Selective coordination is critical for electrical distribution system reliability. A reliable system is not only important for life safety, it’s important from a business perspective as nothing will stop all activity, paralyze production, inconvenience and disconcert people more than a major power failure.
Selectively coordinated overcurrent protective devices address localizing faulted conditions on the power distribution system and is quite often a reliability design goal. In addition, the NEC mandates selectively coordinated OCPDs for circuits that supply power to vital loads in specific building system applications.
A properly engineered and installed system that’s selectively coordinated will allow only the nearest upstream OCPD to open for the full range of overcurrent’s (both overloads and all fault types), leaving the remainder of the system undisturbed and preserving continuity of service. Isolating the circuit’s faulted portion is important for overall system reliability.
Selective coordination isolates the circuit’s faulted portion and only the faulted portion. The OCPD closest to the fault is the only device to open to limit the impact on the balance of the system. To achieve selective coordination, the selection and installation of OCPDs and their ratings or settings are important. This must be addressed in any project’s design phase. Once switchboards, distribution panels, motor control centers, lighting panelboards and OCPDs are selected and installed, retroactively “fixing” a system that does not selectively coordinated can be expensive.
Selective Coordination Marking
- 62 — Selective Coordination for Elevator Circuits
- 27 — Critical Operations Data Systems
- 3(C) — Multi-building Campus-Style Complexes
- 32 — Emergency Systems
- 27 — Legally Required Standby Systems
- 54 — Critical Operations Power Systems