This practical guide to earth / ground resistance testing contains procedures commonly practiced by electrical contractors, electricians and maintenance workers of power utilities.
The information given uses simple language for easy understanding by the users and it is intended to be an educational tool.
Specific procedures may vary with each task and must be performed by qualified people.
Kyoritsu Electrical Instruments Works, LTD informs that this guide is not a substitute for any International, National or Local Standard, which should always be consulted in case of doubt.
Kyoritsu Electrical Instruments Works, LTD is not liable for any claims, damages or losses, including property damage or personal injury incurred on this practical guide.
1. What is Earthing / Grounding?
The planet Earth’s top layer surface is covered with soil and rocks. This layer is usually associated with agricultural use or excavations for construction
foundations. Nonetheless it has an important electrical property, called conductivity (or low resistance) that forms an integral part of many modern day installations in industrial plants and utilities for a variety of reasons.
The Earth is a relatively poor conductor of electricity compared to conductors like copper or aluminum cables. However considering that the Earth’s mass is huge, it provides a large area for the flow of current, and thus resistance to current can be quite low with the result that the earth can be considered a good conductor.
In fact, the whole of the planet may thus be considered as an infinite conductor which is at reference (zero) potential. In Europe it is referred to ‘earth’ whilst in the USA it is called ‘ground’.
People are usually constantly in contact with earth, thus if they touch a charged object whose potential is different from earth, the resulting potential difference across the person will result in an electric shock.
The process of earthing consists of connecting together all objects that may
potentially become charged to the general mass of earth, so as to provide a path for fault currents and to create an equipotential on all objects as close as possible to the earth potential.
In short, an earthing system will
-prevent a potential difference between earth and earthed parts, hence eliminating the risk of shock and
-provide a low path resistance to fault currents thus ensuring that circuit protective systems (e.g. fuse, circuit breaker, residual circuit breaker) can operate.
Earthing/Grounding is thus an indispensable part of an electrical system to ensure it’s safety and integrity.
2. How earthing prevents electrical shocks and fault?
The internal live parts and outside metal chassis of electrical equipment are
insulated and provide protection against electrical shock by direct contact.
In case of an insulation fault, the equipment’s metal chassis potential becomes that of the supply voltage, and hence a potential difference with respect to ground (0V) is created. Thus if there is no proper earthing and protection, when a human being touches this chassis, as a result of the potential difference across the body, a current will pass from the chassis to earth via the human body causing a potentially lethal electric shock. (Human body can be also considered as a conductor with resistance of approx 1000 / 3000Ω)
If the metal case is earthed, the fault current flow is divided. Thus the majority of the fault current flows through the earth protective conductor and into the ground since the earth path resistance is designed to be much lower than the resistance of the human body. (N.B. Current flow is inversely proportional to the resistance value). Thus only a small amount of harmless current flows through the body.
Automatic disconnection of the power supply is required where a risk of harmful physiological effects to a person may arise due to a fault as a result of the value and duration of a dangerous touch voltage.
The proper coordination and design of earth system (or fault impedance) together with the protective device (RCD, MCB, Fuse) will ensure the automatic disconnection of the power supply.
3. What is Earth resistance?
There are 3 combined elements that compose earth resistance:
1. Resistance of earth wire and the earth electrode
2. Contact resistance between the earth electrode and the ground
3. Ground resistance or better the earth resistivity intended as the characteristic of the earth.
Analyzing these 3 elements, the value of element
1. is normally very small and it can be ignored, but of element
2. varies depending on the material, shape and installed depth of the electrode. Meanwhile, the value of element
3. has the greatest effect on the earth resistance value.
The soil composition, temperature and moisture content all have an impact on the earth resistivity. The schematic below shows the effect of soil composition, temperature and moisture on earth resistance.
As a general rule, it is recommended that an earth electrode is placed as deep as possible into the ground, ideally in humid soil , to lower the earth resistance to a minimum level. In addition, the electrode should be installed where there is a stable temperature regardless of seasons change, i.e. below the frost line.
4. Earthing systems: applications
Earth systems are used in various ways. Their purpose is not limited to the
protection of life by providing a safe path for the escape of fault and leakage
currents as mentioned in Chapter 2.
For example, in case the insulation between the primary (6 to 22kV depends on country) and the secondary (100/ 230/400V depends on country) of a distribution transformer is degrading, a dangerous high voltage could appear on the secondary side. In such case, an electrical load connected to the secondary side will be damaged and will expose persons to electrical shock hazards and fire.
A common prevention measure is to connect one end of the secondary side of the distribution transformer to earth (System Earth). In this way, the high voltages can have a path to the ground and the low voltage on the secondary side is kept safe.
