Midlands Power Networks can offer a bespoke Earthing survey, design and installation service which relates to your specific site.

Scope of Works.

Soil Resistivity Surveys.

When designing a substation or generation site it is important to understand the earthing protection system that needs to be installed. As each location is geographically and geologically different, a survey will need to be undertaken to determine the amount of protection required. The illustration below (Fig 1) shows the huge variation in soil resistivity across England, Scotland and Wales.

Fig 1 – Soil Resistivity across England, Scotland and Wales. (British Geological Survey).

In order to design an earthing system for a generation or high voltage substation site, an understanding of the ground conditions is required. Soil resistivity surveys determine the ability of the soil to allow current flow as it changes in depth and soil type.

The earth in some supply networks is used to allow fault current to pass through in order for the protective devices to operate. Generally, there is a value of impedance of which the earth connection must not rise and the maximum step/touch voltage must not be exceeded which could prove dangerous for people and livestock.

Soil resistivity can vary greatly and depends upon factors such as temperature, moisture and chemical content.

Our surveys are conducted in line with ‘BSEN50522:2010 – Earthing of Power Installations Exceeding 1kV A.C.’ A number of tests are carried out and this is usually by the Wenner Array method. Test spikes are driven into an area of available soft dig ground of no less than 60m in length. A buried service report is required in advance to enable this test to be carried out safely. Once data is gathered an Earthing Design Strategy is completed, this will include the earth electrode system required to meet the standards (Fig 2 – Example). The recommended materials and methods to be used will be included in the report.

Fig 2 – Earthing Design Sample

The system is designed for the sole purpose of protecting the electrical distribution system and personnel onsite during a transient event; as such this design may not be suitable for lightning protection, static discharge or Electromagnetic Pulse protection purposes. Where possible the earth electrode system will be designed to return a COLD site as described in Energy Networks Association document S36. Should it not be physically or economically possible to derive a COLD site, you must inform the local DNO prior to energisation of the site.

Prior to any design process, we require, as a minimum, the following pieces of information:

• Highest system voltage on site.
• Highest fault to earth current (Igr).
• Switchgear details including rated short time withstand current rating (Ik) and busbar rating.
• Details of any existing electrical installations on the site.
• Details of any perimeter fence around the site.
• Details of the final finished surface around the site
• Information regarding any telemetry, security or data services likely to be installed across the site.
• Details of feeder substation fault clearance times.
• Details of the site feeder substation including the distance from the site and interconnecting conductors.
• Details of any generation schemes likely to be installed on the site including capacity, switchgear fault clearance times and whether the system is likely to run in island mode.

Installation & Verification Works.

During the installation, visual checks and electrical tests are carried out to ensure conformity to the design. The conductor tape size is checked and that it is installed to the correct depth and within the correct ground medium. Joints and terminations to electrodes can be made either mechanically or by fusion welding. Visual, mechanical and electrical tests are conducted and results are recorded and compared to the design.

Overall System Test.

Prior to energisation, an overall test of the completed earth electrode system is carried out and results compared to the design values.

 

MPN will be pleased to offer initial advice and guidance on any earthing queries that you may have.