AECTP 250 is a NATO Allied Environmental Conditions and Test Publication that defines the electrical and electromagnetic environmental conditions against which defence equipment is to be designed, rather than the tests by which compliance is shown. It belongs to a family of publications in which one document describes an environment and the others describe the test and verification methods appropriate to it, and the separation matters because a programme can then adopt the environment of a specific platform without necessarily accepting the test methods of the country that wrote them. The publication characterises the electrical conditions of the several classes of platform and installation, including the voltage and frequency tolerances of aircraft and vehicle power systems, the transient conditions that accompany switching operations, and the ambient electromagnetic field produced by radio transmitters, radar and the other high power sources present on a military platform. Those conditions then become the design targets for equipment.