NLC Electrical Grid 2 Practice Test

In protection coordination, what does selectivity mean?

Only the faulted zone trips. All zones trips on fault. No zone trips. Protection acts randomly.

Correct Answer:

Only the faulted zone trips.

Explanation:

Selectivity in protection coordination means the protective device closest to the fault operates to clear it, while devices farther upstream remain energized. This keeps only the faulted section isolated, preserving as much of the system as possible. In practice, relays are set with coordinated timing so the downstream zone trips first, and upstream protection backups only if the fault isn’t cleared or a downstream device fails. So, only the faulted zone trips. Options proposing all zones trip, no trip, or random action would cause unnecessary outages or fail to clear the fault, which is not how coordinated protection is designed to work.

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