A circuit breaker sensitive to such faults, will be activated thus disconnecting the power line on the primary side.
However earthing systems are also used to prevent static electricity, for lightening rods, soundproofing purposes, etc. The below table shows some earthing applications.
5. Principle of Earth resistance measurement
Most earth resistance testers carry out measurements based on the
“drop-of-potential” method.
A simple way to measure the earth resistance is to drive an auxiliary earth electrode C, into a point distant from the grounded earth electrode under test E. An AC voltage V is applied between the two electrodes, dividing the AC voltage applied by the current that flows between the electrodes E and C. The earth resistance value R is given by the formula:
However, the earth resistance value obtained, R, includes the earth resistances of electrode E under test but also the resistance of the auxiliary electrode C.
When observing the potential distribution curve (in the above figure), a flat portion can be observed. This corresponds to the drop-of-potential due to the earth resistance of the earth electrode E.
In order to measure only the earth resistance of the electrode under test E, another auxiliary electrode P is driven into the ground between electrode E and C and a voltmeter is used to measure the potential across P-E , that is Vp.
Then the earth resistance RE value is given by the formula:
Electrodes such as P and C driven into the ground for measurement are called as auxiliary earth electrode. (Electrode P is called Potential electrode and C Current electrode.)
The reason why an AC current is used for earth resistance measurements is
because a DC current would cause a chemical reaction*, similar to water
electrolysis, with moisture in the soil and gradually blocking the flow of DC current.
* Bubbles of hydrogen and oxygen arise around electrodes.
The test current frequency employed by earth testers are frequency bands other than the commercial ones (16Hz, 50Hz, 60Hz, 400 Hz) in order to reduce noise effects during testing.
Modern electronic earth testers are almost immune to noise effects by using
special hardware and software filters, including the automatic selection of the test current frequency.
6. Items necessary for Earth testing
The basic equipment for earth measurement is:
1. Earth tester,
2. Auxiliary electrodes (2 pcs) and
3. Test leads (3 pcs). In addition to these items, a simplified test probe
4. Cord reel for long test leads
5. 20m in length may be helpful.
7. Method for the Earth measurement of an earth electrode
Drive the auxiliary electrodes P and C into the ground on a straight line
from the earth electrode under test E spacing them a minimum of 5 to 10 meters. If this is not possible due to the presence of obstacles, the auxiliary electrode P should be positioned on a line not diverging more than 30 degrees from the line between the earth electrode E and the auxiliary electrode C.
Notes:
-The ground into which the auxiliary earth electrodes are driven should be as humid as possible.
-High earth resistance of auxiliary earth electrode can impair the measurement accuracy, in such a case the Earth tester should indicate the problem. If the ground is gravel or sand, or if it is dry, a sufficient amount of water should be poured near the electrodes to ensure the ground has
sufficient humidity.
-If the auxiliary earth electrodes cannot be driven into the ground, such as on
concrete surfaces, lay down the electrode on the ground and pour water or place a wet cloth on the electrode to ensure good contact.
-Measurements cannot be performed if the ground is asphalt since it is a sort of insulator and no current can flow on it.
8. Method for the Earth measurement of a large earthing system
For large sized earth systems, for example those formed by multiple earth electrodes on a wide area, some particular precautions are necessary.
First of all, the earth tester used shall have the highest test current as possible to ensure good accuracy on low resistance measurements that are typical for large earthing systems.
The driven auxiliary earth electrode C should be placed as far from the earthing system as practical, with this distance being at least equal to the estimated diagonal of the earthing system.
The auxiliary earth electrode P is then driven in at a number of points roughly on a straight line between the earthing system and C. The ensuing resistance readings should be logged for each of the points and then drawn on a curve of resistance vs. distance.
The correct earth resistance value is usually obtained from the flat part of curve. See the below figure.
Notes and safety warnings:
-To obtain an ideal earth resistance curve at least 10 readings should be taken at equally spaced intervals.
– True resistance will be obtained where the curve flattens out (typically around the 62% of distance D).
-This method gives a correct value of earth resistance if the earth resisitivity and the soil conditions at the flat point do not vary at other points (assuming there are no other measuring errors).
-Localized reading deviations can be caused by buried metallic objects such as pipes or inhomogeneous soil around construction sites.
– If the curve obtained does not show a flat point, the measurements should be retaken placing the auxiliary earth electrode C at a further distance.
– If the expected earth resistance value is very low, say, lower than 1 or 2 ohms, it is recommended to use earth testers with higher test current since the higher test current creates a greater voltage drop that is more measurable.
