Question

16. A 3-phase motor circuit is proteced against overcurrent by a Type C MCB. Which of the following would result in the maximum fault current for this circuit? A. Three-phase balanced short circuit at the MCB B. Three-phase balanced short circuit at the motor terminal C. Earth fault at the motor terminal D. Phase to phase short circuit at the MCB

Answer 1

Option A is correct ( three phase balance short circuit at MCB)

Fault current is limited by the impedance in series upto the fault that means lesser the impedance higher the fault current and three phase fault is more sevear than any other fault

3 phase Fault at mcb is very high compared to other option because of less impedance at the at the point of MCB

If fault is occur at motor terminal the fault current is limited by connecting cable impedance therefore it is less sevear than fault at mcb

If fault occurred at motor terminal to earth known as LG fault it less in magnitude compare to option A and B

If phase to phase fualt occurred at MCB it short circuit current is higher but less than option A and Option B current

Severity of fault as follows

Optiona : A > B >D >C