This part of the IEC 255 series specifies the 1 MHz burst disturbance test that is applied to measuring relays and protection equipment. The disturbance is a damped oscillatory burst at 1 MHz, chosen because it represents the repetitive transient activity found on the wiring of substations and industrial installations when switching devices, contactors and relay coils operate. The standard defines the generator characteristics, including the burst envelope, repetition rate and output impedance, together with the coupling network used to inject the disturbance into the ports of the equipment under test. Test severity levels are graded so that a manufacturer or a user can select the level appropriate to the installation. Because the coupling mode is common mode, the test simultaneously exercises auxiliary supply, binary input and communication ports. Assessment is based on whether the relay continues to perform its intended protection function during and after the disturbance, with any change of output state recorded in the test report.