Why Earthing Is the Foundation of Switchgear Safety
Earthing (grounding) is the most fundamental safety system in any medium voltage installation. Every other safety feature — arc flash protection, interlocks, personal protective equipment — provides a secondary layer of defense. The earthing system is the primary barrier between fault energy and human life.
A properly designed earthing system ensures that: (1) fault currents are conducted safely to ground without creating dangerous voltages on accessible surfaces, (2) protection relays can detect earth faults quickly and reliably, (3) de-energized equipment remains safely at ground potential during maintenance, and (4) transient overvoltages (lightning, switching surges) are dissipated harmlessly to ground.
This guide covers the earthing aspects specific to medium voltage switchgear — from the earthing switches built into the switchgear to the main earth bus, substation ground grid, neutral grounding method, and portable earthing for safe maintenance.
Built-In Earthing Switches in Switchgear
Every medium voltage switchgear cubicle includes one or more earthing switches as integral components. These switches provide a positive connection between the de-energized circuit and the station earth bus, ensuring that the circuit remains at ground potential during maintenance.
Types of Earthing Switches
Cable-side earthing switch: Located between the circuit breaker and the cable termination. When closed, it earths the cable side of the circuit. This is the most commonly used earthing switch because it makes the cable compartment safe for cable work (termination, testing, replacement).
Bus-side earthing switch: Located between the circuit breaker and the bus bars. When closed, it earths the bus. This switch is used when maintaining the bus bar compartment or when the circuit breaker is withdrawn and the bus-side contacts need to be grounded for safety. In metal-clad switchgear, automatic shutters may serve a similar function by covering the bus-side contacts when the breaker is withdrawn.
Line earthing switch with making capacity (IEC 62271-102): A special earthing switch that has the ability to close onto an energized circuit — i.e., it can make fault current if the section that was supposed to be de-energized is accidentally still live. The making capacity is typically rated at 2.5x the rated short-time withstand current (peak value). This feature provides a safety backup: if the operating procedure fails and the earthing switch closes onto a live circuit, the switch does not weld or explode — it safely makes the fault current, and the upstream protection clears the fault.
Earthing Switch Interlocking
Mechanical and/or electrical interlocks enforce the correct operating sequence and prevent dangerous mis-operations:
- The earthing switch cannot be closed unless the circuit breaker is open and in the disconnected (test or withdrawn) position
- The circuit breaker cannot be closed while the earthing switch is closed
- The cable compartment door cannot be opened unless the earthing switch is closed (some designs allow door opening in the "isolated" position with safety warning)
- The earthing switch handle is padlockable in both the open and closed positions for lockout/tagout (LOTO)
In NAIJI Electric's ASN3-12 switchgear, the interlocking sequence is enforced by a combination of mechanical cam interlocks and key interlocks (Kirk key or Castell type), providing five-point interlock protection between the circuit breaker, isolator, earthing switch, cable compartment door, and padlock.
Neutral Grounding Methods for MV Systems
The method used to ground the transformer neutral has a profound effect on earth fault current magnitude, protection relay requirements, and switchgear earthing duty. The choice of neutral grounding method is a system-level design decision that directly affects switchgear specification.
| Method | Earth Fault Current | Pros | Cons | Typical Application |
|---|---|---|---|---|
| Solidly grounded | High (10-40 kA) | Simple, reliable fault detection | Severe arc flash, high equipment damage | Utility distribution (11-33 kV) |
| Low-resistance grounded (LRG) | Moderate (200-400 A) | Reduced arc flash, fast fault clearing, good selectivity | Requires sensitive earth fault relays | Industrial (6.6-11 kV) |
| High-resistance grounded (HRG) | Low (5-10 A) | Minimal arc flash, can run with single ground fault | Cannot clear double ground faults, requires insulation monitoring | Continuous process, mining |
| Ungrounded (isolated neutral) | Capacitive only (1-10 A) | No interruption on first ground fault | Transient overvoltages (up to 6x), very difficult fault location | Legacy systems (not recommended for new) |
| Resonant grounded (Petersen coil) | Near-zero (compensated) | Self-extinguishing arc faults | Complex, requires coil tuning | European utility networks |
Recommendation for industrial plants: Low-resistance grounding (LRG) with a neutral grounding resistor (NGR) limiting earth fault current to 200-400 A. This provides the best balance of safety (low arc flash energy), reliability (fast and selective fault clearing), and simplicity. The NGR is sized for the maximum single-phase-to-ground fault current and a rated time of 10 seconds (to allow for backup protection clearing).
