Learn selectivity and coordination in LV systems (fuses & circuit breakers curves)

Selectivity example

If the emergency overcurrent protective devices are not selectively coordinated with the normal path overcurrent protective devices (OCPD), a fault in the emergency system can cause the OCPDs to cascade open thereby unnecessarily opening the normal path feeder OCPD or possibly main OCPD.

Figure 1 illustrates this scenario. If this occurs, all the vital loads are unnecessarily without power at least temporarily. Since the power is lost to the ATS normal lineside termination, the generator is signaled to start.

When the generator starts and the loads transfer to the alternate source, some vital loads will continue to be unnecessarily blacked out due to the emergency feeder OCPD’s lack of selective coordination (it is still open).

In addition, this action reduces the reliability of the system since there is some probability that the generator may not start or the transfer switch may not transfer.

Evaluate for the Worst Case Fault Current

In assessing whether the overcurrent protective devices are selectively coordinated in the circuit path for these vital loads, it is important that the available short-circuit current from the normal source be considered (see Figure 2).

Generally, the normal source can deliver much more short-circuit current than the emergency generators. This is required per 700.5(A) Capacity and Rating.

The emergency system equipment shall be suitable for the maximum available fault current at its terminals.

Full Range of Overcurrents

To comply, the overcurrent protective devices must selectively coordinate for the full range of overcurrents possible for the application: overloads and short-circuits which include ground faults, arcing faults and bolted faults.

IMPORTANT NOTE – It is not selective coordination if the fuses or circuit breakers are coordinated only for overloads and low level fault currents. The fuses or circuit breakers must also be selectively coordinated for the maximum short-circuit current available at each point of application.

High- and medium-level faults may not occur as frequently as overloads and very low-level faults, but they can and do occur. High- and medium-level faults will be more likely during fires, attacks on buildings, building failures or as systems age, or if proper maintenance is not regularly performed.

The words “optimized selective coordination,” “selectively coordinated for times greater than 0.1 second,” or other similar wording are merely attempts to not meet the selective coordination requirements.

And terms like “selective coordination where practicable” are unenforceable.