Medium voltages and current values according to IEC Standards
Rated voltage values according to IEC 60071-1 (Insulation co-ordination – Part 1: Definitions, principles and rules)
(see Fig. B5)
Ur = rated voltage, rms value, kV. It is the maximum rms value of voltage that the equipment can withstand permanently. 24 kV rms for example.
Ud = rated power frequency withstand voltage, rms value, kV during 1 mn. It defines the level of rms over-voltages that the equipment may withstand during 1 minute. 50 kV rms for example.
Up = rated lightning impulse withstand voltage, peak value, kV. It defines the level of lightning over-voltages that the equipment may withstand. 125 kV peak for example.
Un = service voltage, rms value, kV. It is the voltage at which the MV utility distribution network is operated. For example, some networks are operated at Un 20 kV. In this case, switchgear of at least 24 kV rated voltage shall be installed.
Rated current values according to IEC 62271-1 (High-voltage switchgear and controlgear - Part 1: Common specifications)
Ir = rated normal current, rms value, A. It is the rms value of current that equipment may withstand permanently, without exceeding the temperature rise allowed in the standards. 630 A rms for example.
Ik = rated short-time withstand current Ik, rms value, kA. It is the rms value of the short circuit current that the equipment can carry during a specific time. It is defined in kA for generally 1 s, and sometimes 3 s. It is used to define the thermal withstand of the equipment 12 kA rms 1s for example.
Ip = rated peak withstand current, peak value, kA. It is the peak value of the short circuit current that the equipment may withstand. It is used to define the electrodynamic withstand of the equipment, 30 kA peak for example.