Main Earth Bus Design
The main earth bus (also called the station ground bus or earth rail) runs along the base of the switchgear lineup and provides the common ground reference for all switchgear frames, CT secondaries, cable screens, surge arresters, and earthing switches.
Design requirements:
- Material: Bare copper flat bar or copper-clad steel. Copper is standard; steel is used only when cost is critical and corrosion is not a concern.
- Cross-section: Sized for the maximum earth fault current x rated duration. Minimum 50x6 mm (300 mm2) copper per most utility standards. For 40 kA, 1-second duty: required cross-section = 40,000 / 143 = 280 mm2 — 50x6 mm (300 mm2) satisfies this.
- Connections: All joints are bolted with Belleville (spring) washers to maintain contact pressure despite thermal cycling. Joint resistance must not exceed 20 micro-ohms per IEC 62271-200.
- Routing: The earth bus runs continuously along the base of the switchgear lineup, with connections to each cubicle frame, and extends to the substation ground grid via at least two independent paths (for redundancy).
Substation Ground Grid (IEEE 80)
The substation ground grid is a mesh of buried copper conductors that distributes fault current into the earth and limits step and touch voltages to safe levels. The ground grid design per IEEE 80 (Guide for Safety in AC Substation Grounding) is a critical part of any switchgear installation.
Key design parameters:
- Grid conductor size: Minimum 70 mm2 (4/0 AWG) copper for most MV substations. Sized for I2t thermal duty of the maximum earth fault.
- Grid spacing: Typical 3-6 m for MV substations. Closer spacing reduces touch voltage but increases copper cost.
- Ground rods: 3-meter copper-clad steel rods driven at grid intersections and at the periphery. The number of rods depends on the soil resistivity and the required grid resistance.
- Target grid resistance: Generally < 1 ohm for MV substations; < 5 ohms for pole-mounted equipment. Achieve by adjusting grid area, conductor length, and number of ground rods.
- Surface layer: 100-150 mm of crushed rock (3,000 ohm-m resistivity) over the grid significantly increases the allowable touch and step voltages by adding resistance between the person's feet and the grid.
Portable Earthing for Maintenance Safety
When maintenance work is performed on de-energized MV switchgear, portable earthing sets provide a visible, physical connection between the conductors and earth — ensuring that even if the circuit is accidentally re-energized, the fault current flows through the earthing set rather than through a person.
Portable earthing set specifications per IEC 61230:
- Rated short-circuit current matching the system fault level (e.g., 25 kA for 1 second)
- Three-phase set with phase connectors and earth connector
- Insulated operating rod for connecting/disconnecting under safe conditions
- Earth clamp connected first (before phase connections) and disconnected last
- Cable cross-section sized for I2t duty: minimum 50 mm2 copper for 25 kA/1s
Procedure:
- Open and lock out the circuit breaker(s) isolating the work area
- Close the built-in earthing switch(es) in the switchgear
- Verify absence of voltage using an approved voltage detector per IEC 61243-5
- Connect the portable earthing set: earth clamp first, then phase clamps
- Issue work permit
- After work completion: remove portable earthing set (phase clamps first, earth clamp last)
- Open the built-in earthing switch(es)
- Remove lockout and restore to service
NAIJI Electric Switchgear Earthing Features
All NAIJI Electric switchgear products include comprehensive earthing provisions:
- Cable-side earthing switches with making capacity rated at 2.5x Ik peak, per IEC 62271-102
- Five-point mechanical interlock system preventing mis-operation
- 50x6 mm copper main earth bus running the full lineup length
- Earth bus connection studs for each cubicle frame, accessible from the front
- CT secondary star-point earth terminals on dedicated terminal blocks
- Cable screen earth terminals in the cable compartment
For earthing system design assistance including ground grid calculations and neutral grounding resistor specification, contact our engineering team.