A gas turbine engine has an electrical harness formed from a flexible printed circuit board which provides a plurality of spaced conductive tracks. The conductive tracks are grouped into plural layers of tracks, the track layers being spaced apart and extending substantially parallel to each other. The tracks of each track layer form respective circuits which all have impedances lying within a respective range for that track layer. The impedances ranges are non-overlapping. The highest voltage differentials between tracks produced during indirect lightning strikes will thus tend to be between tracks of different track layers rather than between tracks of the same track layer.