Functions of an Earthing System
The two primary functions of a safe earthing system are i) ensure that person who is in the vicinity of earthed facilities during a fault is not exposed to the possibility of a fatal electrical shock, ii) provide a low impedance path to earth for currents occurring under normal and fault conditions
Earthing Standards
There are a variety of national and international standards available, which provide empirical formulae for the calculation of earthing design parameters and shock potential safety limits.
BS7354 - 1990 Code of Practice for Design of High Voltage Open Terminal Stations
IEEE Std 80-2000 IEEE Guide for Safety in AC Substation Grounding
EATS 41-24 - Guidelines for the Design, Installation, Testing & Maintenance of Main Earthing Systems in Substations
Ground Potential Rise (GPR)
The substation earth grid is used as an electrical connection to earth at zero potential reference. This connection is not ideal due to the resistivity of the soil within which the earth grid is buried.
During typical earth fault conditions, the flow of current via the grid to earth will therefore result in the grid rising in potential relative to remote earth to which other system neutrals are also connected.
This produces potential gradients within and around the substation ground area - this is defined as ground potential rise or GPR.
The GPR of a substation under earth fault conditions must be limited so that step and touch potential limits are not exceeded, and is controlled by keeping the earthing grid resistance as low as possible.
Step, Touch, Mesh & Transferred Potentials
In order to ensure the safety of people at a substation, it is necessary to ensure that step and touch potentials in and around the substation yard during earth-fault conditions are kept below set limits.
These maximum permitted step and touch potentials are addressed within various national and international standards.
Step Potential
The step potential is defined as the potential difference between a persons outstretched feet, normally 1 metre apart, without the person touching any earthed structure.
Touch Potential
The touch potential is defined as the potential difference between a persons outstretched hand, touching an earthed structure, and his foot. A persons maximum reach is normally assumed to be 1 metre.
Mesh Potential
The mesh potential is defined as the potential difference between the centre of an earthing grid mesh and a structure earthed to the buried grid conductors.
This is effectively a worst-case touch potential - for a substation grid consisting of equal size meshes, it is the meshes at the corner of the earth grid that will have the highest mesh potential.
Transferred Potential
This is a special case of a touch potential in which a voltage is transferred into or out of a substation for some distances by means of an earth referenced metallic conductor.
This can be a very high touch potential as, during fault conditions, the resulting potential to ground may equal the full GPR.
Earthing System Design Considerations
Conductors - a substation earthing grid will consist of a earthing system of bonded cross conductors.
The earthing conductors, composing the grid and connections to all equipment and structures, must possess sufficient thermal capacity to pass the highest fault current for the required time.
Also, the earthing conductors must have sufficient mechanical strength and corrosion resistance. It is normal practice to bury horizontal earthing conductors at a depth of between 0.5m and 1m - this ensures that the earth conductor has the following properties :
- Adequate mechanical protection
- It is situated below the frost line
- The surrounding earth will not dry out
Vertically Driven Earth Rods
Where there are low resistivity strata beneath the surface layer then it would be advantageous to drive vertical earth rods down into this layer - to be effective the earth rods should be on the periphery of the site. The length of the earth rod is chosen so as to reach the more stable layers of ground below.
The earth rods would stabilise the earth grid resistance over seasonal resistivity changes at the grid burial depth.
Substation Fences
The earthing of metallic fences around a substation is of vital importance because dangerous touch potentials can be involved and the fence is often accessible to the general public.
Fence earthing can be accomplished in two different ways :
- Electrically connecting the fence to the earth grid, locating it within the grid area or alternatively just outside
- Independently earthing the fence and locating it outside the earth grid area at a convenient place where the potential gradient from the grid edge is acceptably low.
Other Substation Earthing
The GPR at a substation is reduced by:
- Overhead line earth wires which are connected to the substation earthing grid. This diverts part of the earth fault current to the tower footing earthing.
- Cable entering and leaving the site. The armouring of such cables is usually earthed to the substation earthing grid at both ends. Part of the earth fault current will thus be diverted to a remote earthing grid via the cable armouring.
- Source : www.cablejoints.co.uk
