Difference Between Selectivity and Cascading in Electrical Design
Selectivity
Selectivity is defined as the ability of a protective system to isolate only the problematic portion of the network while keeping the remainder of the system operational. It minimize the impact of a fault by tripping only the nearest breaker or fuse.
Ex: If there is a short circuit in a motor feeder, just the feeder breaker must trip, not the main breaker.
Outcome: Increased supply dependability and continuity.
Types of Selectivity:
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Current selectivity
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Time selectivity
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Energy selectivity (fuses)
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Zone selectivity (for relays and breakers)
Cascading (Backup Protection and Series Rating)
Cascading is the coordination of two protective devices (usually upstream & downstream circuit breakers or fuses), with the upstream device supporting the downstream device’s breaking capacity.
The purpose is to allow the utilization of downstream devices with the lower short-circuit ratings while depending on the upstream device to clear high fault currents.
Ex: A small breaker with a 10 kA rating can be utilized downstream of a 50 kA main breaker, as the main breaker will limit fault current when severe short circuits occurs.
As a result, costs are reduced (smaller devices can be utilized), but the manufacturer must test and approve the pair as one.
Difference Between Selectivity and Cascading
Selectivity vs Cascading
| Aspect | Selectivity (Discrimination) | Cascading (Back-up Protection / Series Rating) |
|---|---|---|
| Definition | Ensures only the faulty circuit breaker/fuse trips, keeping the rest of the system running. | Coordination where the upstream breaker supports the breaking capacity of the downstream breaker. |
| Objective | Improve reliability and continuity of supply by isolating only the fault. | Allow use of downstream devices with lower short-circuit rating, reducing cost. |
| Focus | Who trips during a fault. | Who protects whom during a fault. |
| Fault Handling | The nearest protection device to the fault operates. | Upstream breaker helps interrupt high fault current that downstream breaker cannot handle alone. |
| Types | Current, Time, Energy, and Zone Selectivity. | Series-rated / Back-up protection tested by manufacturers. |
| Application | Critical in industrial plants, hospitals, data centers (where continuity is vital). | Useful in commercial/industrial boards where cost optimization is needed. |
| Result | Selective tripping → continuity of supply. | Cost saving → smaller devices can be used safely. |
| Example | A motor feeder trips without affecting the main breaker. | A 10 kA downstream breaker works safely with a 50 kA upstream breaker. |
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