TECH TIPS Specialized Testing for Line Towers By Je f f J owe tt Megger Because the functions are narrowly defined, ground testing instrumentation exists in only a few fundamental designs. The basic tests performed are ground resistance, soil resistivity, and bonding, or the continuity of the grounding conductors connecting equipment to ground. Traditionally designed testers perform two or all three of these tests, depending on whether 3- or 4-terminal models. The latter add soil resistivity to the repertoire; the former do not. Clamp-on testers are a later addition, designed mainly for speed and ease of use. They do not require walking out hundreds of feet with leads and probes in order to connect the terminals to ground. Clamp-ons perform ground resistance and have a limited continuity function (they can indicate if the return circuit is open or high resistance, but have no means to test specific points). They cannot do soil resistivity. Another later addition doesn’t replace either of these technologies but enhances the capabilities of the traditional terminal design. This is the incorporation of a current clamp into a 3- or 4-terminal tester so that test current can be selected (by clamp placement) between various components of a parallel system, and hence the resistance of that component alone can be measured separate from the total resistance of the parallel system of grounds. Earth Ground Tester Kit These few basic choices cover nearly all ground testing applications. But not all. A particularly difficult challenge is presented by the grounding SPRING 2011 of transmission line towers. Grounding is particularly important for lightning protection not only because of the potential for local damage but because the attendant voltage spikes and surges can travel on the lines and provide a source of widespread damage for miles around. But tower grounds present some unique problems to their testing: they are large, there are four of them, and they are paralleled with the whole transmission line through the overhead wire. Towers are grounded at all four legs, and a grounding conductor elevated above the phase conductors connects each tower to its neighbors and is intended to divert lightning strokes harmlessly to ground rather than striking the lower phase conductors. This construction thwarts ground testing by standard means, and will be examined next against each technology. specialized testing for line towers TECH TIPS Performing a ground test by the familiar fallof-potential method, or any of its related procedures that involve extending leads and probes out into the soil, will be defeated by the overhead wire that parallels the entire transmission line. Test current is not confined, as it should be, to the specific tower, but divides down the line of grounded towers. Any attempt at measurement will probably run off the low end of the tester’s range. Disconnecting the overhead, of course, is not a practical option. Neither is clamp-on technology a solution. One of the major benefits of clampon testing is that it uses parallel grounds as part of a minimal-resistance return path for the induced test current, thereby eliminating the problem of having to isolate the ground under test. Tower footing is much too large, however, to be accommodated by the jaws of a handheld tester, which are meant for clamping a rod or conductor. Flexible CTs are available that can encircle the largest footing, but that’s not the issue. All four legs are grounded in parallel. Test current would merely travel to the other legs to complete the feedback loop, producing an essentially meaningless reading. The technology works well for an application like pole grounds because there is only one per pole. Test current must traverse an expanse of soil to the next pole, and on down the line, in order to complete a circuit. For towers, current wouldn’t have to go to the next tower, merely across to the other legs, making the method essentially useless. What to do? The problem appears unsolvable, and it is…by conventional testers. But specialized applications of technology have been developed to deal with this imperative. The prime solution is to employ a highfrequency test signal. Standard ground testers normally utilize a square wave test current at a frequency close to, but a little off from, a prevalent power harmonic. A common example would be 128 Hz. This frequency is close to the second harmonic of a 60 Hz system, but a little offset. The idea is to approximate the frequency of a fault current while at the same time giving the tester something distinctive to recognize as specialized testing for line towers its own signal. Thereby, interference from other sources will be avoided. Clamp-ons, on the other hand, employ a high frequency, in units of a few kHz. This derives strictly from the design necessities of an oscillator creating an induced test current from a handheld unit, and is not an attempt to simulate any real-world condition on the test item. For specialized tower testing, even higher frequencies are employed than in clamp-ons, in the range of 20-30 kHz. Figure 1 The desired effect of a high-frequency test is to choke the test current to the remote grounds and localize the test onto the designated tower. The variation of Ohm’s law that accounts for impedance shows why. By this rendering, I = E/Z, current equals voltage divided by impedance. So an increase in impedance causes a decrease in current. And inductive impedance increases with frequency. The long runs of overhead ground wire that connect transmission towers in a continuous chain typically offer sufficient inductive impedance to choke off current at frequencies like 20+ kHz. Consequently, the tester will only “see” test