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IEC TS 61936-2: Power Installations Exceeding 1 kV AC — Safety Requirements and Engineering Design
IEC TS 61936-2:2015 is a key technical specification that defines safety requirements for power installations with rated voltages exceeding 1 kV AC. It applies to substations, power distribution systems, and overhead line installations, covering design, erection, and operation. This article breaks down the core safety principles, earthing strategies, and engineering best practices embedded in the standard.
📌 Standard Scope: IEC TS 61936-2 applies to all AC power installations above 1 kV, including industrial, commercial, and utility systems. It addresses protection against electric shock, thermal effects, and mechanical failure.
🔧 Key Safety Requirements and Design Principles
The standard establishes a comprehensive framework for ensuring personnel safety and equipment reliability in high-voltage environments. The key requirements are organized around several core principles:
- Clearance and Creepage Distances: Minimum distances between live conductors and between live parts and earth must be maintained to prevent flashover. These distances depend on the system voltage, altitude, and pollution level.
- Enclosure and Barrier Protection: All live parts must be either enclosed within metallic or insulated housings, or positioned behind barriers that prevent accidental contact. Access to HV compartments must be interlocked.
- Thermal and Dynamic Withstand: Equipment must be rated to withstand the thermal and mechanical stresses of fault currents, including short-circuit conditions. Busbars, conductors, and supporting structures are verified through calculation or type testing.
- Fire Prevention: Materials used in power installations must meet flame-retardant requirements. Segregation between HV and LV circuits reduces fire propagation risk.
| Nominal Voltage (kV) | Phase-to-Earth Clearance (mm) | Phase-to-Phase Clearance (mm) | Pollution Level |
|---|---|---|---|
| 12 | 120 | 150 | Light |
| 24 | 180 | 225 | Light |
| 36 | 250 | 320 | Medium |
| 52 | 350 | 450 | Medium |
| 72.5 | 480 | 630 | Heavy |
⚠️ Important Consideration: Altitude correction factors must be applied to clearance distances for installations above 1000 m. The dielectric strength of air decreases by approximately 1% per 100 m increase in altitude above sea level.
🌍 Earthing Systems and Fault Protection
Proper earthing is critical to the safe operation of HV power installations. IEC TS 61936-2 defines requirements for both protective earthing (to prevent hazardous touch voltages) and system earthing (to define the neutral point treatment).
The standard distinguishes three main earthing topologies:
- Solidly Earthed (TN/TT): The neutral point is directly connected to earth. This provides the lowest overvoltages but requires careful coordination of protective devices to ensure fault clearance.
- Impedance-Earthed: A resistor or reactor is inserted between neutral and earth. This limits fault current while still allowing detection. Common in industrial networks to reduce arc-flash energy.
- Isolated Neutral (IT): The neutral has no intentional connection to earth. A first fault does not cause a trip, but continuity of supply is maintained. This is used in mining, hospitals, and processes requiring uninterrupted power.
💡 Engineering Insight: In practice, the choice of earthing system significantly affects protection coordination. For example, impedance-earthing with a 400 A neutral-earthing resistor (NER) limits earth fault current to a predictable value, enabling sensitive earth fault protection while reducing arc flash incident energy — a critical design consideration in modern switchgear installations.
| Earthing Type | Fault Current Level | Overvoltage Level | Typical Application |
|---|---|---|---|
| Solidly Earthed (TN) | High (up to Ik”) | Low | Utility substations, public supply |
| Impedance-Earthed | Limited (100-1000 A) | Moderate | Industrial plants, mining |
| Isolated (IT) | Capacitive only | High | Hospitals, continuous process |
🏗️ Engineering Design Insights for HV Substations
From an engineering perspective, IEC TS 61936-2 provides the foundational safety framework that underpins the entire substation design process. Key practical considerations include:
Segregation and Accessibility
HV compartments must be segregated from LV and control compartments. The standard mandates that access to HV areas be restricted through locked doors, interlocked cubicles, or physical barriers. In practice, this means a 12 kV switchgear panel must have mechanical interlocks that prevent access to the circuit-breaker compartment unless the breaker is in the disconnected position.
Short-Circuit Withstand Verification
Every current-carrying component in the installation must be verified for both thermal and dynamic short-circuit withstand. For example, a 40 kA rated busbar system must be designed so that the electromagnetic forces do not cause conductor clashing or insulator damage. This is typically verified through type tests per IEC 61439 or through calculation methods specified in the standard.
Arc Flash Mitigation
While IEC TS 61936-2 focuses on design safety, arc flash mitigation has become an increasingly important consideration. Modern installations incorporate arc-resistant switchgear (IEC 62271-200), optical arc protection relays, and remote racking systems to reduce personnel risk during operation and maintenance.
🚨 Critical Safety Note: All HV installations must include five safety conditions before personnel access: (1) complete isolation, (2) verification of dead condition, (3) earthing and short-circuiting, (4) locking and tagging, and (5) authorization. These principles, aligned with IEC 61936-2, form the foundation of safe working procedures worldwide.
❓ Frequently Asked Questions
Q: What is the difference between IEC 61936-1 and IEC TS 61936-2?
A: IEC 61936-1 covers power installations up to 1 kV AC, while IEC TS 61936-2 specifically addresses installations exceeding 1 kV AC. Part 2 is a technical specification that extends the safety principles of Part 1 to high-voltage systems, considering the additional risks associated with higher energy levels and arc flash hazards.
Q: How does pollution level affect clearance distance requirements?
A: IEC TS 61936-2 classifies pollution levels from I (light) to IV (heavy). Higher pollution levels require larger clearance and creepage distances because surface contamination reduces insulation performance. For example, in coastal or industrial areas with heavy pollution (Level III-IV), the creepage distance may need to be doubled compared to indoor clean environments.
Q: Does the standard cover safety requirements for operating personnel?
A: Yes. The standard includes requirements for safe operation, including access control, warning signs, instruction plates, and interlocking systems. It also references IEC 61936-1 for operational safety procedures such as switching schedules, work permits, and earthing before access.
Q: What are the testing requirements for verifying compliance?
A: Compliance verification includes type tests (dielectric tests, temperature-rise tests, short-circuit withstand tests), routine tests (insulation resistance, HV tests on each manufactured unit), and on-site commissioning tests (earthing resistance measurement, insulation coordination verification).
