Dr. Ahmed El-Rasheed - Product Management
Effective earthing is essential for the safe operation of every electrical system and the only way to ensure that earthing installations are effective and that they remain so, is to test them rigorously and regularly.
The vast majority of power distribution systems are designed so that, if an insulation failure or similar fault occurs, the resulting fault current is diverted to earth. This prevents exposed conductive parts from rising to a dangerous potential, while allowing the fault current to flow for long enough and at a high enough level for protective devices to operate and isolate the fault. It is clear from this description that a reliable, effective earth connection is essential for safe operation of the systems, and that if the earthing system fails or becomes inefficient, at best safety will be compromised and at worst there may well be a significant risk to life and property.
The essential function of every earthing system is to provide a dependable, low resistance connection with the bulk of the earth using one or more earth electrodes, which typically take the form of rods or mats. All earthing systems are designed to achieve this, bearing in mind the requirements of the application, such as the level of prospective earth fault current they may be required to handle. Nevertheless, the efficiency of earth systems is affected by so many hard-to-control variables, such as soil type and moisture content, that it is always essential for the performance of new systems to be verified by rigorous testing during commissioning.
And the requirement for testing doesn’t end with the commissioning tests, as many factors can degrade the performance of earthing systems over time. For example, the moisture content of the soil may change. A good design should have taken into account seasonal variations, but other developments, like a change in level of the local water table, are harder to make allowances for. Electrodes and the connections to them may also be affected by corrosion, and it’s by no means unknown for earthing systems to sustain physical damage, either accidentally, as might occur during nearby building operations, or deliberately, in the form of theft and vandalism.
All of this indicates that there can be no certainty that an earthing system, even though its initial performance was entirely satisfactory, will maintain satisfactory performance over time. Once again, the only way to be sure is to test it and, given the vital safety role of earthing systems, regular routine testing must be seen as essential rather than optional.
It is strongly recommended that testing take the form of a comprehensive structured earthing survey comprising seven key steps. The first of these is a close visual inspection of the earthing installation. This should look for any signs of damage, for earthing conductors that have been broken, cut or have otherwise become disconnected, and for signs of corrosion, not only to the electrodes themselves but also at the connections between the electrodes and the earth conductors. All faults will need to be remedied before proceeding with the subsequent testing steps, but it should always be kept in mind that an earthing conductor that has become disconnected may be live, and it is essential to check for this before touching or handling it.
The second step is to measure leakage currents in the earth conductors. Ideally, there should be no current at all flowing in these conductors but filters and similar devices used in modern electronic equipment often produce a small leakage current even when operating correctly. Of more concern, however, is electrical equipment which develops a fault that allows it to continue operating apparently without problems, but nevertheless results in current flowing to earth. Such equipment may continue in use for long periods, with the operator being unaware of the problem but it is clearly essential to detect such leakage current before carrying out further tests on the earthing system, and the most convenient way to do this is usually to use a clamp meter capable of measuring currents in the milliamp range. If significant current is detected in the earthing conductor the source must be traced and the problem rectified before proceeding further with testing.
A final preparatory test is to electrically test the continuity of the earth conductors to confirm the integrity assessment made during the visual inspection of the system. The purpose of this test is to detect and locate high resistance joints, which are a typical result of corrosion in exposed conductor systems. It is important to bear in mind that, in this context, “high resistance” means anything from a hundred micro-ohms or so upward. Resistance values of this order cannot be measured with an ordinary multimeter, so it is essential to use a low-resistance ohmmeter (also known as a micro-ohmmeter) for this test.
After the visual inspection of the earthing system has been completed, the absence of leakage confirmed and the continuity of the conductors verified, it is necessary – for a full earthing survey – to disconnect the earth electrodes. Under no circumstances must earth connections be broken until the safety implications have been fully evaluated and the appropriate steps taken to minimise risks. This will typically involve de-energising and locking out the equipment, which is to be disconnected from earth, but it is also essential to consider the potential hazards of induced voltages, which may be present in unearthed equipment even when it is not energised.