-The earthing system under test has to be temporarily disconnected from the main installation. To avoid possible risk of electric shock while disconnecting the MEC (Main Earth Conductor), switch off the power supply prior to implementing the temporary measures. The power supply should only be restored after the temporary measures have been removed.
9. Simplified measurement with 2-pole measurement method
This method is useful when the auxiliary earth electrodes cannot be driven into the ground and when an estimation of earth resistance is acceptable. Thus instead of the auxiliary earth electrodes one uses existing earth systems (with a sufficiently low earth resistance) such as:
common earth for commercial power supply, buried metal pipes like main water pipe a lightning protection electrode on buildings
With this method, P and C terminals of the earth tester should be shorted together. Connect the P terminal to the existing earth system and E terminal to the earth electrode to be measured.
Then measure the earth voltage and also the earth resistance.
When using the simplified measurement method, the resistance re of the existing earth system to which P terminal is connected is added to the resistance Rx of the electrode under test E, and displayed as the measured result.
Re (measured value) = Rx + re
If the value of re is already known, deduct it from the measured value Re
to determine Rx value.
Rx= Re – re
Example of simplified measurement using the common earth of commercial supply.
Notes and safety warnings:
-The earthing system under test must be far enough away from the earthing system of a commercial supply to be outside its sphere of influence.
-The earth resistance re is normally very low since the earthing system of a commercial supply is usually bonded to the earthing system of the other power transformers (so called common earth).
Given this situation the earth tester can be practically used to measure the resistance of a simple earthing system Rx.
-The MEC (Main earth conductor) has to be temporarily disconnected.
-To avoid possible risk of electric shock due to voltage difference between Main earth bar (MEB) and MEC / Earthing system and neutral conductor switch off the power supply prior to implementing the temporary measures. The power supply should only be restored after the temporary measures have been removed.
Notes and safety warnings:
-The above figure illustrates the measurement of an earthing system in an installation where a metallic water pipe originating from an extensive underground system is available.
-The earth electrode under test must be far enough away from the water pipe to be outside its sphere of influence.
Therefore, under these circumstances, the reading obtained using the Earth tester will practically indicate the resistance of the simple earthing system Rx under test.
-The MEC (Main earth conductor) has to be temporarily disconnected.
-To avoid possible risk of electric shock due to voltage difference between Main earth bar (MEB) and MEC / Earthing system and metallic water pipe, switch off the power supply prior to implementing the temporary measures. The power supply should only be restored after the temporary measures have been removed.
10. Earth resistivity measurements
The most complete Earth testers offer also earth resistivity measurement, which is defined as the resistance of the soil/ground shaped as a cube of 1 x 1 x 1 meter (1m3).
As we already explained, Soil resistance value depends on the nature of the soil and the percentage of water contained in it.
The below figure shows the values of earth resisitivity for different soil types.
Soil resistance value depends on the nature of the soil and percentage of water contained.
The earth resistivity measurement is useful for soil surveys to establish the optimum design, depth and site location of earth electrode system. Such surveys are made, for example, when a new electrical generating station, substation, transmission tower, telecommunication station or tower is under construction. Without such surveys, extra cost of re-working electrode installations may be needed after the construction is finished.
The earth resistivity measurement may be used to indicate the degree of corrosion to be expected in underground pipelines for water, oil, gas, gasoline, etc. In general, corrosions tend to increase where there are spot areas with low resistivity values. This same kind of information is a good guide for installing cathodic protections for underground metallic pipelines.
Finally Earth resistivity measurements can be used conveniently for geophysical investigations.
For instance to locate minerals, clays, and water-bearing gravel beneath the earth’s surface, to determine depth to bed rock and thickness of glacial drift.
11. Principle of Earth Resistivity measurements
The earth testers designed for earth resisitivity measurement have 4 terminals and 4 auxiliary earth electrodes.
According to the Wenner 4-pole method, apply AC current “I” between the “E” (earth electrode) and “H(C)” (current electrode) to find out the potential difference “V” between the two potential electrodes “S(P)” and “ES”. To obtain the earth resistance “Rg (Ω)”, divide the potential difference “V” by AC current “I”; where the distance between electrodes is “a” (m). Then use the formula: ρ = 2 π x a x Rg (Ωm).
Regarding the connections, stick all the 4 auxiliary Earth electrodes into the ground at the same interdistance a [m]. Note: The depth should be 5% or less of a.
With advanced earth resisitivity testers, that use the above mentioned formula, the Earth Resistivity ρ measurement is automatically calculated and shown on the instrument display.
12. Measurement principle of Clamp Earth tester
An earth clamp tester can be used for measuring the earth resistance of a single earth electrode if it is connected to a multiple-earth system, where many earth electrodes are connected in parallel.
