Jeff Jowett - Applications engineer
Electrical grounding is installed primarily for safety reasons but also for the efficient functioning of electrical systems and installed equipment. The grounding system diverts unwanted currents (fault currents) safely into the ground and away from persons and equipment to protect against potential electrocution and fire.
Grounding also diminishes noise and establishes a firm zero reference for voltage, thereby aiding proper and efficient functioning of electrical equipment. These core functions are well known and are commonly implemented by permanent grounding structures in the soil, from a simple rod for a residential ground to a complex and extensive grid underlying a power substation or commercial facility.
However, there are less-known but equally important protective functions of grounding. One of these is HAZMAT (hazardous materials) grounding during transportation. This covers highways and railways in normal and accident situations. Performance isn’t the issue; it’s all about safety. The grounding electrode isn’t normally a permanent part of a larger electrical system, but often just a rod that’s hastily installed in a race against time.
Tanker trucks and tank cars on railways can carry volatile and potentially dangerous materials and they may become involved in accidents. When this happens, the previously well-protected dangerous material can become instantly exposed to potentially catastrophic hazards. One of the worst of these is ignition. Volatile materials can readily catch fire or explode. Given tank-car quantities, the ensuing conflagration can wipe out a small town. The responsibility for averting or successfully containing such potential disasters usually falls upon the local fire department.
A major culprit for the ignition of volatile materials is static electricity. Just picture the shower of sparks flying as a metallic body goes careening down a road surface. Such violent stress in a conductive material like a tanker hull can readily cause a separation of charge. Static charge will usually dissipate on its own as electrons flow to reconstitute a neutral state. But if the separation of charge takes place across an air gap, even of miniscule dimension, an arc may occur. If this happens in the presence of volatile material, the heat of the spark can start a monumental chain reaction and trigger a devastating explosion.
Safe, non-volatile dissipation of charge can be readily accomplished by effective grounding. The sooner a low-resistance ground can be established, the better. A heavy-gauge grounding conductor of negligible resistance is attached to a ground rod driven into the earth with the other end connected to the hull of the stricken tanker.
Firefighters and first responders are trained to move the potential spark or arc out ahead of themselves. They would connect the cable to the tanker first and then take it to the earth ground. This is a safety feature to protect firefighters and first responders. Once a good electron path is established, safe equalization of charge can occur through the earth.
Although soil in small quantities is not considered a good conductive material, planet Earth overall is a good conductor, principally because there is so much of it. The important thing is to have the ground rod make an efficient low-resistance contact with the vastness of the surrounding soil. Simply driving a standard rod may provide a good ground, or it may not. The conductive quality of soil varies considerably and is profoundly affected by local conditions, everything from weather to local construction. To be thorough and rigorous, as well as to conform with various authorities such as insurance coverage and local safety codes, you may be required to test the rod.
At crash sites, time is of the essence. Even though a resistance test for the temporary ground rod may be advised or required, it is possible that it will be skipped over for the sake of expediency. To actually perform the task, the obvious method of choice would be the clamp-on test. A clamp-on ground tester is similar to a clamp-on ammeter; the jaws are opened and clamped around the test item, and voilà — there’s the measurement. Sorry, but it’s not that easy.
A clamp-on ground tester has two circuits and two windings in the jaws, one for current and one for voltage. A test current is induced on the grounding system and the voltage drop measured around the circuit. Ohm’s Law is used for the resistance calculation. The technique works well in utility-grounded systems where the multi-grounded neutral provides a convenient low-resistance return. But on an overturned tanker, no such circuit exists. The tester will merely read an open circuit.
A temporary return can be rigged by running a wire back from a metal fence post, but this is a drop-dead or better-than-nothing alternative at best. It wouldn’t fare well under intense third-party scrutiny. A more traditional method is called for: in this case, a standard three- or four-terminal ground tester. Here, in place of windings in a clamp, the voltage and potential circuits are extended out by long wires from terminals on the tester to probes driven into the ground at discrete distances.
These distances are largely dependent on soil conditions and can be hundreds of feet, hardly amenable to a quick test under pressure. The full procedure, as described in IEEE Std. 81, for example, is to graph a series of readings taken at regular intervals as the potential probe is moved toward the current probe. The graph reveals the maximum resistance at the limits of the electrical field around the ground rod, beyond which no additional resistance is encountered. This procedure is called Fall of Potential (FOP) and is regularly described in the literature. Its limitation is obvious: time. For a permanent structure, like a building ground, it’s the method of choice. But for HAZMAT grounding, a quicker method is the order of the day.