As an aside, it is worth noting that there are techniques for measuring earth resistance without disconnecting the earth electrodes. These include, for example, ART (attached rod technique) and stakeless testing with clamp-on testers. These techniques are useful, but all have limitations and is universally accepted that testing by the fall-of-potential method, which necessarily involves disconnecting the electrode or electrodes under test, delivers the most accurate and reliable results. For definitive earth resistance surveys, therefore, the fall-of-potential test method should be used.
Figure 1
This test is performed with an earth resistance test set that essentially comprises two circuits, as shown in Figure 1 above. The first circuit includes a voltage source and an ammeter, and it is brought out to the instrument’s current terminals. The second circuit includes only a voltmeter, and is brought out to the instrument’s voltage terminals. One of the current terminals and one of the voltage terminals are connected to the electrode under test. The other current terminal is connected to a temporary earth spike that is inserted into the earth a considerable distance away from the electrode (the current spike), while the other voltage terminal is connected to another temporary earth spike (the voltage spike).
The voltage spike is inserted into the soil at various distances along the straight line between the electrode under test and the current spike and, at each distance, the voltage reading is noted. Since the current is also known, it is then possible to use Ohm’s law to calculate a resistance value for each location of the voltage spike. If the resistances are plotted against distance, the curve should show an almost flat region (see Figure 2 below). The resistance value in this region is the resistance of the earth electrode.
Figure 2
The procedure is necessarily more complex for systems with multiple electrodes or with earthing grids but helpful information covering these situations, and explaining earth testing in much more detail can be found in the publication “Getting Down to Earth”, which is available as a free download from the Megger website.
As part of a comprehensive earthing survey, it is also important to carry out tests to determine touch and step potentials, touch potential being the potential difference a person might experience if they were standing on an earth surface and touched an earthed conductive object while a fault was producing a current flow to earth. Step potential is the potential difference a person would experience between their feet over ground in which a fault current existed.
Touch potential is determined by first measuring the resistance to earth of the object in question, using techniques similar to those used to measure the resistance of an earth electrode. When this resistance is known, along with the maximum prospective fault current, Ohm’s law can be used to calculate the worst-case touch potential with a reasonable margin of accuracy. Step potential is evaluated in a similar way, but when the earth resistance measurement is performed, the voltage spikes are hammered into the ground about 1 metre apart, as this is the approximate length of the average person’s step.
The procedures outlined so far provide invaluable data about the condition and performance of an earthing system, but it is often also useful to know about the properties of the soil in which the earthing system is located. Some of this information is obtained by inspecting and examianing the soil to determine its type, but it is also important to carry out earth resistivity measurements. Note that these measurements relate only to the intrinsic resistivity of the soil, whereas the earth resistance measurements discussed earlier relate to the resistance of a specific earth electrode (or electrodes).
Earth resistivity testing can usually be carried out using the same instrument used for earth resistance testing, with one proviso: the instrument must be a four-terminal type with the voltage and current connections brought out to separate terminals. Three terminal instruments are not suitable for earth resistivity testing.
Earth resistivity is usually measured using the Wenner method, which involves the use of four temporary earth spikes. The spikes do not need to be moved as part of the testing procedure however – their location and spacing is determined by the depth at which it is required to determine the earth resistivity.
Earthing is a fundamental requirement for the safety of electrical installations but all too often the effectiveness of earthing systems receives scant attention, especially after initial performance has been verified. This is dangerous and unnecessary. As we have seen, the performance of earth systems can be reliably evaluated by adopted structured, step-by-step approach and, while it can be argued that the procedures involved are time consuming and, to a certain extent disruptive, surely this is a small price to pay for protecting human life?
Electrical safety testing is essential to ensure safe operating standards for any product that uses electricity. Various governments and agencies have developed stringent requirements for electrical products that are sold world-wide. Several tests are conducted to check the safety of products. One among that is earth test.
Potentially the most dangerous appliances are Class I appliances (earthed appliances), example: microwave ovens/bench grinders and the like, but also in this category are extension leads. Class I appliances are designed to have a connection to the ground via an earth conductor. This may, or may not, be a suitable low resistance path for electric current to protect personnel and equipment. If this conductor is damaged anywhere then the consequences can be virulent.
