User manual 09/2009 - Schneider Electric

The Protection FunctionDiscrimination of Overload ProtectionFor overload protection, the discrimination rules between electronic trip units are as follows:1. Current discrimination:• A ratio of 1.3 between the Ir pick-up for long time protection of the trip unit on the upstream circuitbreaker Q1 and that of the trip unit on the downstream circuit breaker Q2 is usually sufficient.• The tr time delay for long time protection of the trip unit on the upstream circuit breaker Q1 isidentical or higher than that of the trip unit on the downstream circuit breaker Q2.2. Time discrimination:• A ratio of 1.5 between the Isd pick-up for short time protection of the trip unit on the upstream circuitbreaker Q1 and that of the trip unit on the downstream circuit breaker Q2 is usually sufficient.• The tsd time delay for short time protection of the trip unit on the upstream circuit breaker Q1 ishigher than that of the trip unit on the downstream circuit breaker Q2.• If the upstream circuit breaker is in position I 2 t OFF, the downstream circuit breakers must not bein position I 2 t ON.3. Energy discrimination:• Energy discrimination is provided by the circuit breaker design and build characteristics. Thediscrimination limit can only be guaranteed by the manufacturer.• For circuit breakers in the Compact NSX range, a ratio of 2.5 between the upstream circuit breakerQ1 and that of the downstream circuit breaker Q2 guarantees total discrimination.Ground Fault Protection DiscriminationFor ground fault protection, only the rules for time discrimination should be applied to the protection Igpick-up and tg time delay:• A ratio of 1.3 between the Ig pick-up for ground fault protection of the trip unit on the upstream circuitbreaker Q1 and that of the trip unit on the downstream circuit breaker Q2 is usually sufficient.• The tg time delay for ground fault protection of the trip unit on the upstream circuit breaker Q1 is higherthan that of the trip unit on the downstream circuit breaker Q2.• If the upstream circuit breaker is in position I 2 t OFF, the downstream circuit breakers must not be inposition I 2 t ON.Discrimination LimitDepending on the staging of circuit breaker ratings and protection parameter settings, discrimination canbe:• Limited (partial discrimination) up to a value Is of the short-circuit current• Total (total discrimination), performed irrespective of the value of the short-circuit currentDiscrimination TableSchneider Electric provides discrimination tables showing the type of discrimination (partial or total)between each circuit breaker for its entire range of circuit breakers (see the Compact NSX 100-630 A -Catalogue). These coordinations are tested in accordance with the recommendations of standard IEC60947-2.38 LV434104 09/2009