Protection Engineering Basics and Schemes Protection Engineering | Page 113

We used these relays for motor protection and
downstream protection.
In motors, at time of motor starting, motor draw more
current then normal current due to torque. We need to
introduce time delay at the starting of motor.
5.3.2 Inverse Time Relay(51)
These Relays are introduced on the principle of Time vs
fault intensity.
We follow simple rule here, if intensity of fault is high
then Relay tripping time will be low.
We called it Time current characteristics (TCC), TCC help
us in Relay coordination
We as per IEC have multiple TCCs
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Normal or Standard Inverse
Very inverse
Extreme Inverse
Long time earth
Customized