Answer / atul naik

Its not 100% true. Actually Motor has been protected many
a times before it actually burns. But either we have
ignored the actual reason of tripping by resetting OL
Relay or by removing the Over Load on Motor.
Still Motor Burns. So it is observed that following reasons
are there. 1 Dips in Supply Voltage, 2 Frequent Over Loads
reduces the performance/ sensitivity of OL Relay 3 Short
Ckt Protection is Not there (ANSI 50 & 50N )4 Short Ckt
Protection is Not Fast or delayed ( to avoid Nusence
tripping in Starting )5 Impairing of an Insulation due to
entry of unwanted gases,liquids etc,Reducing the Insulation
Strength of the Motor.,6 Wornout bearings, it will not only
oppose the motion and create resistance but also it
increases the heat / temp. 7 In case of CT Operated
Relays , CTs may reduce senstivity. Now, the cumulative
effect is that Fault Clearing Time is More than the Temp
rise time due to Fault , due to which Motor insulation
fails and motor winding no longer remain healthy ie Motor
burns.

why ct is testing using dc.. y cant we use AC insted of dc on POLARITY TEST.

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