Relay Setting Calculation 2

[PoliPoliCarps]

Hello,

I have question regarding protection relay
1. How to select pick-up time IEC for 51 protection relay? Inverse Time Overcurrent Relay. Where they get t=0.06?

2. How to select pick-up time IEC for 50 protection relay? Definite Time Overcurrent Relay. Where they get Operating time for instantaneous current setting = 0.03 second?

Thank you,
I will really appreciate any response.

 

[Engineer1916]

OP,
You need to perform a coordination study to select the pickup values. Cant look at each relay in isolation, have to make sure it is coordinated with upstream and downstream devices. See attached document for recommended margins.

 

[RRaghunath]

It seems this is a motor feeder. The Instantaneous Overcurrent element is being set 2.5 times the motor starting (/ locked rotor) current and it seems fine.
IDMT overcurrent element is being set 2.8 times full load current which means the element will pickup every time the motor starts. To prevent this protection element tripping the motor, TMS needs to be chosen so that the operating time is more than the motor start time (at estimated minimum voltage at motor terminals). Apparently, this is not done and needs to be corrected.