To bond or not to bond a substation perimeter fence to the HV switchyard earthing conductors is a key decision in the earthing system design process for a major substation. The answer is not straightforward, and more than one design approach can be implemented with different outcomes in terms of cost, as well as initial and ongoing safety and robustness.
Although not supporting HV equipment, how a substation fence is earthed has impact, as touch voltages and surface potential gradients are highest at the periphery of the earthing system where utility staff and the public may easily contact the fence. Also as many fences play a critical role in securing a substation from illegal entry and are vulnerable to vandalism or damage, they will require a high level of condition monitoring. While determining the design that minimises shock risk with least capital cost is a primary goal, additional factors should be considered. Some of these factors are critical to maintaining optimal safety at the same time as minimising ongoing replacement and maintenance requirements and may not be able to be defined at the time of initial design. For instance ensuring ongoing robustness in the face of unforeseen environmental changes, both internal and external to the fence.
Fence earthing design is informed by answering questions such as the following:
- Is the fence intended to be earthed separately to the main HV earth grid or common bonded?
- What is the earth potential rise (EPR) of the HV grid?
- Do any overhead lines run above the fence? • How much distance is there between the HV equipment and the fence being designed?
- Is the fence near where people are expected to move (especially gates)?
- Is there room outside the fence for a touch voltage gradient control conductor?
- Is the fence close to air cored reactors or very high voltage equipment?
- Are transfer hazards an issue or likely to become an issue?
- Are there any LV supplied assets connected onto or near the fence?
- What is the intended fence construction?
- Are there any conductive materials in the soil between the HV equipment and the perimeter fence?
- Are there any code requirements?
This paper aims to describe the hazard mechanisms that impact fence earthing effectiveness and provide a step by step decision structure to enable a designer to determine and justify the most effective, fit for purpose fence earthing configuration.