Protective Relay (Overcurrent / Differential / Distance)
Protection
The brains behind every big circuit breaker: relays watch CT/PT signals and decide within milliseconds whether a fault exists — then fire the breaker's trip coil. A full unit of every Switchgear & Protection syllabus, from induction-disc classics to numerical relays.
What it looks like
Protective Relay (Overcurrent / Differential / Distance)
A typical protection — exact shape, colour and markings vary between manufacturers.
Types & variants
Key specs
ANSI device number
—The universal shorthand: 50 instantaneous OC, 51 IDMT OC, 87 differential, 21 distance, 64 earth fault, 86 lockout. Panels and exams both speak this language.
Plug setting / pickup
× InCurrent at which the relay starts. Classic PSM (plug setting multiplier) = fault current ÷ pickup setting.
Time multiplier setting (TMS)
0.1–1.0Scales the IDMT curve for coordination — upstream relays get higher TMS so the nearest relay trips first.
Characteristic curve
—Normal inverse, very inverse, extremely inverse, definite time — chosen to grade with fuses and downstream relays.
CT/PT ratio inputs
A / VRelays read scaled signals: 1 A or 5 A CT secondaries, 110 V PT secondaries. Settings are entered in these terms.
Operating time
msNumerical relays decide in ~20–40 ms; the breaker adds 3–5 cycles — total fault clearing time drives all stability studies.
Markings
ANSI numbers on the panel front (e.g. '51N'), setting dials on electromechanical units (plug bridge + TMS dial), LCD + LEDs on numerical relays showing pickup/trip flags and fault records.
Standard values
IDMT normal inverse curve per IEC 60255: t = TMS × 0.14/(PSM^0.02 − 1). CT secondaries 1/5 A, PT 110 V. Numerical families: ABB REF/RET, Siemens 7SJ, SEL, Schneider MiCOM — all teach the same settings.
How to choose
1) Feeder protection: IDMT overcurrent + earth fault (51/51N) coordinated by grading margin (~0.3–0.4 s per stage). 2) Transformer: differential (87T) + Buchholz + winding temperature, with OC backup. 3) Long lines: distance (21) with 3-zone stepped reach. 4) Motors: thermal overload + stall + single-phasing (better done by a motor protection relay than a bimetal block). 5) New installs: numerical — one box does many ANSI functions plus fault recording.
Pinout & package
Draw-out or flush panel cases: CT inputs (star/delta per scheme), PT inputs, DC auxiliary supply (24–220 V), output contacts to the trip coil, and comm ports (RS485/IEC 61850) on numerical units. Buchholz mounts in the transformer's conservator pipe with alarm + trip float switches.
Example circuits
- 11 kV feeder: CTs → 51/51N numerical relay → VCB trip coil, graded with the upstream station relay
- Transformer differential: CTs on both sides compared; through-fault current balances, internal fault trips instantly
- Buchholz alarm stage warning on slow gassing, trip stage on oil surge
- Lab experiment: plotting an IDMT relay's time–PSM curve with an injection kit
Common failures
Wrong settings (the #1 'failure' — relay healthy, coordination wrong), open CT secondary (dangerous high voltage + relay blind), sticky discs/dirty contacts in electromechanical units, dead DC trip supply (breaker never trips — why trip-circuit supervision exists), and aged capacitors in static relays.
How to test
Secondary injection: a test kit feeds calibrated current/voltage into the relay and times its operation against the curve — done routinely on maintenance. Primary injection proves the whole CT-to-breaker chain. Always verify the trip actually opens the breaker, not just lights an LED.
Substitutes
Numerical relays replace electromechanical one-for-one at the panel (mind CT ratings and DC supply). Within a function, any brand implementing the same IEC curves and ANSI functions substitutes after a settings study. Never substitute protection with 'a bigger fuse'.