Real OHE Loop Impedance Values (at 70° Phase Angle)
For Mho relay calculation purposes (the distance protection relay covered in earlier chapters), loop impedance with earth return for the OHE is taken at these real, standardised values:
| OHE Configuration | Loop Impedance (ohm/km) |
|---|---|
| Single track OHE | 0.41 |
| Double track OHE | 0.24 |
| Single track OHE with RC (Return Conductor) | 0.70 |
| Double track OHE with RC | 0.43 |
Exam Focus — Adding RC INCREASES Impedance, Not Decreases It
A genuinely counter-intuitive real fact: adding a Return Conductor (RC) INCREASES the loop impedance per km — single track goes from 0.41 to 0.70 ohm/km (about 71% higher), and double track goes from 0.24 to 0.43 ohm/km (about 79% higher). This seems backwards if you assume 'more conductor = less resistance,' but the RC's real purpose is reducing INDUCTION effects on adjacent communication circuits (covered in the earlier Safety Precautions on Electrified Sections chapter) — not reducing loop impedance for protection calculations, which is a genuinely separate concern from what RC is actually optimising for.
Governing IS Specifications — The Real Table
| Component | IS Standard |
|---|---|
| Electrical relays for power system protection | IS:3231-1965 |
| Static protective relays | IS:8686-1977 |
| Electrical indicating instrument | IS:1248-1968 |
| Control cable | IS:694-1977 |
| Interposing current transformer | IS:2705(4)-1981 |
Memory Aid — Chronological Spread of Standards
These 5 governing standards span 16 years (1965 to 1981) — a genuinely wide chronological range for standards all cited together in one specification, reflecting how electrical protection standards were adopted/updated at different paces for different component types (electromechanical relays first in 1965, static/electronic relays later in 1977 as that technology matured).
Relay Construction & Mounting — Real Requirements
- Relays must be DRAW-OUT (plug-in) switchboard type, back-connected, suitable for semi-flush mounting, with dust-tight covers of DULL BLACK ENAMEL finish — a real, specific cosmetic/functional spec.
- Relays must have test plugs or separate test blocks for secondary injection tests (the same secondary injection test covered in the earlier Protective Relay Maintenance chapter).
- When a relay is DRAWN OUT, the CT secondaries' current terminals must AUTOMATICALLY SHORT — a real, critical safety feature preventing an open-circuited CT secondary (which is genuinely dangerous, capable of producing lethal voltages) whenever maintenance staff remove a relay.
Exam Focus — Self-Reset vs Hand-Reset, and Why
Since substations are normally UNATTENDED, relays are generally of SELF-RESET type — EXCEPT for exactly 2 real alarm types, which must be HAND-RESET: Low Oil Level alarm, and Winding Temperature Indicator alarm/trip. This isn't arbitrary — these two conditions represent genuine equipment-health concerns that need a HUMAN to actually verify and acknowledge before resuming normal operation, unlike routine transient faults that self-reset relays can safely clear on their own.