MIL-HDBK-274 Electrical Grounding For Aircraft SafetyThis handbook has been prepared to provide U.S. Navy and Marine Corps aircraft maintenance and operations personnel with the background and technical details of the different types of aircraft grounds and how they are applied. In addition, this handbook provides background information for aircraft grounding, static electricity theory and how it affects aircraft, and techniques used for measurement of grounding points. This handbook is intended for use by all U.S. Navy and Marine Corps aircraft maintenance and operations personnel for the purposes of ensuring that aircraft are properly and safely grounded. This handbook is for guidance only and cannot be cited as a requirement.

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MIL-HDBK-274A

5.2.1 Earth's shell.  Visualize a ground rod being surrounded by shells of  Earth; all of equal thickness (see Figure 8).  The Earth's shell closest to the rod has the greatest resistance due to the fact that it has the smallest surface area.  Each successive shell has a larger area than the previous one and thus less resistance.  Farther out, a point is reached where the inclusion of additional Earth shells have no significant effect on the resistance of the Earth surrounding the rod.  Therefore, a measurement can be made (within a reasonable distance) between the ground rod and a test rod.

5.2.2 Measuring ground points.  In measuring ground points, the measurement is taken of the resistance between the driven ground rod and the main body of the Earth.  If a test connection could be made on the centerline of the Earth (see Figure 9), Ohm's law could again be applied to the circuit.  Since such a connection is impractical, an alternate method should be used.

5.2.3 Resistance of the ground rod.  In reality, the rod has a resistance (Rc) associated with it.  To exclude this resistance from the measurement, a three-terminal test technique called the Three-Terminal Fall of Potential Method is used, as shown on Figure 9.  The use of a potential probe enables a voltage measurement to be made across R.  Using the meter reading for the current through R, we can compute the value of R using Ohm's law.  The resistance of Rp  is very high, a characteristic of all voltmeter probes.

5.3 Measurement equipment.  The following measuring equipment has proven useful in measuring ground resistance in the past: Three-terminal Ground Resistance Tester, Transcat Order No. 8175E, Analog Display, 0.5-500 Ω Range and the Earth Testing Kit, Two or Three- Terminal, Transcat Catalog No. DET3TD, Digital Display, 0.01-199.9 Ω/0.001-19.9 Ω Range. Similar testers by other manufacturers that have equivalent characteristics may also be used. Volt-Ohm Meters (VOMs) have not proven suitable for this task, in past experience.

5.3.1 Null balance Earth tester. The null balance Earth tester includes batteries for its voltage source and an ohmmeter to measure the resistance directly.  The tester generates an AC test current that is passed between the ground rod and the current rod.  AC is used to eliminate errors due to existing direct currents and voltages that may be present in the Earth.  The voltage drop at the potential rod is applied to a bridge circuit and nulled with a three-decade variable resistance.  When at balance (no current flowing through the potential rod), the ground resistance is obtained from the readout of the digital decade switches.  Although other equivalent meters may have different types of power supplies (such as hand-cranked generators) and different readout devices, the principles of operation are still the same.

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