Sag, sometimes known as pre-sag, is introduced on the contact wire to compensate for greater elasticity of the contact wire at midspan between two supports. This allows the pantograph to exert upward pressure on the contact wire without any resultant contact loss.

The amount of sag introduced into the contact wire is dependent on the system design range and wire properties.

Lineardraft doesn't currently account for any sag or pre-sag in the catenary system, but does account for sag in the auxiliary wire. Auxiliary wires are drawn as catenary curves taking into account tension and mass - they are not parabolic. Although the wire is of no volume, its profile is precise.

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