This article will give an introduction to electrical earthing (or grounding). In order to understand what earthing is and the role it plays in a power system, its important to first appreciate power systems faults and the hazards they can cause.
Electrical Hazards Around Substations
It is generally known that electricity presents an inherent risk to anyone who comes too close to live conductors in normal operation, and this document will not discuss those risks. This document focuses on risks associated with electrical infrastructure when the electrical system has a fault, an abnormal event in the power system that results in power flow in undesirable locations that may include a human body.
There are a number of fault types, including:
- Phase to phase faults
- Three phase faults
- Double line to ground
- Single line to ground (earth faults)
As in normal operation, for current flow there must be a circuit from the source transformer, through a load, and back to the neutral of that source transformer thus completing the circuit. During a fault, the current escapes from the circuit, but still must return to the source to complete the circuit, doing so through other conductors, objects, people, local earth grids and the soil. When fault current enters the ground, it creates an Ground Potential Rise (PR) at that point.
Minimising risks during fault events typically focuses on two things:
- Reducing or eliminating current through the human body
- Reducing the overall voltage during a fault
Hazards During Earth Faults
Single line to ground faults, are generally considered the worst-case fault. While three phase faults have a higher fault level, they are generally balanced faults producing very little current in the soil, hence very little voltage rise, and double line to ground faults are much less common than single line to ground faults, making them the most common and generally a relatively high fault current magnitude.
When a single line to ground fault occurs, a Ground Potential Rise (GPR) is produced. The faulted ground grid/ground mat and the local soil rises in voltage with respect to a remote point, and the soil voltage decreases with distance from the fault point. Local voltage hazards may be present to a human body coming in contact with an object at a high voltage, and the soil at a lower voltage. These hazards are present across a body between hands and feet, across the chest, and various combinations where a voltage across the body may cause current to flow through the heart, of which only a relatively small amount of current can cause the heart to go into ventricular fibrillation.
What is Earthing?
Earthing can broadly be described as a connection to the greater mass of earth. Earthing can be used in a number of contexts within the electric power industry:
- Low voltage wiring protective earth conductor
- Ground stake installed at LV switchboards
- Ground grids installed at transformers and substations consisting of bare conductors and earth rods
- Overhead ground wires on high voltage lines
- Cable screens on HV cables
Sometimes earthing can be metallic items that are not originally intended as such, for example telecommunications cables, pipelines and fences.
In power systems, earthing exists for a number of reasons:
- Transient energy dissipation (such as lightning strikes)
- Allowing protection to operate during faults
- Minimise equipment damage during faults
- Minimise voltage differences and protect people during faults
Each of these earthing systems may be stand-alone, or they may be connected together to a form a larger, common bonded grounding system.
Mitigating Hazards During Earth Faults
In order to mitigate hazards during ground fault events, two methods can be employed:
- Reducing the overall hazard magnitude
- Addressing local hazards in an effort to reduce the current that may flow through a body
To reduce the overall hazards, commonly would mean reducing fault clearing times, reducing the fault level (often with a neutral earthing resistor), or reducing the system impedance (by making more connections or installing a larger ground grid / ground mat).
Where the above options are not available, local hazards in specific location may be addressed through engineering controls such increased resistance/insulation, isolation, equipotential bonding, and administrative controls such as procedural changes and PPE.
Important: In order to understand the hazard levels and any necessary controls, appropriate testing and analysis is required to determine the performance of the installed earthing system.
Zero Sequence Earthing are happy to spend some time to discuss your design or testing needs, provide advice, and can also assist by undertaking earthing system design and testing of complex earthing systems, including:
- Distribution and zone substation earthing
- Private HV, mining and industrial earthing
- High voltage transmission lines and cable fed systems
- Electromagnetic induction (Low Frequency Induction) assessments (Right of Way assessments)
- Electrostatic induction problems
Not sure? Get in touch and we will help you with any earthing related queries.
Zero Sequence Earthing
+61 2 4039 8000 / +61 434 190 272