. A crawler such as Detectra follows the conductor route up to 80 % of the blade length and records it in 8K 360° with a distance reference on every take.