Guide to Power System Earthing Practice - New Zealand Electricity
Introduction:
The guide to power system earthing practice in New Zealand provides a comprehensive framework for the design, installation, testing, and maintenance of earthing systems in electrical installations. Earthing in the New Zealand power system context is governed by industry standards and regulations to ensure electrical safety, system reliability, and protection of both personnel and equipment. The guidelines align with New Zealand-specific conditions, such as unique soil resistivity profiles, environmental considerations, and the country's regulatory requirements.
Purpose of Earthing in Power Systems:
The primary purpose of earthing in power systems is to ensure safety and stability. It involves creating a direct electrical connection between the power system components and the earth to:
- Protect Against Electric Shock: Safeguard individuals from electric shock by ensuring that exposed conductive parts do not reach dangerous voltage levels during faults.
- Facilitate Fault Current Dissipation: Provide a low-resistance path for fault currents, allowing protective devices like fuses and circuit breakers to operate effectively.
- Stabilize Voltage Levels: Maintain stable voltage levels throughout the system by providing a consistent reference point for electrical potentials.
- Minimize Equipment Damage: Protect equipment from damage due to overvoltages caused by lightning strikes, switching surges, or insulation failures.
Standards and Regulatory Framework:
New Zealand's earthing practices are guided by several key standards and regulations, including:
- AS/NZS 3000 (Wiring Rules): The standard specifies general requirements for electrical installations, including earthing and bonding arrangements.
- AS/NZS 3017: Outlines testing and inspection guidelines for earthing systems in electrical installations.
- Electricity (Safety) Regulations 2010: Provides legislative requirements for electrical safety, including earthing provisions for all electrical installations.
- SNZ HB 6117: A handbook that offers guidance on earthing systems specifically for high-voltage installations.
Earthing System Configurations in New Zealand:
In New Zealand, earthing system configurations vary depending on the application and voltage level. The most common configurations include:
1. TN Systems (Terra-Neutral):
The TN earthing system connects the neutral point of the power supply directly to the ground. In this system, the exposed conductive parts of the electrical installation are connected to the neutral or protective earth conductor.
- TN-S (Separate Earth and Neutral): The neutral (N) and protective earth (PE) conductors are separate throughout the entire system.
- TN-C-S (Combined Neutral and Earth): The neutral and earth conductors are combined in a single conductor (PEN) from the power source but separated into PE and N at the consumer’s installation.
Advantages:
- Provides effective fault clearance.
- Minimizes the risk of electric shock.
Disadvantages:
- Requires robust protective measures, particularly in TN-C-S systems, to prevent potential electric shock due to the combined PEN conductor.
2. TT Systems (Terra-Terra):
In TT systems, the neutral of the power supply is earthed at the transformer, while the consumer's installation uses an independent earth electrode.
Advantages:
- Simple and effective, especially in rural and remote areas.
- Reduced risk of faults propagating through shared earthing conductors.
Disadvantages:
- May require additional protection devices like Residual Current Devices (RCDs) due to the higher earth resistance.
3. IT Systems (Isolated Terra):
The IT earthing system features an isolated or impedance-grounded neutral point, with exposed conductive parts connected to a separate earth electrode.
Advantages:
- High continuity of service, as the system can continue to operate with a single ground fault.
- Reduced risk of electric shock due to the isolated neutral.
Disadvantages:
- Complex fault detection and insulation monitoring are required.
- Higher design and maintenance costs.
Design and Installation Guidelines:
1. Soil Resistivity Considerations:
- New Zealand's diverse geological conditions make soil resistivity a critical factor in earthing design. Conducting soil resistivity tests is essential to determine the most effective type and placement of earth electrodes.
2. Earth Electrode Design:
- Earth electrodes in New Zealand installations can be rods, plates, or ground grids. Copper or galvanized steel is commonly used due to its resistance to corrosion.
- For urban installations, driven rod electrodes are popular, while in rural areas, ground plates or mesh grids may be preferred due to lower resistivity requirements.
3. Selection of Earthing Conductors:
- The earthing conductors must be sized appropriately to handle expected fault currents without overheating. AS/NZS standards provide guidelines for conductor sizing based on current-carrying capacity and environmental conditions.
4. Bonding Requirements:
- All exposed conductive parts, metal structures, and non-electrical metallic systems (e.g., water pipes, gas pipes) should be bonded to the earthing system to ensure a common potential and reduce the risk of electric shock.
Testing and Maintenance Practices:
Regular testing and maintenance are crucial to the effectiveness of earthing systems. Key practices include:
1. Earth Resistance Testing:
- Conduct regular earth resistance tests to verify that the earthing system provides a sufficiently low resistance path. Values below 5 ohms are generally acceptable for most installations, with critical systems aiming for less than 1 ohm.
2. Visual Inspections:
- Regular inspections are necessary to identify physical damage, corrosion, or loose connections in earthing components. This helps prevent deterioration of the earthing system over time.
3. Continuity Testing:
- Ensures that all bonding and earthing conductors provide a continuous low-resistance path. Continuity testing helps confirm that all parts of the system are properly interconnected.
4. Soil Resistivity Monitoring:
- Periodic soil resistivity testing is advised to detect any changes in soil conditions that might affect the earthing performance, particularly in areas with seasonal variations.
Lightning Protection and Surge Protection:
New Zealand's climate makes lightning protection an important aspect of earthing practice, especially in high-risk areas. Effective lightning protection systems are integrated with earthing systems to dissipate high voltages safely into the ground.
1. Lightning Arresters:
- Installed at critical points to intercept lightning strikes and channel the energy safely to the earth.
2. Surge Protection Devices (SPDs):
- Protect sensitive electrical and electronic equipment from transient overvoltages caused by lightning or switching surges.
Compliance and Safety Requirements:
To ensure compliance with New Zealand's regulatory framework:
- Follow AS/NZS Standards: All earthing practices must comply with the relevant AS/NZS standards, including AS/NZS 3000 for general wiring rules.
- Engage Licensed Professionals: Only licensed electricians and electrical engineers should design, install, and test earthing systems to meet safety regulations.
- Maintain Accurate Records: Keep detailed records of testing results, maintenance activities, and any modifications to the earthing system. This documentation is crucial for ongoing compliance and safety assurance.
Best Practices in Earthing System Design and Maintenance:
- Consider Site-Specific Conditions: Tailor the earthing design to local conditions, including soil resistivity, environmental factors, and site-specific risks.
- Use High-Quality Materials: Opt for corrosion-resistant materials like copper or galvanized steel to ensure long-term reliability.
- Regularly Inspect and Test: Conduct routine inspections and testing to maintain the effectiveness of the earthing system and ensure compliance with safety standards.
- Plan for Future Expansion: Design earthing systems with scalability in mind to accommodate future upgrades and expansions in the power network.
Conclusion:
The guide to power system earthing practice in New Zealand emphasizes the importance of effective earthing for safety, reliability, and compliance. By adhering to established standards and guidelines, electrical installations can minimize the risk of electric shock, protect equipment, and enhance system stability. Regular testing, proper design, and adherence to local conditions are essential to maintaining a robust and effective earthing system in New Zealand's diverse environments.
