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.
There are many excellent tests and references available, which give sound designs for earthing systems for various purposes. The references quoted at the beginning of this section is found to be of considerable value.
Lightning Protection Earths
It is to be noted that some further research should be undertaken into the design of lightning protection earths. In the past, many authors have recommended that the D.C. resistance of such an earth be of not more than some specified (low) value. The need for achieving a low D.C. resistance is now being questioned, particularly in view of the difficulty and experience of achieving a low value in areas where the need for an effective lightning earth is greatest, viz. exposed area of high soil resistively. It is the author’s opinion that the achievement of a low value for the resistance of a lightning protection earth is of less importance than the achievement of a low value of surge impedance. Thus, it is important that a lightning earth electrode system by :
as close as practicable to the plant or equipment to be protected;
- connected to that plant via lightning protectors of adequate current carrying capacity and an appropriate value of breakdown voltage;
- connected to the protectors via conductors having minimum surge impedance (i.e., no sharp bends or coils in the conductor);
- of such a configuration in the ground as will achieve minimum surge impedance (in this regard it is to be noted that for trench electrodes, a system with four electrodes radiating in four directions at right angles from the connection point to the earthing conductor has a much lower surge impedance than a single trench earth of the same total length).
Service earths which carry current (e.g. teleprinter earths which may carry 20-25 mA) must be capable of surviving the discharge of such current to ground for their designed working life. It is of interest to note that a direct current of 1 ampere flowing through a steel earth electrode will consume approximately 10 kg of steel per annum. Thus, if an earth electrode is to carry 25mA for 20 years it must contain in excess of 5 kg of steel. In order to ensure that the electrode is still in working order after this period, a factor of safety of two would mean that the earth electrode must contain at least 10 kg of steel.
Earth Electrode Materials
Since copper is a good electrical conductor, there is a great temptation to use copper as the material for earth electrodes, e.g. bare copper wire, copper plates, copper-clad steel rods, etc. However, due to the position of copper in the electrochemical series, it rapidly causes corrosion of steel, zinc, lead, aluminum, etc. Thus, the use of copper as an earthing material requires very careful attention to the prevention of corrosion and it is usually better to choose some other metal, e.g. galvanized steel, plain steel, stainless steel, etc.
Types of earth electrode systems
Spike earth electrode system
The present standard for spike earth-electrode system consists of twenty 25.0 mm or 38.00 mm diameter G.I pipes, each of 275 or 375 cm length. Each pipe is used as an earth spike driven to its full length into the ground, the spacing between any two being not less than 375 cms. This spacing between adjacent electrodes has a significant influence on the potential distribution over the earth around the electrode system; the condition of minimum resistance to earth in a multiple earth system, requires that this spacing be as a large as possible. In practice a spacing of 375 cms is found adequate and in no case should this spacing be permitted to be less than 250 cms. A typical layout is as shown in fig 1.
The actual layout depends on the space available for driving the electrodes and may be modified to suit. The spacing, of not less than 375 cms between any two electrodes is important and must always be kept up as for as possible.
The resistance of a single spike buried to its full length into the ground is given by the formula.
- R = (r/2L).log e (4L/a) -1. Where
- r = soil resistively in ohms/cm2
- L = length of spike in cm.
- a = radius of spike in cm.
Generally the soil resistively in most of the locations in the country is less than 10,000 W /cm3. For this value of soil resistively, standard spike length of 375 cms and diameter 38.0 cm the resistance works out to 24.6 ohms per spike. For 275 mm long and 25.0 mm diameter spikes the value is 32.2 ohms.
The effective resistance of a number of (N) electrodes in parallel can approximate to the ideal of R/N CR = resistance to earth of single electrodes, only when the electrodes are very far away from each other. In practice, however, the distance between electrodes is dictated by condition like availability of space, number of electrodes that can be used etc. For most application with electrodes spacing of 378 cms and above, a coupling efficiency 50 percent may be assumed and the effective resistance calculated more accurately by the formula R/0.5 x N. For the standard spike earth electrode system, assuming a soil resistively of 10,000 ohms/cm3 the effective earth resistance would be as the order of 2.46 ohms.
The positions for the spikes should be marked out on site and a trench no wider than is necessary should be excavated to take the bending cable which will be used to connect all the spikes in parallel for leading into the premises. The trench should be about 45 cms deep if dug in ground which will be within the walls of the building or 75 cm deep if dug in a yard or in spare ground. The spikes should be driven vertically into the trench until the top of the driving head is 30 cm above the bottom of the excavation.
