Direct started asynchronous MV motor switching by mv circuit breaker characters acc to IEC

• high current(I =6…7 × motor rated current).
• low power factor(cosϕ≈0,1…0,2).
• infrequent operation.
• due to the higher interrupted current the choppingcurrent can higher values.
• the arcing time for some switching devices may be longer than when the motor is running.
• overvoltages when breaking the starting current.

are usually higher than when breaking no-load or load-current

• the no-load current of a small HV motor may be lower than the chopping current level of the circuit breaker.
• the power-frequency recovery-voltage is very small compared to the case of breaking the starting current.
• re-ignitions are not likely as the recovery voltage is low.
• low powerfactor(cosϕ≈0,1).

• high power factor(cosϕ=0,7 – 0,9).
• power-frequency recovery voltage is low.
• low overvoltages, no re-ignitions.

With regard to breaking capability and overvoltage generation, it is clear that the breaking of starting current is the most severe case.
Motor switching is analogous to the non-effectively earthed shunt reactor switching case.
The motor switching equivalent circuit is shown in Figure.