Protection and Discrimination

Speed limits I²t damage and keeps the system stable. Selectivity normally means waiting to see whether a downstream device acts first — which is exactly what speed forbids.

The two design virtues are stated as dependability (never fail to operate) and security (never operate when you should not). They also trade against each other, and different applications weight them differently.

Time grading gives each upstream device a longer delay. It is simple and completely reliable, and it puts the longest delay at the source end where fault current is largest — correct discrimination achieved in the worst possible way.

IDMT relays operate faster the larger the current, so a close-up fault is also cleared sooner. Two settings — pick-up current and a time multiplier that shifts the whole curve — give discrimination without punishing severe faults with long delays.

A relay measuring V/I sees an impedance proportional to the distance to the fault, so it grades by position rather than by waiting.

Zone 1 stops at 80% because measurement error must never let it overreach into the next line. The remaining 20% is cleared by the far end's zone 1, or instantaneously by both if a communication channel is available.

Compare the current entering a zone with the current leaving it. Equal means healthy; a difference means current is escaping inside the zone. It is instantaneous and perfectly selective — a fault outside the zone produces no difference at all — which resolves the central conflict completely for the zone it covers.

• Ideal for transformers, generators and busbars, where both current transformers are metres apart.
• For a transmission line it needs a communication channel; fibre has made line differential routine.
• CT saturation during heavy external faults creates a false difference, handled with a percentage-bias characteristic.
• Transformer differential must also block on magnetising inrush, which looks like an internal fault and is not.

Relays, breakers and trip supplies all fail, so every scheme has a backup sharing as little as possible with the primary — different manufacturer, separate battery, separate CT core.

Breaker-fail protection is the last line: if current still flows after a trip command, it opens every breaker around the stuck one, deliberately disconnecting healthy circuits. A stuck breaker is rare and its consequence unbounded, which justifies a dedicated scheme.

Roughly 80% of overhead faults are transient — a bird, a branch, a lightning flashover — and clear once the current stops. Auto-reclose waits a dead time long enough for the arc to deionise and short enough to preserve stability, typically 0.3 to 1 second, then recloses.

On a permanent fault it trips again and locks out. It is never used on underground cable, where every fault is permanent and reclosing simply causes a second, larger failure.