This can be made without using any auxiliary earth electrodes and without disconnection of the single earth electrode from the rest of the installation.
Let’s regard earth resistance under test as Rx, and the other earth resistances as R1, R2, …Rn, see the below figure.
Normally in a distributed power line system as in the above figure, the earth electrodes R1, R2 Rn can be considered as resistors connected in parallel.
The total earth resistance (Rs) of this circuit will usually be very small compared to the resistance of a single earth electrode (Rx) because there are many electrodes in parallel.
Following is an equivalent circuit diagram of this circuit.
If we consider this equivalent circuit diagram, when the voltage injection transformer CT1 of the earth clamp meter induces a voltage V on the conductor joining the two resistances, a current I will flow through the conductor and the earth resistance Rx and Rs.
The amount of current I flowing is in inversely proportional to resistance R (combined resistance: Rx+Rs).
Such current can be measured by the detection current transformer CT2 and then the value of R can be obtained by Ohm’s low calculation.
The resultant R can be considered equal to the Rx under test since the Rs can be negligible enough against Rx.
Kyoritsu Earth Clamp Kew 4200/4202 are instruments that includes the voltage injection transformer CT1, the current detection transformer CT2 and all the necessary electronics for obtaining the measurement result in ohms.
13. Limits of Kew 4200/4202 Earth Clamp tester
Earth clamp tester Kew 4200/4202 cannot be used for earth measurements in following situations.
●Single earth systems (isolated from the other earth systems) like in many TT systems.
●Earth systems on which large fault current (over 2A) flows (This current can be checked by AC current range of our Kew 4200/4202)
●Earth systems with earth resistance values larger than 1500Ω
●When earth resistance under test is smaller than the total earth resistance (very rare case).
14. Practical applications of Earth clamp tester.
Illustrated below are applications where the earth clamp tester is most suitable to be employed.
Earth resistance measurement of a pole earthing electrode:
Earth resistance measurement of a pole earthing electrode in Railway:
Earth resistance measurement of an earthing electrode in a street lighting system:
Earth resistance measurement of an earthing electrode in a lightning protection system:
Earth resistance measurement of a simple earthing system using the main water pipe:
The above figure illustrates the measurement of an earthing system in an installation where a metallic water pipe originating from an extensive underground system is available. However the main earthing conductor (MEC) has to be temporarily disconnected and a temporary link between the water pipe and the earthing system introduced. (Refer to safety warnings below).
Therefore, under these circumstances, the reading obtained using the Earth Clamp indicates the resistance of the simple earthing system under test.
Safety Warnings!
To avoid possible risk of electric shock due to voltage difference between:
-Main earth bar (Meb) and Mec
-Earthing system and metallic water pipe
switch off the power supply prior to implementing the temporary measures. The power supply should only be restored after the temporary measures have been removed.
Earth resistance measurement of a simple earthing system using the Neutral conductor:
The above figure shows a possible field application when a very low Ro is present. Ro is normally very low since the earthing system of a transformer is usually bonded to the earthing system of the other transformers.
Given this situation the Earth Clamp can be used to measure the resistance of a simple earthing system. However the Mec has to be temporarily disconnected and a temporary link introduced between the neutral conductor and the earthing system (Refer to safety warnings below).
Therefore, under these circumstances, the reading obtained using the Earth Clamp indicates the resistance of the simple earthing installation under test.
Safety Warnings!
To avoid possible risk of electric shock due to voltage difference between:
-Main earth bar (Meb) and Mec.
-Earthing system and neutral conductor
switch off the power supply prior to implementing the temporary measures. The power supply should only be restored after the temporary measures have been removed.
15. Work safely!!!
Your safety depends on the combination of the right tools and your safe Work Practices
There is no tool that can guarantee your safety if you do not follow a safe work
procedure.
Here are a few tips to help you in your work:
– Work on de-energized circuits (dead circuits) whenever possible.
– Use proper specific practices and procedures of Lockout/Tag out to safeguard against unexpected energisation or start-up of machinery or equipment, or the release of hazardous energy during service or maintenance activities.
– If the above procedures are not in place or not enforced, consider the circuit as energized or “live”.
– On live circuits, use the following protective gear:
Wear insulated gloves.
Wear safety glasses or better, a face shield.
Use insulated tools.
Remove watches, bracelets or other jewellery.
Stand on an insulated mat or insulating footboard.
Wear specific flame resistant clothing.
16 Kyoritsu Earth tester Line-up
Kyoritsu reserves the rights to change the information described in this guide without prior notice and without obligations.
No part of this guide may be reproduced or utilised in any form or by any means without the permission from Kyoritsu.
