When designing an electrical distribution system, the use of circuit breakers contributes to ensuring safety and power availability. Yet the cost of the electrical infrastructure is also important to control. Selecting breakers for cascading circuit breaker protection, can reduce the cost of downstream devices by up to 40%. Here we consider the reasons for using cascading circuit breakers.
Under overload conditions, both selective and cascading circuit breakers systems are selective, with only the breaker nearest the fault tripping. The difference is under short circuit conditions.
Selectivity
Circuit breaker selectivity minimises the impact of electrical faults by ensuring that only the downstream breaker closest to the fault trips. This preserves continuity of power for the rest of the facility and makes it easier to locate the fault. Selectivity is critical for power facilities in hospitals and data centres, as well as any operations, but less so in other installations.
Cascading
Cascading, also called backup protection, or combined short-circuit protection, allows the upstream circuit breaker to act as a barrier against short-circuits. Unlike selective tripping, when a short circuit flows during cascaded protection both the upstream and downstream devices operate simultaneously. This enables the selection of downstream breakers with lower breaking capacities than the prospective short-circuit at their point of installation.
How do cascading circuit breakers work?
Cascading protection uses the tripping of the upstream circuit breaker to help the downstream circuit breaker to clear the fault. The upstream breaker does this by letting through a much lower fault current that all downstream breakers can cope with. Moreover, as the upstream current limiting circuit breaker limits the fault current throughout, cascading applies to all switchgear downstream. For example, if the system short-circuit current is 25 kA, you can choose a lower-rated device of 10kA for the downstream device.
It is important to use cascading selection tables provided by the manufacturer as they test these combinations. For this reason, cascading protection between breakers from different manufacturers are not normally available. Schneider Electric provides a free guide to makecascading tables fast and easy. IEC 60947-2 standard covers combinations of circuit breakers in cascading configurations.
Is selectivity possible for cascading circuit breakers?
With traditional circuit breakers, cascading between two devices generally results in the loss of selectivity. Yet, with ComPacT circuit breakers, the selectivity characteristics in the tables remain applicable and are in some cases even enhanced. This results from the unique current limiting effect of the ComPacT NSX Roto-active breaking technique.
For systems requiring selectivity, Schneider Electric recommends using EcoStruxure Power Design (EcoDial) software. It will optimise the network design and calculate and size up the electrical installation.
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