EN 50121-5 is Part 5 of the European railway electromagnetic compatibility series and specifies the emission and immunity requirements for fixed power supply installations and apparatus. The installations concerned are the traction substations and the feeder stations that convert and distribute the power supplied to the railway, together with the switchgear, transformers, converters and auxiliary apparatus they contain. These installations are unusual in that they are both a source and a victim of disturbance: a traction substation produces harmonic currents and transient overvoltages that are injected into the public supply network and into the railway system, and its own control and protection equipment must continue to operate while that is happening. The standard therefore sets emission limits at the interface with the public network and at the interface with the railway, and defines the immunity requirements for the apparatus with levels appropriate to an installation of this type, together with the reference conditions and the assessment criteria.