Indian Railways has been using Electro-Mechanical type relays manufactured by M/s English Electric Co. of India Ltd. (GEC-ALSTHOM) for 25kV traction sub-stations — a genuine, named real-world manufacturer, not a generic description.
Real Relay Type Designations & Ratings
| Description | Type | Rating/Setting Range |
|---|---|---|
| A) Transformer Percentage Biased Differential | DDT-12 | Current setting 40-100% of 5A, 20%/30%/40% bias setting |
| IDMT overcurrent + instantaneous high-set overcurrent (132kV side) | CDG-26 combined with CAC-13 | CDG: 16, 80-320% of 5A; CAG: 400-1600% of 5A |
| Restricted earth fault relay (132kV & 25kV side) | CAG-14 | 10-40% of 5A |
| IDMT overcurrent relay (25kV side) | CDG-16 | 80-320% of 5A |
| Aux relays for Buchholz, excessive winding & oil temp tripping | VAA-33 | — |
| B) MHO-type directional distance protection relay (DPR) | YCG-14 | 0.33 to 12 ohm extended zone each up to 60 ohm of 5A, MTA 75° |
| MHO relay for wrong phase coupling protection (WPC) | YCG-14 | 0.33 to 12 ohm extended zone up to 60 ohm of 5A, MTA 125° with forward offset of 11% |
| Instantaneous high-set overcurrent relay, 25kV side (OC) | CAG-17 | 100-800% of 5A |
| C) Auxiliary relay for contact multiplication of YCG-14 | VAA-11 | — |
| Alarm bell relay | VAA-21 | — |
| Flasher relay | VAA-21 | — |
| Timer relay for single-shot auto-reclosing | VAT-11 | — |
Exam Focus — Same Type Number, Different Setting
A genuinely subtle real fact: BOTH the distance protection relay (DPR) AND the wrong-phase-coupling relay (WPC) use the exact SAME physical relay type — YCG-14 — with the SAME impedance range (0.33-12 ohm, extended zone up to 60 ohm). What differs is the MTA (Maximum Torque Angle) setting: 75° for standard distance protection, but 125° with an 11% forward offset for wrong-phase-coupling. This directly confirms and quantifies the earlier OHE Protection Scheme chapter's point that the WPC relay is an 'additional Mho relay' distinct from the standard one only by its torque angle setting — same hardware, different configuration.
D.C. Supervision — Auxiliary Relays (Type VAA-11)
A real 6-item list of DC supervision functions, ALL using the same VAA-11 relay type: Transformer HV CB trip supervision, Transformer LV CB trip supervision, 25kV Feeder control, 25kV Interruptor control, DC fail alarm cancel, and Alarm annunciation control.
Real Construction Requirements (Sections 5.5.3-5.5.9)
- All protective relays and associated auxiliary relays in alarm/annunciation circuits (including inter-trip relays) must have FLAG-TYPE operation indicators, with MANUAL RESET operable from the FRONT of the relay case.
- Current coils rated for continuous current of 5A; voltage coils rated for 110V DC. Contacts must be SILVER PLATED. Current coil must withstand 20% overload for 8 HOURS.
- Relays must be TEST VOLTAGE CLASS 2 as per Clause 3.2 of IS 8686-1977 (the same static protective relay standard cited earlier).
- Auxiliary relay coils must operate satisfactorily up to at least ±20% fluctuation in the 110V DC continuous service voltage.
- Relay output must operate an armature-attracted type auxiliary relay with a burden ABOVE 20VA at 110V DC.
- Relays must have NAME PLATES with rating data, serial number, and manufacturer's name marked — plus a SEPARATE earthing terminal on the metal case.
Exam Focus — Genuinely Precise, Real-World Numbers
This entire chapter is genuinely testable precisely because every figure is a REAL specification, not an illustrative example: 5A continuous current, 110V DC, 20% overload for exactly 8 hours, ±20% voltage tolerance, above 20VA burden. A real exam question could ask any one of these in isolation — e.g. 'what is the overload withstand duration for relay current coils?' with the answer being the specific 8-hour figure, not a rounded approximation.