The measurement of ground resistance for an earth electrode system should be done when the electrode is first installed, and then at periodic intervals thereafter. This ensures that the resistance-to-ground does not increase over time. The International Electrical Testing Association specifies ground electrode testing every three years for a system in good condition with average up-time requirements.
Poor grounding not only increases the risk of equipment failure; it is dangerous too. Facilities need to have adequately grounded electrical systems so in the event of a lightning strike, or utility overvoltage, current will find a safe path to earth. Although the ground system when initially installed had low earth ground resistance values, the resistance of the grounding system can increase if the ground rods are eaten away by corrosive soils with high moisture content, high salt content, and high temperatures.
If our technician finds an increase in resistance of more than 20 percent, we will investigate the source of the problem and make the correction to the grounding system to lower the resistance.
These factors emphasize the importance of a continuous, periodic program of earth-resistance testing. It is not enough to check the earth resistance only at the time of installation.
Resistance to earth can vary with changes in climate and temperature. Such changes can be considerable. An earth electrode that was good (low-resistance) when installed may not stay that way; to be sure, you must check it periodically. We cannot tell you what your maximum earth resistance should be. For specific systems in definite locations, specifications are often set. Some call for 5 Ω maximum; others accept no more than 3 Ω. In certain cases, resistances as low as a small fraction of an ohm are required.
Nature of an Earth Electrode Resistance to current through an earth electrode actually has three components:
Electrode Resistance: Rods, pipes, masses of metal, structures, and other devices are commonly used for earth connections. These are usually of sufficient size or cross-section that their resistance is a negligible part of the total resistance.
Electrode-Earth Contact Resistance: This is much less than you might think. If the electrode is free from paint or grease, and the earth is packed firmly, contact resistance is negligible. Rust on an iron electrode has little or no effect but if an iron pipe has rusted through, the part below the break is not effective as a part of the earth electrode
Resistance of Surrounding Earth: An electrode driven into earth of uniform resistivity radiates current in all directions. Think of the electrode as being surrounded by shells of earth, all of equal thickness. The earth shell nearest the electrode naturally has the smallest surface area and so offers the greatest resistance
The resistance to earth of any system of electrodes theoretically can be calculated from formulas based upon the general resistance formula:
R = ρ LA
Where ρ is the resistivity of the earth in ohm-cm, L is the length of the conducting path, and A is the cross-sectional area of the path. All such formulas can be simplified a little by basing them on the assumption that the earth’s resistivity is uniform throughout the entire soil volume under consideration.
Soil Resistivity Testing:
Wenner four-pole equal method [19] has been considered in measuring the soil resistivity. The correct design of an earthing system is dependent upon detailed knowledge of the local ground resistivity. This is measured as a function of depth at a series of locations around the site, using an expanding four electrode Wenner array (BS EN 50522). The procedure is known as soil resistivity or earth resistance testing. Correct measurement is particularly important in areas of high resistivity ground, where electrical currents are not able to dissipate. In these conditions obtaining an earth can be problematic, and information on ground resistivity is required to much greater depths for the successful installation of an earthing system.
Fall-of-Potential Method:
With a four-terminal tester, P1 and C1 terminals on the instrument are connected to the earth electrode under test. With a three-terminal instrument, connect X to the earth electrode. Although four terminals are necessary for resistivity measurements, the use of either three of four terminals is largely optional for testing the resistance of an installed electrode. The use of three terminals is more convenient because it requires one lead to be connected. The trade-off is that the resistance of this common lead is included in the measurement. Normally, this effect can be minimized by keeping the lead short, to accommodate simple test requirements. The small additional resistance thus introduced is negligible. When performing more complex tests or meeting stringent requirements, however, it may be better to use all four terminals by a lead from the P1 terminal to the test electrode (connecting it inside the lead from C1). This is a true four wire test configuration which eliminates all lead resistance from the measurement.
