Answer / rajendran m

Single phase:
a)3 wire system
b)up to use minimum ratting electrical equipments
c)Do not use heavy capacity of motors

Three phase

a) 4 & 5 wire system
b) To be use maximum capacity of motors

Answer / prashant verma

THE MAIN DIFFERENCE BETWEEN 3 PHASE AND SINGLE PHASE SYSTEM IS THE AVG POWER IN TWO SYSTEM FOR 3 PHASE SYTEM THE AVG POWER IS NEVER ZERO BUT IN SINGLE PHASE SYSTEM IT BECOMES ZERO ONCE PER CYCLE SO ITS A CURSE FOR HIGH TORQUE MOTORS N ALSO ITS REQUIREMENT IS FOR SELF STARTING OF SUCH MOTORS.

My question is whether bellow formula is correct to find bus size for 46kA for 0.5 second & what is RT in bellow formula. formula to calculate the size of busbar to withstand the thermal stresses due to currents with duration of 0.2 sec to 5 sec. Sp = RT( I x I x t) / K Where Sp – is the cross sectional area of bus bar in millimeters. I -    is the value of AC fault current. t -    is the operating time of disconnecting device in seconds. K –  is the factor depending on the material of the conductor, insulation and initial and final temperature. Value of ‘k’ for copper is 143.

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