Protection Engineering Basics and Schemes Protection Engineering | Page 137

selected time current characteristics will be normal or standard inverse of IEC then,

TMS t = 0.14 ∗

I 0. 02 − 1 TMS

0.6 = 0.14 ∗

17.5 0. 02 − 1 TMS = 0. 25sec

Instantaneous Over current Relay setting( 50);

We will select 130 % of I shortcircuit as our instantaneous current.

= 130 % of I PSC = 130 % of 552.5A I ≫ = 718.25A As per CT ratio 200 / 1, relay will trip instantaneously when I ≫,

718. 25 I ≫ = = 3. 59125A ≅ 4A

200

1 Tripping nature of I ≫ is definite minimum time

( t≅ 0 sec)

Earth Fault Relay Setting( 51N):

Pick up earth = 40 % of I PFL = 40 % of 26.24 = 10.496A pg. 137