What does 'selectivity' refer to in protection devices?

'Selectivity' in protection devices specifically refers to the ability of a protective device to coordinate its operation so that only the portion of the system experiencing a fault is disconnected, while leaving the rest of the system operating normally. This is crucial in power distribution networks because it helps to minimize outages and maintain service reliability by limiting the impact of faults to the smallest possible area.

When selectivity is effectively implemented, devices such as circuit breakers and fuses work in a hierarchical fashion — upstream devices will not trip unnecessarily when a downstream device identifies and isolates a fault. This coordination ensures that only the affected section is interrupted, allowing for quicker service restoration and reducing the chances of widespread outages.

For example, if a fault occurs in a particular feeder line, a properly coordinated protection scheme will ensure that only the breakers feeding that line will trip, thus maintaining power to unaffected areas.

The other aspects mentioned, such as the ability to withstand high voltages, the speed of response, and ease of maintenance, while important in the context of protection devices, do not accurately define 'selectivity.' Instead, selectivity is exclusively focused on the coordination of fault interruption within a system.