The voltages and the overvoltages that stress the insulation shall be determined in amplitude,shape and duration.
For maximum value overvoltage of the various classes shall be:
- For the continuous power-frequency voltage: a power-frequency voltage with r.m.s. value equal to the highest voltage of the system, and with duration corresponding to the lifetime of the equipment.
- For the temporary overvoltage: a standard power-frequency short-duration voltage with an r.m.s. value equal to the assumed max of the temporary overvoltages divided by √2
- For the slow-front overvoltage: a standard switching impulse with peak value equal to the peak value of the assumed max of the slow-front overvoltages.
- For the fast-front overvoltage: a standard lightning impulse with peak value equal to the peak value of the assumed maximum of the fast-front overvoltages phase to earth.
- For the very-fast-front overvoltage: the characteristics for this class of overvoltage are specified in relevant standards.
- For the slow-front phase-to-phase overvoltage: a standard combined switching impulse with peak value equal to the peak value of the assumed maximum of the slow-front phase to phase overvoltages .