Grounding Regime for Power Supply and Distribution Systems in Mines – DGMS Guidelines & Safety MCQs

🧾 DETAILED NOTES

1️⃣ Introduction

Electrical power systems in mines and oilfields are exposed to hazardous and damp conditions, making proper grounding (earthing) regimes essential for safety. Grounding ensures that fault currents safely discharge to earth, preventing electric shocks, equipment damage, and fires. DGMS mandates specific grounding regimes depending on the mine type (belowground, opencast, or oilfield) to ensure maximum electrical safety.

2️⃣ DGMS & Legal Provisions

Regulation / Circular	Description
CMR 2017 – Regulation 130	Precautions relating to earthing and grounding.
DGMS Circular (Tech) 1 of 2010	Grounding regime to be adopted for power distribution systems in mines.
DGMS Circular (Tech) 2 of 2013	Guidelines for earthing in oilfields and hazardous zones.
IS 3043: 2018	Indian Standard Code of Practice for Earthing.
Indian Electricity Rules, 2003 – Rule 90	Earthing of mining installations.

3️⃣ Purpose of Grounding

Prevent electric shock to personnel.
Protect electrical equipment from overvoltage and faults.
Provide stable reference potential for control and communication circuits.
Facilitate operation of protective relays and circuit breakers.

4️⃣ Grounding Regime Classification

System	Neutral Connection	Earth Connection	Typical Application
TT System	Source neutral earthed	Equipment body separately earthed	Surface installations
TN System	Source and body share same earth	Common earth	Non-hazardous zones
IT System	Neutral isolated or earthed via impedance	Equipment earthed	Underground gassy mines, oilfields

5️⃣ DGMS-Recommended Grounding Regime

For Belowground Gassy Mines: → IT System with isolated neutral and sensitive earth leakage protection (≤30 mA).
For Opencast / Surface Mines: → TN-S or TT System based on substation design.
For Oilfields / Hazardous Areas: → IT System with insulation monitoring device (IMD).

6️⃣ Components of Grounding System

Earth Electrode: GI/Copper rod or plate buried 3 m deep.
Earth Conductors: Copper or GI strip connecting all metallic parts.
Main Earth Bus: Common bonding point for all earth connections.
Equipotential Bonding: Interconnection of all exposed metallic structures.
Earth Leakage Relay (ELR): Detects leakage >30 mA and trips supply.

7️⃣ Ground Resistance Standards

Location	Max. Earth Resistance	Test Frequency
Transformer Substation	≤ 2 ohms	6 months
Portable Electrical Equipment	≤ 4 ohms	Monthly
Oilfield Installation	≤ 1 ohm	Quarterly
Lightning Protection Earth	≤ 10 ohms	Half-yearly

8️⃣ Testing and Maintenance

Test earth resistance before monsoon and every 6 months.
Record readings in the Earthing Test Register.
Ensure all joints are tight, corrosion-proof, and labeled.
Maintain separate earthing for lightning, power, and signal circuits.
Replace damaged or corroded earth conductors immediately.

⚙️ QUICK ONE-LINERS (Revision Points)

Reg. 130 (CMR 2017): Precautions relating to earthing.
DGMS Circular 1/2010: Grounding system for mines.
IS 3043:2018: Earthing standard.
IT System: Used in underground gassy mines.
TN/TT Systems: Used in surface areas.
Max Earth Resistance: 2–4 ohms (power systems).
ELR Trip Limit: 30 mA for portable apparatus.
Tests: Half-yearly or before monsoon.
Two earth connections mandatory.
Ground resistance logbook must be maintained.

🧩 DESCRIPTIVE MODEL QUESTION

Q. Describe the grounding regime recommended by DGMS for power supply and distribution systems in mines.

Answer:
DGMS Circular (Tech) 1 of 2010 and Regulation 130 of the CMR 2017 specify the grounding regime. For underground gassy mines, the IT system is recommended, where the neutral is isolated (or earthed through high impedance) to reduce fault current and explosion risk. For surface mines, the TN-S or TT system is adopted. Every circuit must have two separate earth connections, and earth resistance must not exceed 2 ohms (for substations). Periodic testing (half-yearly) ensures compliance with IS 3043:2018 and DGMS safety standards.

