Earthing - an integral part of Electrical Safety
Besides human safety risks, sensitive computers and electronics devices fail to operate properly and/or sustain damage at lower voltages which usually cannot be felt by humans, resulting in loss of productivity and expensive troubleshooting & repairs
DEFENITION OF EARTHING
Earthing means any connection given to the earth mass by means of a conductor connected to the earth electrode buried in soil.
GOOD EARTHING Means
There are various formal definitions of a good Earthing, e.g., how many amps of current it can drain at a particular voltage, or resistance per surface meter.
Good Earthing must have impedance low enough to ensure that sufficient current can flow through the safety device so that it disconnects the supply ( <0.4 sec ) and that voltage of earth doesn't rise more than 50V.
In simple theory this will prevent a potential difference between earth and earthed parts, as well as permitting the flow of fault current which will cause the operation of the protective systems.
IS: 3043-1987 Clause 9.2.8 & 9.3 says…
The material, selected for earthing electrode, should exhibit a galvanic potential with respect to the ground as nearly equal to that exhibited by the structure in its natural or unprotected condition, the material used should be such that it resists the corrosion.
Tests in a wide variety of soils have shown that copper is entirely satisfactory and taking the life of copper earthing as max. 12 years, the average loss is weight of copper, buried for 12 years, is 0.2 percent per year and average loss in weight of galvanized steel , a little inferior to copper, 0.5 percent per year “
QUALITIES OF GOOD EARTHING
The quality of Earthing you need depends upon your application
The qualities of a good earthing system are :
- Must be of low electrical resistance
- Must be of good corrosion resistance
- Must be able to dissipate high fault current repeatedly
The quality of Earthing you need depends upon your application.
NEED OF GOOD EARTHING
- To save human life from danger of electrical shock or death by blowing a fuse i.e. To provide an alternative path for the fault current to flow so that it will not endanger the user
- To protect buildings, machinery & appliances under fault conditions ie. To ensure that all exposed conductive parts do not reach a dangerous potential.
- To provide safe path to dissipate lightning and short circuit currents.
- To provide stable platform for operation of sensitive electronic equipments i.e. To maintain the voltage at any part of an electrical system at a known value so as to prevent over current or excessive voltage on the appliances or equipment .
- To provide protection against static electricity from friction
How EARTHING works?
The basic idea of a safety from electrical disaster the same everywhere. The case (chassis) of the equipment (and except for special situations, the internal electronics) is connected to an earth pin on the mains outlet. This is then connected through the house wiring and switchboard to an electrically solid earth point, which is called EARTHING or GROUNDING.
Should a fault develop within the equipment that causes the active (live) conductor to come into contact with the chassis, the fault current will flow to earth, and the equipment or main switchboard fuse or circuit breaker will blow. This protects the user from electric shock, bypassing the dangerous current directly to earth, rather than through the body of the unsuspecting poor bastard who just touched it.
Earth leakage circuit breakers (RCD - residual current detectors) measure the current in the active and neutral conductors. If these differ by more than a few milliamps, the circuit is disconnected. The principle is simple - if the current in the two wires differs, some of it must be going somewhere that is undesirable, so the supply is interrupted almost instantly. While these are mandatory in some countries (or under some circumstances), it is best not to rely on any advanced technique, but provide a system that is intrinsically safe.
TYPES OF EARTHING
There are various ways of doing EARTHING
- Conventional EARTHING
- Pipe Earthing,
- GI Plat Earthing,
- Cast Iron plat Earthing,
- Copper plat Earthing
- Maintenance Free EARTHING
The Conventional system of Earthing calls for digging of a large pit into which a GI pipe or a copper plate is positioned amidst layers of charcoal and salt. It is cumbersome to install only one or two pits in a day.
The Conventional system of GI pipe Earthing or copper plate Earthing requires maintenance and pouring of water at regular interval.
MAINTENANCE FREE EARTHING
It is a new type of earthing system which is readymade, standardized, scientifically developed. Its Benefits are
- Maintenance Free: No need to pour water at regular interval- except in sandy soil.
- Consistency: Maintain stable and consistent earth resistance around the year.
- More Surface Area: The conductive compound creates a conductive zone, which provides the increased surface area for peak current dissipation. And also get stable reference point.
- Low earth resistance. Highly conductive. Carries high peak current repeatedly.
- No corrosion. Eco Friendly.
- Long Life.
- Easy Installation.
- As per IS: 3043- 1987.
TECHNICAL DETAILS OF MAINTENANCE FREE EARTHING MAINTENANCE FREE EATHING consists of two parts:
Two ‘B’ class mild steel pipes, one inside the other, are subjected to Hot dip Galvanization : 80-100 micron on the secondary electrode and 250 – 300 micron on the primary electrode. Empty space inside the primary electrode and the secondary electrode is filled with CRYSTALLINE CONDUCTIVE MIXTURE and then sealed.
