Earthing and Measurement | Earth Electrode System For Telephone Exchange
Earth electrode systems are installed at telephone exchanges
- To provide an earth connection to the battery circuit to stabilize the battery potential of the lines and equipment with respect to earth, thus reducing the risk of cross talk due to lines and equipments assuming an indefinite voltage with respect to earth, and enabling single pole switching to be used on the exchange power plant. This also reduces the number of fuses required in the circuit and avoids the need of insulating the earthed conductor i.e. positive bus bar.
- To provide a direct connection with earth for lightning protective apparatus.
- To provide protection to persons and plant against leakage from station power wiring to metallic apparatus, frames etc.
- To provide a means of earthing electrostatic screens on apparatus and of earthing lead sheaths of cables.
- To complete the circuit of telephone systems, employing a common earth path for signaling purposes.
Standards for resistance of earth electrode systems
The resistance of the earth electrode system should be as low as possible and in any case should not normally exceed 2 ohms at any time for the year. The worst condition occurs in winter when specific resistively of soil increases sharply with temperature near or below 0 C, where it exceeds 2 ohms, two or more of similar or any one of the alternative system described below should be installed and spaced as far as away from each other as possible but in no case within a distance less than 375 cm from the first electrode system. The system must be paralleled at the earth bar on M.D.F so that the overall resistance of the earth system is below 2 ohms where the space available does not permit two systems in parallel, special treatment of soil may be necessary to reduce its resistively and the problem should be referred to the additional Chief Engineer.
The earth resistance of Earth electrode system for Electronic exchanges should be less than 0.5 Ohms.
Classes of Earthing Systems
Earthing systems are provided to serve many different purposes. They may be divided into two major categories, viz.
- Service Earthing Systems, e.g.
- switching equipment earth
- transmission equipment earth
- measuring equipment earth
- C. power supply earth
- Corrosion mitigation earth
- Miscellaneous equipment earth (e.g. telegraphs).
- Protective Earthing System, e.g.
- Power system earth to provide protection against excessive current;
- Lightning protective earth to provide protection against excessive voltage.
Requirements for Service Earthing Systems
In general, service earthing systems should have a low D.C. resistance to the general body of the earth, in order to ensure that the potential drop across the earth connection is low. If any current flows through the service earthing system, a potential difference will be developed across the earth connection. This can introduce interference in the form of electrical noise into any telecommunication circuit connected to that earthing system.
The value of resistance which must be met by a service earthing will depend on the purpose for which the earth likely to be carried by the earthing system, and the tolerable voltage drop across the earth connection. The value chosen by most administrations is usually not more than ten ohms, although in some isolated cases higher values are acceptable.
Requirements for Protective Earthing System
The requirements to be satisfied by a protective earthing system are governed by the purpose for which the earth is being provided. Earth which protects against excessive current (e.g. power supply protective earth) must have a low resistance in order to :
- carry the anticipated value of over-current without overheating and “burning out”.
- Enable sufficient current to flow to ground to ensure that any over current protective devices (e.g. fuses, circuit breakers, etc.) will operate to disconnect the current after a very short time.
- prevent hazardous potential difference to develop across the earth connection (excessive potential differences can cause breakdown of insulation, and danger to life and limb).
The foregoing requirements can be satisfied by heavy gauge conductors of very low resistance connected to earthing systems of very low resistance (typically less than one ohm).
- Able to withstand indefinitely the corrosive action of soil.
- Inert in relation to the system to be protected (i.e. must not be a source of galvanic corrosion currents).
- The resistance of the earth connection must remain reasonably constant throughout the various seasons of the year.
Earths which protect against excessive voltage (e.g. earths connected to lightning protection systems) must possess a low surge impedance in order to enable the lightning surge currents to be easily conveyed to the earth and thus diverted away from the equipment which is to be protected from the lightning.
It now considered that it is better to install a common earth than go for different earthing systems for different purposes as this may cause currents to flow through them because of potential differences between them. The common earth must be designed and installed to suit the requirement of various earthing systems which are required at the site.
The main advantage of a common earthing system are :
- By carefully bonding the various earthing systems together, the potential difference between one earth connection and another is negligible. Thus, no excessive currents or voltages will be developed within the earthing system.
- If, due to excessive voltage or current, the potential of the protective earth rises, then so will all other earths rise in potential, thus once again preventing the development of potential differences within the earthed environment of the installation (This is the so-called “Faraday Cage” effect).
Design Principles for Earthing Systems
Earthing systems should be designed to achieve the following :
- adequate current carrying capacity (DC or AC as appropriate).
- adequate mechanical strength to withstand the rigors of service without fracturing.
- In the case of lightning protective earths adequate-surge-current carrying ability.
Earthing System Designs
It is not appropriate to specify or to recommend the designs or dimensions of earthing systems which are to be provided for various purposes. It is appropriate, however, to draw attention to the principle embodied in good design, and this has been done.