This is the safety document for European protection equipment, and it is used by manufacturers during design and by certification bodies during assessment, in both cases alongside the insulation coordination part of the same series. Its distinguishing feature is that it treats a protection relay as an interface rather than as a simple appliance: the device is wired permanently into the power system at one end and into the station auxiliary supply and communication network at the other, so the safety requirement is not only that accessible parts be safe but that a fault on one side must not propagate to the other. The document therefore governs separation between circuits as carefully as it governs external insulation, and it adds the requirements for enclosure strength, temperature rise and marking. The part most often examined during a factory audit is the list of routine tests that must be applied to every unit produced, since a type test alone does not discharge the manufacturer's obligations under the directive.