- Empty space inside the primary and the secondary electrode is filled with Conductor rich crystalline mixture which contains metal alloys and natural compounds
- Contains metal alloys and natural compounds
- High conductive, Anticorrosive
- Does not disintegrate or collapse when
- Outer electrode becomes inactive
Back Fill Compound:-
- Empty space around the electrode is filled with a compound which Contains eco-friendly materials.
- Maintains moisture and enhances conductivity around the electrode.
- Does not mix with or leach in to the soil.
- Absorbs moisture 13 times its dry volume.
- No need to recharge pit. Except in sandy areas.
- Improves electrode performance and protects the system in corrosive environment.
INSTALLATION OF MAINTENANCE FREE EARTHING SYSTEM
Hands augur a 10 inch or 8 inch dia pit 2 m or 3 m deep to install electrode of required length. Fill the space between soil and electrode with specially developed BACKFILL COMPOUND mixed with dug out soil in small quantities along with water. Continue till pit is filled up to the neck of electrode as shown in the diagram. After installation pour a few buckets of water in and around electrode for few days for the entire system to set. About 4 electrodes can be installed in a day in normal soil conditions.
- Electricity takes the least path of resistance
- Earth Resistance Value (Ohmic value) of an earth pit depends solely on soil resistivity at the location.
- It is the resistance of soil to the passage of electric current
- It varies from soil to soil.
- It depends on the composition of soil, its Moisture content, & electrolyte content, Dissolved salts, grain size and its distribution, seasonal variation, temperature and current magnitude.
EARTH RESISTANCE: DEPENDS ON SEVERAL FACTORS
- It is the resistance of soil to the passage of electric current
- It varies from soil to soil
- In depends on the composition of soil, Moisture content, Dissolved salts, grain size and its distribution, seasonal variation, current magnitude.
- Different soil conditions give different soil resistivity.
- Most of the soils are very poor conductors of electricity when they are completely dry.
- Soil resistivity is measured in ohm-meters or ohm-cms
- Soil plays a significant role in determining the performance of Electrode.
- Soil with low resistivity is highly corrosive.
- If soil is dry then soil resistivity value will be very high.
- If soil resistivity is high, earth resistance of electrode will also be high.
- Moisture significantly influences soil resistivity
- Conduction of electricity in soil is through water.
- Soil resistivity drops significantly in soil with moisture content.
- Moisture is the most important element in conductivity.
- In many locations water table goes down in dry weather conditions. Therefore it is essential to pour water in and around earth pits to maintain moisture in dry weather conditions.
- Pure water is poor conductor of electricity.
- Resistivity of soil depends on resistivity of water which in turn depends on the amount and nature of salts dissolved in it.
- Small quantity of salts in water reduces soil resistivity by 80%.
- Common salt is most effective in improving conductivity of soil. But it corrodes metal and hence discouraged.
GRAIN SIZE & DISTRIBUTION
The grain size, distribution and closeness of packing also contribute to retention of moisture in the soil.
- Increase or decrease of moisture content determines the increase or decrease of soil resistivity.
- Thus in dry whether resistivity will be very high and in monsoon months the resistivity will be low.
A current of significant magnitude and duration will cause significant drying condition in soil and thus increase the soil resistivity.
MEASUREMENT OF EARTH RESISTANCE- Through: Earth Tester
For measuring soil resistivity, and earthing device resistance, Earth Tester is used. It is also called the “MEGGER”.
It has a voltage source, a meter to measure Resistance in ohms, switches to change instrument range, Wires to connect terminal to Earth Electrode and Spikes.
It is measured by using Four Terminal Earth Tester Instrument. The terminals are connected by wires as in illustration.
|P = Potential spike
||C = Current spike
The distance between the spikes may be 1M, 2M, 5M, 10M, 35M, 50M.
All depending on space available.
All spikes are equidistant and in straight line to maintain electrical continuity. Take measurement in different directions.
Soil resistivity ‘P’ may be obtained from the following formula :
|P = 2 Л LR
R = Value of Earth resistance in ohm
L = Distance between the spike in cm
π = 3.14
P = Earth resistivity ohm-cm
Earth resistance value is directly proportional to Soil resistivity value.
The following illustration describes how resistance is measured Spikes must be in straight line to maintain electrical continuity.
Resistance must be verified by increasing or decreasing the distance between the tester electrode and the spikes by 5 meter. Normally, the length of wires should be 10 and 15 Mtr or in proportion of 62% of ‘D’.
Suppose, the distance of Current Spike from Earth Electrode D = 60 ft,
Then, distance of Potential Spike would be 62 % of D = 0.62D i.e. 0.62 x 60 ft = 37 ft