🧮 25 DGMS-Based MCQs (5 Options Each)

Q1. Regulation for earthing and grounding in coal mines:

Solution: Regulation 130 of CMR 2017 specifically outlines the precautions relating to earthing.

Q2. DGMS Circular prescribing grounding regime:

Solution: DGMS Tech Circular 1 of 2010 provides specific guidelines for the grounding regime to be adopted in mines.

Q3. Standard for earthing design:

Solution: IS 3043 is the comprehensive Indian Standard "Code of Practice for Earthing" referenced by DGMS.

Q4. Recommended system for underground gassy mines:

Solution: The IT system (isolated neutral) is recommended for gassy mines to limit fault currents to a safe level, preventing ignition.

Q5. Neutral in IT system is:

Solution: The "I" in IT system stands for isolated (or high-impedance earthed) neutral.

Q6. Earth leakage protection for portable apparatus:

Solution: 30 mA is the standard sensitivity for ELRs protecting portable apparatus (hand tools) to prevent fatal electric shock.

Q7. Earth resistance for transformer substation:

Solution: A low resistance path is crucial. For major substations, it should not exceed 2 ohms.

Q8. Testing frequency of earthing in mines:

Solution: Earth pit resistance must be tested at least once every six months (half-yearly), especially before the monsoon.

Q9. Each circuit must have:

Solution: Regulations mandate two distinct and separate earth connections for redundancy and safety.

Q10. Grounding regime for oilfields:

Solution: Oilfields are hazardous areas (like gassy mines) where an IT system is preferred to limit fault currents.

Q11. Earth leakage relays detect leakage current above:

Solution: 30 mA is the standard sensitivity for personnel protection (e.g., portable apparatus). Higher settings (e.g., 100mA, 300mA) are used for equipment/fire protection.

Q12. Earthing conductors made of:

Solution: Copper or Galvanized Iron (GI) strips are used for their high conductivity and corrosion resistance.

Q13. DGMS recommends earthing conductor cross-section of (minimum):

Solution: While varying by application, 25 mm² (for copper) or 50 mm² (for GI) is a common minimum requirement for main earthing conductors.

Q14. Earth fault current in IT system is:

Solution: Because the neutral is not directly earthed, the first earth fault current is very low, limited by the system's capacitance or impedance.

Q15. Equipotential bonding prevents:

Solution: It connects all metallic parts, ensuring there is no dangerous voltage (potential difference) between them during a fault.

Q16. Earth resistance for oilfield installations:

Solution: Due to the high hazard, highly sensitive installations like oilfields often require a very low earth resistance, typically 1 ohm or less.

Q17. IS 3043 specifies:

Solution: IS 3043 is the Indian Standard "Code of Practice for Earthing".

Q18. Separate earthing required for:

Solution: It is mandatory to keep lightning protection earthing separate from the power system earthing to prevent surges.

Q19. DGMS testing interval for portable earthing:

Solution: Earthing for portable and transportable apparatus requires more frequent testing, typically monthly.

Q20. Earth resistance test equipment:

Solution: A dedicated Earth Resistance Tester (commonly known by the brand name Megger) is used.

Q21. Earth continuity conductor should be tested:

Solution: For portable machinery, the earth continuity conductor must be tested before the start of every shift.

Q22. Ground fault trip time not to exceed:

Solution: Protective devices must operate quickly to prevent injury. 0.5 seconds (500 ms) is a common maximum time limit for such relays.

Q23. Grounding improves:

Solution: Proper grounding is essential for all three: personnel safety, equipment protection, and stable system operation.

Q24. Earth electrode depth:

Solution: A typical depth for driven rods or buried plates is around 3 meters to reach stable, moist soil.

Q25. Earthing logbook maintained by:

Solution: The competent Electrical Engineer or Supervisor is responsible for conducting tests and maintaining the statutory earthing logbook.

🔗 INTERNAL LINKS

Related Topic	Read More On...
Electrical Safety	Intimation of Electrical Accidents in Mines
Electrical Training	DGMS Training Syllabus for Electrical Engineers
Lightning Protection	Lightning Protection System (LPS) in Mines
CMR 2017	CMR 2017 Part 2 - Working & Supervision Rules

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