Protective Relaying, Instrumentation, and Control
Device numbers and schemes
IEEE C37.2 numbers appear constantly: 50 (instantaneous overcurrent), 51 (AC time overcurrent), 87 (differential), 27/59 (under/overvoltage), 81 (frequency), 49 (thermal), 86 (lockout), 25 (synch check), 64 (ground fault). Differential protection (87T, 87B, 87G) compares currents entering and leaving a zone and is inherently selective; it does not provide backup for downstream faults.
CTs, ratios, and burdens
Relay settings are usually expressed in secondary amperes. Always convert: secondary current = primary current × (CT secondary rating / CT primary rating).
Worked example
A 1200:5 CT sees a 9600 A primary fault. Ratio = 240, so the relay receives 9600 / 240 = 40 A secondary. If the pickup is set at 4 A secondary, that is 4 × 240 = 960 A primary. Mixing primary and secondary amperes when reading a coordination study is the most common error in this domain.
Coordination
Time-current curves must show a coordination time interval - typically 0.3 s - between the downstream device's clearing time and the upstream device's pickup, accounting for breaker clearing plus relay overshoot and CT error. CT saturation, DC offset, and misapplied instantaneous elements can defeat an otherwise correct setting.
Instrumentation and control
Know 4–20 mA current loops (live zero allows broken-wire detection), two-wire versus four-wire transmitters, RTDs and thermocouples, loop power, and the difference between accuracy and resolution. In SCADA and PLC work, understand analog input scaling and the meaning of fail-safe state for a control output. Traps include assuming 0 mA represents zero process value and confusing setpoint with deadband.