Protective relays

1. The 3 most imp. characteristic for any relay:

i.	Pickup Current value ii. Time Delay iii. Relay Coordination is ultimately determined by the type of Grounding used in Distribution Systems: a.	Isolated/ Unearthed Neutral b.	Impedance-earthed Neutral c.	Solidly (or low impedance) earthed Neutral- In this, the network is generally earthed on the Neutral of an i/coming delta/star tx.former. When this neutral is not accessible, the network is earthed either through and zig-zag tx.former or through a star/delta tx.former connected to the main busbar.

2. When a Fault occurs, Electrodynamic and Thermal stresses built up.

3.	For an Induction. motor, a starting current of 6 times the Rated Current(Full Load Current) and a Locked Rotor Resistance, R1 of 3 times the R0 (Running Resist.) causes I2 * r to be 6 X 6 X 3 or 108 times normal.

4.	All the elements of the universal relay are organized under the application of Feeder Motor Breaker

5.	Feeder application group: It provides 3 separate sets of  Instantaneous, Definite-Time & Inverse-Time elements.

Instantaneous: Their operating time does not depend upon the current level. It responds to Phase current and provides conventional Phase fault protection.

Definite-Time OR Independent Time relays: Their operating time depends on the current level. It responds to Negative-Sequence Current and provides Phase to Phase fault protection.

Application: 	Independent Time relays are used when the short-circuit level is very high OR when short circuit current is likely to vary widely at and given point (eg- when and the network is supplied from small generators whose short circuit decrement curve falls off rapidly.) Inverse-Time OR Dependent Time relays: It responds to Residual Current and provides conventional ground fault protection.

Application: Dependent Time relays are used when:

i.	The operation of the network includes high-level short-time overloads. ii. Magnetizing inrush currents at switch-on may be considered for several tenths of and second. iii. Relay operation must be coordinated with a large no. of fuses.

6.	Motor application group: It shares the Phase, Negative-Sequence, and Ground Instantaneous and Definite-Time elements to protect the motor of winding faults or faults in the connecting leads. Another element, responding to both Positive-Sequence & Negative-Sequence current, provides for Overload, Locked rotor, or unbalanced current conditions