The added accuracy may prove significant when meeting very low resistance specifications or using test methods that necessitate an extra digit of measurement in order to meet the mathematical requirements. The decision is optional, based on the operator’s testing goals and the method used. The driven reference rod C should be placed as far from the earth electrode as practical; this distance may be limited by the length of extension wire available, or the geography of the surroundings. Leads should be separated and “snaked,” not run close and parallel to each other, to eliminate mutual inductance. Potential-reference rod P is then driven in at a number of points roughly on a straight line between the earth electrode and C. Resistance readings are logged for each of the points.
Dead Earth Method:
When using a four-terminal instrument, P1 and C1 terminals connect to the earth electrode under test; P2 and C2 terminals connect to an all-metallic water-pipe system. With a three-terminal instrument, connect X to the earth electrode, P and C to the pipe system. If the water system is extensive (covering a large area), its resistance should only be a fraction of an ohm. You can then take the instrument reading as being the resistance of the electrode under test. The dead earth method is the simplest way to make an earth resistance test. With this method, resistance of two electrodes in series is measured — the driven rod and the water system. But there are three important limitations:
Under these circumstances, if conditions 1 and 2 above are met, you can connect to the water-pipe system and obtain a suitable earth electrode. As a precaution against any possible future changes in the resistance of the water-pipe system, however, you should also install an earth electrode.
Clamp-on Method:
Fall-of-potential testing, and its modifications, is the only ground testing method that conforms to IEEE 81. It is extremely reliable, highly accurate and can be used to test any size ground system. Additionally, the operator has complete control of the test set-up and can check or proof his/her results by testing at different probe spacing. Unfortunately, the Fall of Potential method also comes with drawbacks:
The clamp-on ground testing method, although it does not conform to IEEE 81, does provide the operator with the ability to make effective measurements under the right conditions. The clamp-on methodology is based on Ohm’s Law (R=V/I). A known voltage is applied to a complete circuit and the resulting current flow is measured. The resistance of the circuit can then be calculated. The clamp-on ground tester applies the signal and measures the current without a direct electrical connection. The clamp includes a transmit coil that applies the voltage and a receive coil that measures the current.
Selective Measurement Testing:
Selective testing is very similar to the Fall-of-Potential testing, providing all the same measurements, but in a much safer and easier way. This is because with Selective testing, the earth electrode of interest does not need to be disconnected from its connection to the site! The technician does not have to endanger himself by disconnecting ground, nor endanger other personnel or electrical equipment inside a no grounded structure.
When you find that your earth electrode resistance is not low enough, there are several ways you can improve it:
Effect of Rod Size:
As you might suspect, driving a longer rod deeper into the earth, materially decreases its resistance. In general, doubling the rod length reduces resistance by about 40 percent.
Use of Multiple Rods:
Two well-spaced rods driven into the earth provide parallel paths. They are, in effect, two resistances in parallel. The rule for two resistances in parallel does not apply exactly; that is, the resultant resistance is not one-half the individual rod resistances (assuming they are of the same size and depth).
Treatment of the Soil:
Chemical treatment of soil is a good way to improve earth electrode resistance when you cannot drive deeper ground rods because of hard underlying rock, for example. It is beyond the scope of this manual to recommend the best treatment chemicals for all situations. You have to consider the possible corrosive effect on the electrode as well as EPA and local environmental regulations. Magnesium sulfate, copper sulfate, and ordinary rock salt are suitable non-corrosive materials. Magnesium sulfate is the least corrosive, but rock salt is cheaper and does the job if applied in a trench dug around the electrode. It should be noted that soluble sulfates attack concrete, and should be kept away from building foundations. Another popular approach is to backfill around the electrode with a specialized conductive concrete. A number of these products, like bentonite, are available on the market.
Not much information has been collected on the effects of temperature. Two facts lead to the logical conclusion that an increase in temperature will decrease resistivity:
Grounding testers are indispensable troubleshooting tools to help you maintain uptime. It is recommended that all grounds and ground connections be checked at least annually as a part of your normal predictive maintenance plan. Should an increase in resistance of more than 20% be measured during these periodic checks, the technician should investigate the source of the problem and make the correction to lower the resistance by replacing or adding ground rods to the ground system.