Experienced ground testing technicians often take a shortcut around Fall of Potential by merely moving the potential probe back and forth five or ten feet and taking two or three additional readings, as opposed to plotting and graphing the entire distance. The measurement displayed on the test instrument is the soil resistance to the point of placement of the potential probe. The three or four readings collected in this way may vary by a small amount due to localized inconsistencies in the soil, but they can be averaged to get an acceptable measurement.
What one does not want to see are steadily rising numbers as the potential probe is moved away from the ground under test. This would indicate that the maximum resistance that defines the quality of the ground connection has not been reached. The test is being conducted within the electrical field of the test ground, not beyond it. The probes would have to be moved to greater distances and the test rerun.
As every such measurement may not be exactly the same to the last decimal, a degree of operator interpretation is involved in accepting or rejecting the result. Therein lies a possible source of error. To make such a test more objectively reliable, one can go to the Simplified Fall of Potential. The test procedure is basically the same, involving only three measurements. But instead of the operator deciding, a brief mathematical proof separates an acceptable test from a spurious one.
The three readings are taken halfway and at 40 percent and 60 percent of the distance to the current probe. The readings are averaged, and the one that deviates most from the average is expressed as a percentage of the average. This figure is then multiplied by a correction factor of 1.2. The resulting calculation is the percentage accuracy of the average, like an accuracy statement for a meter. The same general rules apply.
For example, the result is 10.2 Ω and the calculated accuracy is 1.5 percent, that’s good. If, however, the accuracy is worse than 10 percent – which is the industry standard – the test should be considered unacceptable and repeated with new probe spacing. Including the accuracy calculation in a test report gives the test objectivity and removes the stigma of possible operator misjudgement in the eyes of third parties.
Taking the reduction of test time one step further by resorting to a single measurement brings us to the familiar 62-percent rule. Seen widely in ground-testing literature, the method consists of placing the potential probe at 62 percent of the distance to the current probe and taking a single measurement. Yes, this actually works, but it is based on ideal test conditions. The theory behind it states that a FOP graph will coincide with the probe position for the perfect measurement at 62 percent (actually 61.8 percent) of the distance to the current probe.
As a complete FOP graph covers all the points from virtually zero at the test ground to some value higher than that of the test ground, because of the superimposed resistance of the current probe, the graph must coincide at some point with the correct measurement. This point is at 62 percent.
However, this isn’t a universal test procedure because it relies on ideal conditions. Among numerous factors, these include soil uniformity, absence of underground objects, and current probe placement at a sufficient distance that its own resistance is not included. These conditions are often not met in practical testing, which requires other methods. Nonetheless, the 62-percent method does have reference in IEEE Std. 81 and in many cases, will yield the correct measurement, more or less by luck. Under the time pressures that often accompany HAZMAT clearance, it may be the best choice.
The ground rod has now been verified, but the grounding conductor that runs from the tank to the rod must also be verified as providing a continuous, low-impedance path. This is relatively easy with a three- or four-terminal ground tester. It does not require an additional piece of test equipment.
Modern testers have a selector switch, allowing the operator to engage the desired number of terminals for the required test. Physical jumpers between terminals are no longer required as they were in the old days; just turn the selector to the two-terminal position and run test leads from the tank to the rod. In seconds, the resistance of the grounding conductor will be displayed and should be less than one ohm. If a high reading appears, take measures to tighten the contact at both ends. Alternatively, replacement of the conductor may be necessary.
The site has now been grounded and static charges dissipated harmlessly into the soil. But the job is not finished. Dangerous materials must be off-loaded into safe containers or tanks. Normally, fluids passing through hoses are considered benign, but in fact, the friction involved can again separate the charge in the hose material, and the site reverts to a dangerous condition with a risk of arcing. To guard against this, hoses have to be electrically tested.
Another piece of test equipment, an insulation tester, is now required. Fortunately, none of the requirements are demanding, and so an economical basic-function tester may be employed. Industry standard is to perform a 500-volt test, end to end. Resistance of the hose material must be low enough to permit movement of charge to counter the effects of the rapidly passing fluid within, so that dangerous separation of charge doesn’t develop on the surface and build voltage to the point of arcing.
Since the hoses are made of insulating material, much higher resistances are involved, and hence the need for greater test voltage. Different materials have slightly different properties, and manufacturers should be consulted on their recommendations. Resistance must be adjusted to hose length, but in general, an industry standard of less than 1 MΩ is recognized.
HAZMAT sites can be volatile and dangerous, but all the procedures are established and available to render sites safe provided that safe working practices are diligently applied.
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