How is friction and sparking during power transfer from the overhead cable

The power supply to locomotive is from two overhead electric wires – the lower, called contact wire and the upper, called catenary.

The contact wire which is generally made of copper is hung from the catenary wire in such a way that its level is always horizontal. The collection of electric current is done by pantograph fixed on the roof of the locomotive. The pantograph is held by pneumatic pressure in such a way that it always exerts a vertical pressure against the contact wire. At the top of the pantograph a horizontal arm with a carbon strip is provided which is the current collecting arm.

An electric spark is produced when there is a gap in the electrical path. In this case, there is no gap between contact wire and pantograph current collecting arm. The contact wire and the catenary wire are kept under tension of 2,000 kg force.

The overhead wires are generally one km long. At the end of the section there is overlap in such a way that before the contact with previous wire is lost, the pantograph is already pressing the contact wire of the section ahead. However, due to some vertical kinks in the contact wire or due to oscillations in the locomotive on account of track defects, sparks can occur some times.

There is always friction between the contact wire and the pantograph as the pantograph is pressing the contact wire all the time. But it is minimised by the surface of the contact wire as well as that of current collecting arm of the pantograph. Further, the contact wire alignment is not kept straight but there is a zig zag of 100 mm from the centre of the track.

This ensures that the contact wire moves laterally in a width of 200 mm of the collecting arm. This is to ensure that there is no groove formation in the current collecting arm due to continuous movement of the current collecting arm under the contact wire. However, the carbon strip of the collecting arm needs frequent replacement and so does the contact wire.