Electronic Interlocking
Electronic Interlocking (EI) – Railway GK Capsule
1. What is Electronic Interlocking?
Electronic Interlocking (EI) is a microprocessor-based fail-safe signalling system that replaces traditional relay-based interlocking.
It continuously monitors the position of points, signals & track circuits and prevents any unsafe route setting through software logics certified to SIL-4 (Safety Integrity Level-4 – 10⁻⁹ probability of dangerous failure/hour).
2. Technical Architecture (Typical RDSO/CRIS Design)
| Sub-system | Hardware | Safety Standard | Redundancy |
|---|---|---|---|
| Vital Logic Controller | Triple Modular Redundant (TMR) CPUs | EN-50128 / EN-50129 | 2-out-of-3 voting |
| I/O Modules | Fail-safe FSE (Field-Side Electronics) | SIL-4 | Dual I/O cross-checked |
| Communication | Ethernet (UDP profile), RSSP-II safety protocol | IEC-61508 | Dual LAN, hot-stand-by |
| Power | 24 V DC / 60 V DC battery bank | — | 2×30 min autonomy (UPS) |
| HMI | LCD/LCP panel with soft switches | EN-50155 | Hot-stand-by MMI |
- Scan cycle time: ≤ 250 ms
- Reaction time (input→output): ≤ 500 ms
- Temperature range: –20 °C to +70 °C
- MTBF target: > 1 lakh hours
3. Key Indian Specifications
| Document | Clause | Salient Point |
|---|---|---|
| RDSO Spec: IRS:S-99/2017 | Rev-3 | Mandatory for all new Panel/Route-Relay interlocking since 01.04.2022 |
| IRS:S-74 | — | Interface protocol for point machines (single/ dual detection) |
| IRS:S-36 | — | Track circuit interface (Electronic TC proved in EI) |
| Amendment 2023 | — | Provision for ETCS Level-1/2 and T-12 axle counter direct connect |
Maximum yard size cleared: 8192 I/O points (2³² possible but limited by code)
Largest live installation: Mathura (NCR) – 967 routes, 1277 I/O (commissioned Mar-2023)
4. Historical Milestones
| Year | Event |
|---|---|
| 1986 | First solid-state interlocking (SSI) trial by British Rail – technology base for later EI |
| 1998 | RDSO issues draft “Computer Based Interlocking (CBI) Guidelines” |
| 2003 | Gooty (SCR) – First indigenous EI by BHEL-RDSO (80 routes) |
| 2008 | White-Labelling Policy – IR opens to private vendors (Alstom, Siemens, Kyosan, Ansaldo, Medha) |
| 2016 | Mission-Rashtriya – Decision to convert all CLIs to EI by 2025-26 |
| 2020 | “Make-in-India” – Indigenous TMR CPU card “Utkarsh” developed by CRIS & C-DAC |
| 2022 | Gazette notification – EI mandatory for all new/modified yards > 50 routes |
| 2023 | 950+ stations on Indian Railways fitted with EI (IRCA data, Dec-23) |
5. Benefits over Relay Interlocking
- ~40 % reduction in cabling (multiplexed bus)
- Software-based route cancellation – no mechanical lever locking
- Remote diagnostics & logging – failure pin-pointed to card-level through SNMP
- Easy expansion – add I/O modules without new relays
- Energy saving – 70 % less coil power consumption
- Integration ready – ACD, TCAS, ETCS, LSS, Smart yard IoT
6. Recent Updates (2023-24)
- Kavach compatibility: All EI being provided with SIL-4 safe input for Kavach RFID interrogator
- Satellite Health Monitoring: CRIS pilot at Jhansi – EI health packets uplinked to Bhuvan-IRIMS
- AI-based predictive maintenance: Nagpur division trial with i-EMR software (reduced failure by 27 %)
- Green EI: Solar-powered EI commissioned at Mhow (WR) in May-2023
- Production target 2023-24: 750 stations (Railway Board Green-Book)
7. Quick-Fire Data Points (Memory Hooks)
- “3-2-1” Rule – 3 CPUs, 2-out-of-3 voted, 1 unsafe failure not possible
- “250 ms” – One complete safety scan
- “SIL-4” – Same safety level as aviation fly-by-wire
- “RSSP-II” – Safety protocol borrowed from European ETCS
- “8192” – Maximum I/O currently type-tested
- “2025-26” – 100 % CLI-to-EI switch-over target
8. 15+ MCQs for Railway Exams
Q1. What is the Safety Integrity Level (SIL) achieved by Electronic Interlocking?
Answer: SIL-4Q2. The voting logic used in the vital CPU of EI is:
Answer: 2-out-of-3 (2oo3)Q3. Maximum I/O capacity currently type-approved under IRS:S-99 is:
Answer: 8192Q4. Which station holds the record for largest EI installation on IR as of 2023?
Answer: Mathura (967 routes)Q5. The mandatory cut-off date for using only EI in all new yards exceeding 50 routes is:
Answer: 01-April-2022Q6. The scan cycle time of EI should not exceed:
Answer: 250 msQ7. First indigenous EI on Indian Railways was commissioned at:
Answer: Gooty (2003)Q8. The safety communication protocol between EI and RTUs is:
Answer: RSSP-IIQ9. Which organisation developed the indigenous TMR CPU card “Utkarsh”?
Answer: CRIS & C-DACQ10. Electronic Interlocking reduces yard cabling by approximately:
Answer: 40 %Q11. As per IRS:S-99, the minimum UPS autonomy required for EI is:
Answer: 30 minutesQ12. Which of the following is NOT an advantage of EI over relay interlocking?
Answer: Increases relay room space (✓ it decreases space)Q13. The temperature range specified for EI outdoor equipment is:
Answer: –20 °C to +70 °CQ14. Which of the following signalling systems can be directly integrated with EI?
Answer: All – ACD, TCAS, ETCS, KavachQ15. Mission Rashtriya aims to convert all existing __________ interlocking to EI by 2025-26.
Answer: CLI (Colour-Light & Relay)9. One-Liner Revision Chart
- EI = Microprocessor + Software + 2oo3 Voting + SIL-4
- IRS:S-99/2017 → Bible for EI
- Mathura → Largest; Gooty → First
- Scan 250 ms, Reaction 500 ms
- 2025-26 → Full EI coverage target
Best of luck for your Railway exams – always keep the “2oo3” safety rule in mind!
BETA
