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ATS48 Type 2 coordination up to 250kW we can choose the type of breaker according SCCR, but 315KW and above the breaker SCCR is fixed at 150KA? if feeder SCCR is less, can we use less SCCR Breaker?

Published date: 28 November 2018

Yes, it is correct, till 250KW power rating, we have the option to select the beaker type N(50KA), H(70KA), L(150KA). But for higher power ratings 315KW and above, the circuit breaker type "L(150KA) with Micrologic 5.0E" is only allowed. 

It is understand that the question is: why it is not allowed to use lesser short circuit current rating breaker, if the upstream supply feeder short circuit current rating is less than 150KA?
Type 2 Coordination not only ensures that there is no danger user, but it also complies there is "No-Damage" to the controller from downstream faults. It means, it is not only the supply feeder's short circuit current rating, it is also considered the short circuit current from the motor during fault condition.

IEC 60947-4-1 defines "Under short circuit conditions, the controller shall not result in danger to persons or the installation, and after the short circuit test the controller shall be suitable for further use..." The only allowable damage under Type 2 Coordination is light contact welding provided that the contacts are easily separable".

To qualify for Type 2 Coordination (No-Damage Protection) the motor starter manufacturers must test all combinations of starters, contactors, overload relay and short circuit protective devices (SCPD). The tests include:
• A low-level short circuit test to confirm proper operation of the SCPD(MCCB, MPCB etc..).
• A high-level short circuit test to check for No Damage protection of the contactor and overload relay etc...
• A Dielectric Voltage-Withstand Test to prove the adequacy of the insulation after both short circuit current tests have been performed.
• An Overload Relay Calibration Test to verify that the overload relay conforms to published tripping characteristics.
• A discrimination test to ensure proper coordination between the overload relay and the MCCB/Fuse.
Achieving "No-Damage" short circuit protection of a motor starter requires that the overcurrent protective device limit both the peak current and the I2t let - through energy. Type 2 coordination is typically obtained with tests at 100,000A with every current limiting devices.

The combinations given in the above attachment are calculated to comply the above standard and passed through above testes and certified for type 2 coordination. Hence the breaker rating given in the chart only should be used to comply the type 2 coordination, other combination will not confirm the type 2 coordination.

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