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As explained in my last post, the design of an electrical distribution system, including the protection devices chosen, contributes directly to ensuring safety and power availability. We saw how coordination of circuit breakers using ‘selectivity’ helps to minimize the impact of an overload, short circuit, or ground fault by making sure that only the downstream breaker closest to the fault trips. This helps preserve continuity of power for the rest of the facility, while making it easier for the facility team to locate the fault. These are important advantages for critical power facilities like hospitals and data centers, as well as any operations where power interruptions or outages can cause costly losses or customer dissatisfaction.
However, organizations also need to keep the cost of their electrical infrastructure under control. Fortunately, investment can be optimized with circuit breaker ‘cascading’, sometimes referred to as ‘backup protection’ or ‘series rating’. This can reduce the cost of downstream devices by up to 40%.
How does cascading work?
Cascading between two circuit breakers is achieved by using the tripping of the upstream circuit breaker A to help the downstream (load side) circuit breaker B to break the current. This allows the selection of downstream devices with lower breaking capacities, and therefore lower cost.
In other words, the upstream circuit breaker helps downstream devices break short circuit currents that are greater than the highest short circuit currents that they can break.
The upstream breaker does this by letting through a much lower current that all downstream devices can cope with.
So, let’s say the short-circuit current is projected to be 25 kA. You can still choose a lower-rated device of 10kA for the downstream device, for example. But this requires a combined upstream and downstream breaker current capacity that is higher than the downstream breaker on its own.
Choosing circuit breakers for cascading
But how to do you make sure you’re choosing the right circuit breaker models and ratings for safe and reliable operation? This requires the use of cascading tables that provide the reinforced breaking capacity for a downstream breaker according to the upstream breaker.
Schneider Electric provides an online tool that makes using cascading tables fast and easy. And our MasterPact, ComPact, and Acti9 series circuit breakers offer a limited number of frame sizes and models to this make this process even simpler. The upstream and downstream circuit breakers should be tested by the manufacturer to ensure that in combination they can withstand the full projected short circuit current.
Optimizing cost and continuity
By principle, cascading is in contradiction with selectivity. But can you get the cost benefits of cascading, plus the continuity advantages of selectivity? Yes, there are special tables called ‘selectivity enhanced by cascading’. This combination can work, especially with the energy selectivity technology implemented in Compact NSX circuit breakers, which can improve the breaking capacity of downstream circuit breakers while delivering high selectivity performance.
How does this work? Take the example of a downstream circuit breaker B, which sees a very high short-circuit current and trips very fast. At the same time, the upstream circuit breaker A sees a limited short-circuit current compared to its breaking capability, but this current induces a repulsion of its contacts which, in turn, causes an arcing voltage that increases the current limitation. But this arc energy is not high enough to trip the upstream circuit breaker, so it’s helping breaker B to trip, without tripping itself.
Finally, when selectivity is required, I recommend specifying either:
- Total Selectivity is required, and back-up is not allowed
- Total selectivity is required, if back-up is applied, enhanced selectivity up to maximum short-circuit current shall be checked
To learn more about circuit breaker coordination, we recommend you use EcoStruxure Power Design software to optimize your network design, calculate and size up your electrical installation components. Alternatively, you can use Electrical Calculations Tool, or download our Selectivity, Cascading and Coordination Guide.
What are your thoughts? Continue the discussion on the Power Availability Forum.