current flowing on the specific tower to which it is connected, the desired effect. A ground test can be conducted without disrupting the protective conductor overhead. As an added benefit, the test frequency can also be considered as simulating the frequencies associated with a lightning stroke and thereby giving a better indication of how the tower’s grounding system will perform its primary function, lightning protection. NETAWORLD TECH TIPS Figure 2 A high-frequency tester otherwise operates the same as a general-purpose model, with current and potential terminals to which are connected long leads and probes extended far enough to perform fall-of-potential and all related tests (See Figure 1 on previous page). It represents a vast improvement, but still has some limitations in that while it significantly reduces the effects of adjacent towers, it doesn’t necessarily completely eliminate that contingency. There are uncontrolled variables, including the proximity of neighboring towers and the length of wire connecting them, that could still allow some leakage of test current and introduce some extraneous element into the reading. An extension of the technology incorporates a possibility that has already been suggested: clamping the legs with flexible CTs. A type of instrumentation has been developed that can incorporate four flexible current sensors, one for each tower leg, into a single measuring unit. Flexible CTs up to 30 feet are available, and these will encircle an eight-foot diameter footing. Both the footing and its associated grounding electrode should be encircled. With a quality model, the sensors can be calibrated individually against the measurement module and always used in the same relative positions so as to assure accuracy and repeatability of SPRING 2011 test results. The tester should accommodate multiple turns of the CTs (typically up to four), with a matching selector switch, so as to maximize sensitivity to both test current and leakage current. Just as a high-frequency model performs the rest of its functions in standard four-terminal manner, so does a flexible-sensor model (Figure 2). But the positioning of test connections is critical. One pair of current and potential leads is attached to a tower leg in order to establish test current through the structure and sense the requisite voltage drop. In this application, the tower footing is the item under test, and so one side of the test connections are made to the tower just as they would be to a ground rod. The essential element of this connection is to make it above a CT so that current will be monitored going to ground. For the return side of the test, two probes are driven into the ground on opposite sides of the tower and at a substantial distance (100 feet or more). It is best to install these probes at a right angle to the transmission line where possible in order to minimize interference. The current probe completes the test circuit from the tower leg, through the soil, and back to the tester. The potential probe senses voltage drop. In this manner, the tester behaves as a standard ground tester, but with the specialized positioning of the probes around the tower. If the two probes cannot be positioned on opposite sides, then it is best to rely on the familiar 62 percent rule in locating the potential with respect to the current, at least at a 30° angle off the power line to minimize interference. When the tester is energized, the flexible current sensor measures the current going to ground on a particular leg, and makes its measurement based only on that accommodation. So far, this is essentially the same as the tester-withcurrent-clamp function described in the opening paragraph. There are three more legs, and if only one were clamped, the information would only yield the grounding condition of that leg. This would mean little in the overall evaluation of the tower. But as we have seen, this specialized testing for line towers TECH TIPS highly specialized and sophisticated technology addresses this issue thoroughly. With all four legs encircled simultaneously, a selector switch enables the measurement to be taken on any one of them, any combination, or all four together, from one setup. Thus, the operator is provided a complete look at the effective ground resistance of the tower while still energized and without disconnecting the overhead ground or the counterpoise. Frequency selection and sweep enable assessment of tower condition and the connections to neighboring towers as well. By utilizing power frequency and moving a flexible sensor above the lead connections, current is measured going up the tower and the quality of the bond to the overhead ground wire can be assessed. Measuring current at different frequencies can also indicate the amount of leakage from the system to different legs and aid in evaluating the overall grounding condition of the tower. This advanced technology illustrates how various functions initially intended to serve more general usage can be integrated into a single application to produce previously unattainable results. Next, we will move from instrumentation to technique and examine some procedures for determining earth resistivity. Sources of information: AEMC® Instruments, Dover, NH MEGGER®, Getting Down To Earth Megabras Industria Electronica LTDA, Sao Paulo, Brazil Jeffery R. Jowett is a Senior Applications Engineer for Megger in Valley Forge, Pennsylvania, serving the manufacturing lines of Biddle, Megger, and multi-Amp for electrical test and measurement instrumentation. He holds a BS in Biology and Chemistry from Ursinus College. He was employed for 22 years with James G. Biddle Co. which became Biddle Instruments and is now Megger.