WHAT IS type 2 coordination chart?

Answer 1

To understand various considerations for proper co-ordination, time-current characteristics of thermal overload relay (curve B), H.R.C. fus (curve C), MCCB with only instantaneous release (curve D) and MPCB (curve E) are superimposed on motor starting characteristics (curve A) in Fig. 3. Intersection of characteristics

of thermal overload relay and Fuse / MCCB is termed as 'cross-over point' and corresponding current as 'cross-over current' lco.

Following points are to be ensured while selecting components to have properly co-ordinated motor protection:

lContactor rating (AC-3) should be more than or equal to motor full load current (if application is AC-3 duty)

Thermal overload relay of appropriate 'Trip Class' is selected. Time current characteristics of the relay should remain above motor starting characteristics as shown in Fig. 3.

For fault currents lower than 'cross-over current lco', relay will respond faster than SCPD and hence contactor will interrupt the fault current. Fault currents higher than lco will be interrupted by SCPD. Hence,

rating of contactor is so chosen that lco is less than rated breaking capacity of the contactor

Relay and contactor should be able to withstand lco for a duration equal to trip time of the relay. IEC / BS / EN standards require that the contactor should be able to withstand at least current equal to 8 times AC-3 rating (6 times for ratings higher than 630A) for 10 seconds

While using MCCB or MPCB, attention needs to be given to motor peak starting current. To avoid operation of MCCB / MPCB during starting, instantaneous release is chosen as 12 times the full load current of the motor. This thumb rule assumes motor starting current equal to 6 times full load current

In case of high efficiency motors, starting currents could be about 8 times full load current. For such

application, MCCB rating need to be selected such that instantaneous release setting is higher than 12 (about 14) times full load current to avoid nuisance tripping during starting.

Similarly, while using MCCB as a SCPD for Star-Delta starter, consideration needs to be given to peak current associated with change over from Star to delta. Instantaneous release of MPCB is normally set at 12 times the rating. Hence, possibility of nuisance tripping needs to be considered while using MPCB for protection of high efficiency motors or for star delta starter.