Discrimination (selectivity)
Arranging protection so only the device nearest a fault trips — not the whole board above it.
The idea
When a fault or overload happens, you want the protective device closest to it to operate, and everything upstream to stay in. That way a fault on one final circuit takes out only that circuit — not the sub-board, and certainly not the main switch feeding the whole installation.
Achieving that is discrimination (also called selectivity): choosing and grading the protective devices so their operating characteristics don't overlap in a way that lets an upstream device beat a downstream one to the trip.
How it's graded
Devices are graded on two axes: current (an upstream device is rated higher than the ones below it) and time (an upstream device is a little slower for a given fault level). For low-level overloads, the ratio of ratings usually gives discrimination. For high fault currents the fast magnetic trips can overlap, so time-grading or devices designed to coordinate are used.
The classic symptom of poor discrimination is an upstream breaker tripping for a fault that should have been cleared below it — a sub-main trips when a single circuit faults, or the main goes out for a downstream short. It's annoying domestically and serious on a site where one fault drops far more than it should.
Related faults
Sub-main keeps tripping the main board
A sub-board's incoming protective device (or the main feeding it) trips — taking out everything downstream — and you need to tell overload from a fault on the sub-main or sub-board.
MCB (circuit breaker) keeps tripping
A circuit breaker trips repeatedly — instantly on reset, or after a load runs for a while — and you need to tell a short from an overload from a faulty breaker.
Whole circuit / board nuisance tripping
A whole circuit or board protective device trips intermittently with no obvious single cause — affecting several loads — and you need a systematic way to corner it.
Related definitions
Circuit breaker (MCB)
Automatically disconnects a circuit on overload or short circuit, and can be reset rather than replaced.
Distribution & sub-mains
How power is split from the main board into final circuits and sub-boards, with protection at each level.
Earth-fault loop impedance
The path a fault current takes back to the source, and why its impedance decides whether protection trips in time.