Why do you sometimes see sparks on the catenaries?

This phenomenon is part of the normal functioning of the train, causing only very minor wear
to material and presenting no risk for passengers. How can it be explained?

Why do you sometimes see sparks on the catenaries?

The sparking from catenaries can be explained by the flexibility of the (spring loaded) pantograph and the catenary (wire), sometimes causing the pantograph to separate from the cable. When the two components are less than one centimetre apart, the current continues to flow, due to the very high voltage which prevents the air from acting as an insulator. The passage of the current leads to ionisation of the air, i.e. a modification of the electrical charge of the atoms present in the air, which are no longer electrically neutral. It is this reaction that creates a spark, known as an “electrical arc”.

More information - Train operation