Coordination between the upstream and downstream devices, especially selectivity, is essential to optimize continuity of service. The large number of options for setting the protection functions on MicroLogic X control units improves the natural coordination between circuit breakers.
Three selectivity techniques can be used:
oCurrent selectivity, which corresponds to staging of the long-time overcurrent protection threshold.
oTime selectivity, which corresponds to staging of the short-time overcurrent protection threshold.
oEnergy selectivity, which corresponds to staging of the circuit breaker energy levels: this applies for high intensity short-circuit currents.
The selectivity rules depend on:
oThe type of control unit on the circuit breakers installed upstream and downstream: electronic or thermal-magnetic.
oThe accuracy of the settings.
Selectivity of Overcurrent Protection
For overcurrent protection, the selectivity rules between electronic control units are as follows:
oCurrent and time selectivity:
oA ratio of Ir Q1/Ir Q2 greater than or equal to 1.3 is sufficient between the Ir threshold for long-time protection of the control unit on the upstream circuit breaker Q1 and that of the control unit on the downstream circuit breaker Q2.
oThe tr time delay for long-time protection of the control unit on the upstream circuit breaker Q1 is identical or greater than that of the control unit on the downstream circuit breaker Q2.
oA ratio of 1.5 is sufficient between the Isd threshold for short-time protection of the control unit on the upstream circuit breaker Q1 and that of the control unit on the downstream circuit breaker Q2.
oThe tsd time delay for short-time protection of the control unit on the upstream circuit breaker Q1 is greater than that of the control unit on the downstream circuit breaker Q2.
oIf the upstream circuit breaker is in the I2t off position, the downstream circuit breakers must not be in the I2t on position.
oEnergy selectivity is provided by the circuit breaker design and build characteristics. The selectivity limit can only be specified by the manufacturer.
Ground-Fault Protection Selectivity
For ground-fault protection, only the rules for time selectivity should be applied to the Ig protection threshold and tg time delay:
oA ratio of 1.3 is sufficient between the Ig threshold for ground-fault protection of the control unit on the upstream circuit breaker Q1 and that of the control unit on the downstream circuit breaker Q2.
oThe tg time delay for ground-fault protection of the control unit on the upstream circuit breaker Q1 is greater than that of the control unit on the downstream circuit breaker Q2.
oIf the upstream circuit breaker is in theI2t off position, the downstream circuit breakers must not be in the I2t on position.
Depending on the staging of circuit breaker ratings and protection parameter settings, selectivity can be:
oLimited (partial selectivity) up to a value lower than the maximum expected short-circuit current.
oTotal (total selectivity), performed irrespective of the value of the short-circuit current.
Schneider Electric provides selectivity tables showing the type of selectivity (partial or total) between each circuit breaker for its entire range of circuit breakers. For more information, refer to LVPED318033EN Complementary Technical Information.
Use the I2t inverse time curve function to improve circuit breaker coordination. Use it when a protection device using inverse time only is installed upstream or downstream, for example a fuse protection device.
I2t OFF I2t ON
