Question

(e) Complete the following:

1. Desired relay operating time = TMS × ....................

2. Saturation can be a feature of CTs and ....................

(f) State the two rules of successful time grading of relays.

Answer 1

1. Desired relay operating time = TMS * PSM

2.Saturation can be a feature of CTs and relays

TWO RULES OF SUCCESSFUL TIME GRADING OF RELAYS :

• relay timing error : All relays have errors in their timing compared to the ideal characteristics as defined in IEC 60255 . For a relay specified to IEC 60255a relay error index is quoted that determines the maximum timing error of the relay. The timing error must be taken into account when determining the grading margin .
• overshoot :When relay is de-energized operation may continue for a little longer until any stored energy has been dissipated . For example, an induction disc relay will have stored kinetic energy in the motion of disc ; static relay circuits may have energy stored in capacitors . Relay design is directed to minimizing and absorbing these energies , but some allowance is necessary