Published 05 Dec 2017
by Chris Dodds T&D - estimated reading time 6 minutes
Thorne & Derrick International are distributors for the ABB Furse and AN Wallis range of Earthing & Lightning Protection Systems – this includes earth rods, earth bars, earth tapes and a full supporting range of accessories to comprise the complete earthing system for electricity transmission and distribution systems.
The copper products are used typically to provide electricity substation earthing to medium/high voltage grids containing electrical infrastructure, cables, switchgear and transformers. Furthermore, it is usual for all exposed and normally un-energised metalwork inside the substation perimeter, including doors, staircases, ventilation ducts, cable supports etc to be be bonded to the main earth grid to avoid any potential differences between different items of metalwork.
Guidelines and policies vary according to the Distribution Network Operator – specialist contractors using modern earthing techniques compliant with relevant British and international standards, including BS 7430:2011+A1:2015 Code of Practice For Earthing and IEEE80-2000 Guide for Safety in AC Substation Grounding should be employed to calculate specific substation earthing requirements.
A final design of the earthing system can only be undertaken when sufficient knowledge is available of the proposed physical and electrical requirements of the substation – as an absolute minimum the earthing system desginer must have knowledge of:
- value of fault current and supply arrangements (overhead and/or underground cable)
- fault duration (or protection settings)
- soil resistivity
- substation dimensions
The specification for fixed earthing systems within EHV, HV, HV/MV and LV substations also applied to terminal towers adjacent to substations and cable sealing end compounds, pole mounted transformer or air-break switch disconnector installations and pole mounted reclosers with ground level control.
Copper Earthing Products
For MV & HV Substations
Earthing products and systems can be supplied by T&D for all substation voltages:
- MV Medium Voltage 11kV 33kV
- High Voltage 66kV 132kV
- Extra High Voltage EHV 275kV 400kV
T&D provide expert technical support, competitive prices and delivery from stock for substation earthing projects – all earthing products are available to meet the design criteria and construction specification of the earthing systems for substations and equipment for 33kV and 132kV substations.
This include copper earth tapes with typical dimensions 32mm x 4, 40mm x 6mm, 50mm x 4mm and 50mm x 6mm.
With respect to high voltage earthing applications in the power utility sector the following items would normally be earthed or bonded to the main earth grid:
- Overhead line termination structures including towers, gantries and earthed wood pole structures adjacent to the substation
- Power cable sheaths and armours
- Transformer and reactor banks, cooler and radiators, tap changers, earthing resistors, earthing reactors, high voltage transformer neutral connections
- Metal clad switchgear assemblies and cases, isolators and earth switch bases
- Metal gantries and structures and metalwork mounted on wood structures
- Metallic structures including steel frames (bonded at each corner) rebars and piles
- Panels, cubicles, kiosks, LV AC equipment, lighting and security masts
It is usual for critical items such as medium/high voltage transformer tanks and terminal towers to have duplicate connections to the main earth grid.
Pictured above is a diagram showing the typical layout of a high voltage GRP electricity substation distributing 11kV power – an outer ring of 70sqmm copper earthing conductor plus 4 earth rods of standard depth are shown with a 10 metre tail in the HV cable trench accompanied by a layout for the concrete rebar. Here, the HV substation is contained within a GRP enclosure with potential touch and step voltage risks.
Earth tapes are available stamped with the UK DNO ownership details to discourage copper metal theft – earth tapes printed “National Grid Property” are also available.
Full range of copperbond earth rods, couplings and driving studs are available for grid and primary substation earthing depending upon the calculated fault level.
- E – Earth electrode (copper earth rod)
- UE – Earth potential rise (EPR)
- S1, S2, S3 – Potential grading earth electrodes connected to the earth electrode E
- US Touch – Source voltage for touching (Touch Voltage)
- US Step – Source voltage for stepping (Step Voltage)
The potential on the surface of the soil near a copper earth rod during the time that fault current flows. If a person were to touch a rod (or any exposed metalwork connected to it), then the potential difference experienced between hands and feet is termed the touch voltage. Touch voltages are calculated assuming that the feet of the person are 1 metre away from the metalwork being touched. Touch voltages are normally reduced by using potential grading electrodes.
Potential differences are established on the surface of the soil whilst fault current is flowing. Step voltage in a particular direction is defined as the potential difference between two points a metre part. Step potentials can be reduced by using potential grading copper electrodes or installation electrodes, such as copper earth rods, at a greater soil depth.
A potential rise of an earthing system caused by a current flowing to earth, transferred by means of a connected conductor (eg metallic sheath or pipe), which enters a substation area, with little or no potential rise relative to reference earth, and which is not connected to earth in that area. This results in a potential difference occurring between the conductor and its surroundings at the remote position.
Electrical Safety – Maintaining & Repair Of Earthing Systems
Substation earthing systems are the victim of operational and intentional damage – copper theft is rife and earthing maintenance work is often identified during routine site checks and inspection. Earth faults can be weekly occurrences on primary substations supplying extended high voltage rural overhead line networks. During these maintenance tasks there is serious risk of injury or even fatal electric shock when working on depleted or damaged earthing systems – specialist electrical safety equipment including insulated sticks, poles or rods, portable earth shorting leads, earthing clamps (substation or line-end), insulating footwear (shoes or boots) and insulating gloves depending on the voltage class of the substation earthing system.
High Voltage Substation Earthing 275kV
All images courtesy: Ian Forster (EHV Site Manager)
More Substation Earthing Applications
Fig.1 3 x Copper earth cables connected to main aluminium substation earth tape
Fig.2 Copper earth tape connected to main aluminium earth tape
Fig.3 Aluminium to aluminium tape connection
- IEEE Guide for Safety In AC Substation Earthing Grounding Size: 15.99 MB
- Earthing & Lightning Protection Systems – AN Wallis Handbook Size: 676.61 KB
- AN Wallis – Earthing & Lightning Protection Size: 3.88 MB