A driving head, or a suitable size bolt, is supplied. With each earth spike and this must be used to prevent the top of the spike being damaged during driving operation. When the spike has been driven fully into the ground, it will probably be found that the driving head will be a tight fit on the spike and in these circumstances it may be left in position, no forcible attempts being made to remove it.
The spikes should be connected together by a continuous main earth lead, nominally of size 19/0.64 inches bare tinned copper, protected by a lead pipe to prevent corrosion. The lead pipe containing the earth lead should be lightly dressed down and wiped on to the bare tinned copper conductor at each side of the binder. The binder should be of soft copper wire 0.056 inches diameter, wound over the earth lead at the points where it is held in the clamp on the spike.
The lead pipe used should be 19 mm nominal diameter and 10 kg per 460 cms length in weight. The binder and the exposed part of earth lead should be thoroughly tinned. The earth lead should be clamped at a point as low as possible on the spike and bent so as to lie along the bottom of the trench. All bolts, clamps, tinned copper wire adjacent to the wiped joints should be well coated with black paint suitable for iron work.
The main earth lead should be run as straight and as short as possible from the electrode system to the earth bar on M.D.F via the earth bar on cable chamber without breaking its continuity. From the MDF, a separate load of 19/064 inches bare tinned copper should be run along the shortest route and terminated on the earth bar of the exchange D.C distribution board.
Outside the building the underground earth load should be drawn into 19 mm lead pipe. The pipe should be brought into the building through the floor or main wall and terminated at not less than 30 cm above the floor level. A 38 mm steel pipe through the wall or floor should be provided in the building to allow the lead pipe to be lead in. Within the building the earth lead should be bare uncovered and unpainted, but short length of conduct may be used where protection against mechanical damage is considered necessary such as where it passes through a floor or a wall.
Lead Strip Electrode System
This consists of a lead strip 1 mm wide and 6 gms/square cm buried at a depth of from 60 to 90 cms. The strip should be preferably laid in one continuous length of 2450 cms. Otherwise two lengths of 1225 cm should be laid at least 250 cm apart and overlapped by at least 152 mm, the two electrode being parallel at the M.D.F earth bar. The earth lead protected by a load pipe as described in spike earth electrode system should be connected to the lead strip for at least 152 mm by a plumber is wiped joint as shown in the diagram below.
All exposed surface of the tinned copper wire and the adjacent lead surface should be liberally coated with black paint suitable for iron work.
Earth plate electrode system;
This consists of four galvanized iron plates of 145 SWG 76 cms square arranged as shown in the diagram below.
These four plates are placed vertically and at diagonally opposite in an excavation 185 cm square and of a depth sufficient to reach damp soil. The depth should never be less that about 250 cms and need not be greater than 500 cms. The lead pipe carrying the main earth lead should be as close to the tails of the plates as possible.
Conditions determining the type of earth electrode system to be used.
- Spike earth electrode system: This system is generally used at all new auto exchange installations where adequate space is available around the exchange building and where subsoil suitable for driving in pipes to the prescribed depth exists.
- Lead strip electrode systems. This is used when adequate space around the exchange building is available but where rock is encountered at a depth less than 375 cm below ground level.
- Earth plate electrode system: This is employed when the layout of the exchange site is such that adequate space is not available to install an earth electrode system of types (a) and (b) mentioned above.
Location of the earth electrode system
In most cases it is possible to provide the earth electrode system on the space around the telephone exchange building. The following points need careful attention before choosing the site for the earth electrodes.
- The electrodes must be located in undisturbed soil i.e.; not in made up soil or loose soil or
- If new buildings or extensions to existing building are to be erected on the site, the area to be covered by these must be avoided.
- The electrodes should not be buried in a position where damage is likely to result from heavy vehicular traffic near by.
- The position chosen for the electrodes should be such as to give the shortest and meet direct run practicable to the cable chamber, or to M.D.F where cable chamber does not exist.
- As far as practicable, the electrodes should be placed at least 185 cm from the intended or probable position of underground metal pipes or cable belonging to other undertaking to reduce risk of damage.
- It should, as far as possible, be far removed from other similar earth in area like AC, earth etc. and in no case should the separation between the two be less than 375 cms.
After the electrodes have been located properly bent and covered over, suitable visible markers should be laid over the electrodes for future identification. A route map for the bonding wire should be drawn up and displayed in the M.D.F room . Copies should be made available to the authorities responsible for planning the building operation in the premises.Download