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It's convenient to think of the earth electrode as being surrounded by a series of increasingly-larger 'shells' of soil, through which any earth-fault current will radiate outwards. The resistance of each 'shell' is inversely-proportional to its cross-sectional area. The shell immediately surrounding the earth electrode (1, in the following diagram) will have the smallest cross-sectional area and, therefore, the greatest resistance value; as we more further and further away from the earth electrode, each subseqent shell (2, 3, 4, etc.) increases in cross-sectional area and, therefore, reduces in resistance.
The further we move away from the earth electrode, the difference between the size of each shell becomes less and less significant and, therefore, the resistance between adjacent 'shells', will become less and less until, eventually, the increase in resistance, too, will become negligible.
The resistance of each of these 'shells' is, of course, cumulative. So, while the rate of change in resistance is greatest in the area immediately surrounding the earth electrode, the cumulative resistancecontinues to increase until it eventually become more-or-less constant. And it is this constant value that is taken as being the resistance of the earth electrode.
In practice, we cannot use an ordinary ohmmeter to measure the resistance of the earth electrode. Instead, the basis of the test is actually as follows.
A small spike is laid out in a straight line radiating away from the earth electrode. The resistance is then measured between the earth electrode and the spike, using an earth megger. The test is repeated several times, with the spike moved further and further away from the earth electrode. A graph drawn from the results shows a curve which is steepest (representing the greatest rate of change of resistance) where the test spike is closest to the earth electrode, and which eventually becomes horizontal (no further rate of change of resistance). The cumulative resistance increases, until there is no further significant increase in resistance, and this value is taken as the earth-electrode's resistance. The same results will be seen in whichever radial direction the resistance is measured, relative to the earth electrode. The area, immediately surrounding the earth electrode, in which the resistance value changes is termed its 'resistance area'.
For the UK, the wiring regulations, BS 7671:2008, specifies that the value of the earth-electrode resistance must be 'low enough to ensure that the potential of any exposed metalwork, with respect to earth, during an earth fault does not exceed 50 V for normal, dry, conditions'.
The 'On-Site Guide', a supplement to BS 7671:2008, further specifies (section 10.3.5) that the earth-electrode resistance should 'in any event, not exceed 200 Ω'.
What else can I help you with?
What is the difference between pipe earthing and plate earthing?
Assuming your reference to "earthing" is grounding (Grounding Electrode Conductor), a pipe is, as it sounds, a pipe (i.e. water pipe). A plate would be, as it sounds, a square (or rectangular) copper plate which would be buried in the ground.
How we get 0.5 ohm resistance for Earth pit?
Type your answer here... how to get ohm in earth pit
Which method of earthing is used in electrical engineering laboratory?
TT system is normally used for consumer power supply.No earthing system provided by power distributor.The owner must install the earthing protection by their own connection to the earth.They must installing a suitable electrode and safe arrangement for which they are responsibility to their installation.
When is earthing required?
As it's a safety factor, earthing is essential and mandatory. It's an electrical connection between the exposed metallic parts of an electrical appliance or installation and the earth, regarded to have zero potential. Proper earthing provides an alternative and easy path for leakage or faulty current to flow. It ensures that any exposed conductive part of the appliance does not reach a dangerous level of potential or voltage that endangers the user's life. A proper earthing system should have least electrical resistance, good corrosion resistance and ability of dissipating high faulty current.
What is the difference between insulation resistance and earth resistance?
insulation resistance can be measured by means of megger, with built in d.c generator. It is measured between the conductor and the earth. earth resistance can be measured by means of earth resistance tester. For good effective earthing, th earth resistance should be of order 0.5 ohm
What is an earthing electrode?
one or more conductive parts that are embedded
What is the standard dept of an electrical earthing pit?
A plate electrode can be installed at a minimum of 24 inches (600mm).
What is the difference between pipe earthing and plate earthing?
Assuming your reference to "earthing" is grounding (Grounding Electrode Conductor), a pipe is, as it sounds, a pipe (i.e. water pipe). A plate would be, as it sounds, a square (or rectangular) copper plate which would be buried in the ground.
What is the difference between neutral earthing resistance and solid earth?
The star point of a primary*-transformer's secondary winding is frequently connected to earth via an impedance, in order to limit the return current in the event of an earth fault close to the transformer. The impedance may be a resistor or an inductor, (choke) depending on the nature of the earth return. In general, if the earth resistance is low, then resistive earthing is used; if the earth resistance is high, then inductive earthing is used.(*e.g. 33/11 kV)For resistive earthing, the transformer secondary's star point is often connected to the centre electrode of a galvanized-steel tank which is solidly earthed. The tank is then filled with water, and it is the resistance of the water that provides the required resistance to earth. The actual resistance of the water is modified to the desired value by dissolving sodium carbonate into the water while measuring its resistance. The solution has a negative temperature coefficient of resistance, which means that, in the event of an earth fault, as the fault current increases, the increasing temperature causes the resistance of the water to decrease, thus accelerating the response time of the protective system.
What is Earthing electrode?
one or more conductive parts that are embedded
How we get 0.5 ohm resistance for Earth pit?
Type your answer here... how to get ohm in earth pit
Which method of earthing is used in electrical engineering laboratory?
TT system is normally used for consumer power supply.No earthing system provided by power distributor.The owner must install the earthing protection by their own connection to the earth.They must installing a suitable electrode and safe arrangement for which they are responsibility to their installation.
Why measurement of resistance of electrode is accurate in ac not in dc?
Assuming you are talking about an earth (or ground) electrode, then applying d.c. results in electrolytic action which separates charges within the soil, giving a false indication of resistance.
Why does an thick earthing wire?
A thick earthing wire, or a wide, flat conductor, is preferred, because it's more important to minimize henrys (inductance) in the earthing system than to reduce ohms (resistance). For the same reason, corners and small angles in an earthing conductor should be avoided, and the conductor should approach any connections from a parallel direction.
What is the difference between chemical earthing and conventional earthing?
Chemical Earthing and Conventional Earthing are different types of earthing techniques. Conventional Earthing is based on plate earthing and pipe type earthing but Chemical Earthing on GI or Copper.
What should be the earth resistance of a good earth?
The lower the resistance you get in the grounding/earth system the better. In offshore a resistance from 1-2 ohm is a good earthing system. In Onshore a miximum of 10ohm is acceptable.
Why should a earthing circuit have low resistance?
Oh, dude, like, you want that earthing circuit to have low resistance so it can quickly divert any unwanted electrical currents to the ground, you know? It's like giving those pesky electrons a speedy highway to escape instead of causing chaos in your appliances or, worse, in you! So yeah, low resistance in the earthing circuit is crucial for safety and keeping things running smoothly.
