Wind turbines should not rotate more than sixty times a minute to avoid provoking seizures, researchers from the Universities of Essex and Aston have found.

They were looking to identify what type of flicker from wind turbines could bring on seizures and as a result have come up with guidelines to minimise impact. Planning permission for wind farms often consider flicker, but current guidelines relate to annoyance and are based on physical or engineering considerations rather than the danger to people who may be photosensitive.

Arnold Wilkins from the University of Essex and Graham and Pamela Harding from the University of Aston calculated different shadow flicker effects on wind turbines to measure the seizure provoking possibilities.

Large wind turbines usually rotate at between 30 and 60 revolutions per minute, many are three-bladed and operate at constant speed, while some are two-bladed.

Turbines that rotate faster or have more blades will produce unacceptable levels of flicker. Smaller variable-speed turbines range between 30 and 300 revolutions a minute and some have more than three blades, so their flicker is within the range for which seizures are likely.

In addition, photosensitive people would need to be at least four kilometres away to significantly reduce the possibility of seizures, a distance approximately 100 times the height of an average turbine.

The researchers also found that on wind farms the shadows cast by one turbine on another should not be in public sight if the cumulative flash rate exceeds three per second and that turbine blades should not be reflective.