GROUND RESISTANCE TESTING IN THE MINING INDUSTRY
Presented at the Annual Technical Conference of the
Ground beds provide safety grounding of mine electrical equipment. The lower the resistance of the ground bed, the better protection it provides. While ground beds may have a low resistance when first installed, corrosion of ground rods, breaks in interconnecting wires, and water table changes can all increase the resistance of the safety ground bed. For this reason it is important that the resistance of the ground bed be measured not only when first installed, but also periodically to ensure that it remains low in value. For many contractors the testing and recording of these measurements has not been completely understood.
The purpose of this paper is to describe a preferred method for measuring ground bed resistance that can give confidence in the value of the resistance obtained. Methods are also presented for lowering the resistance value in high resistivity areas.
A reliable equipment grounding system that connects all the metallic frames of electrical equipment together must be kept at a safe reference potential. Since earth ground is considered to be at zero potential, making an electrical connection to earth is a logical choice. The earth grounding electrode should provide the lowest impedance connection possible to earth and maintain this reference at a low value. The objective is that in the event of a fault to ground, sufficient current will flow through the ground path to allow the protective equipment to operate and isolate the circuit.
In the real world, however, the ground system does have resistance. All ground beds, even the very largest, have some measurable amount of resistance. "Earth resistance" means the resistance of the earth to the passage of electric current. In comparison with metal conductors, soil is not a good conductor of electricity. Resistances in the two to five ohm range are generally found suitable for industrial plant substations, buildings and large commercial installations.
The National Electrical Code requires that made electrodes shall have resistance to ground not to exceed 25 ohms and that where the resistance is not as low as 25 ohms, two or more electrodes connected in parallel shall be used. They should not be less than six feet apart.
"The 25 ohms value noted in the National Electrical Code applies to the maximum resistance for a single electrode. There is no implication that 25 ohms per se' is a satisfactory level for a grounding system."
The Institute of Electrical and Electronics Engineers Standard 142, Recommended Practice for Grounding of Industrial and Commercial Power Systems states:
"The most elaborate grounding system that can be designed may prove to be inadequate unless the connection of the system to the earth is adequate and has a low resistance. It follows, therefore, that the earth connection is one of the most important parts of the whole grounding system. It is also the most difficult part to design and to obtain... For small substations and industrial plants in general, a resistance of less than 5 ohms should be obtained if practicable."
However, from a practical standpoint, no grounding electrode no matter how low its resistance can be depended upon to clear a ground fault. If equipment is effectively grounded as pointed out in the National Electrical Code under 250-51, a path of low impedance (not through the grounding electrode) must be provided to facilitate the operation of the overcurrent devices in the circuit. While the lowest practical resistance of a grounding electrode is desirable and will better limit the potential of equipment frames above ground, it is more important to provide a low-impedance path to clear a fault promptly to ensure safety. To obtain the lowest practical impedance, the equipment grounding circuit must be connected to the grounded conductor within the service equipment.
For maximum safety, one grounding electrode system should be used with everything connected to that grounding system. If multiple grounding electrodes comprise the system, they must be bonded together to form a common grounding electrode.
One topic which needs to be stressed is that the resistance of a ground bed, as shown in figure 1, cannot be accurately measured unless it is isolated from other parallel ground paths. The current generated by a test instrument will be split among all the paths. Therefore, the meter reading on a test instrument will not represent the ground bed resistance accurately. Also, the "effective ground bed" will include the mine, the mill, and the pole line as well as the substation to be tested. The auxiliary current and potential electrodes would have to be miles away to make an accurate measurement on such a large ground bed.
Since many variable factors contribute to the earth electrode resistance, it is not practical to expect a precise or repeatable measurement over different seasons. Such factor as moisture content, soil temperature and dissolved salts may vary considerably from summer to winter. When the moisture content of dry soil is increased by 15% the resistivity can decrease by a factor of 50,000. When water in the soil freezes, the earth resistivity will increase since ice is not a good conductor. They type and grain size of each soil also contributes to the resistance value.
Through research conducted by the U.S. Department of Interior, Bureau of Mines, the most reliable and accurate method for determine the earth electrode resistance was identified as the "fall-of-potential" method.
This method involves passing a current into the electrode to be measured and measuring the voltage between the ground electrode under test and a test potential electrode (P). A test current electrode (C) is driven into the earth to permit passing a current into the electrode to be tested. Potentials are measured with respect to the ground electrode under test which is assumed to be at zero potential.
A graph is then made of the resistance measured with the instrument as a function of potential electrode distances (X). The potential electrode is moved roughly on a straight line from the electrode under test in enough steps to plot a smooth curve. The value in ohms at which this plotted curve appears to flatten out is taken as the resistance value of the earth ground bed under test. This value is usually about 62% of the distance (D) from the electrode under test to the current probe.
The current probe (C) should be far enough away from the electrode under test to be out of the "sphere of influence" of the earth electrode. Usually a distance of five times the rod length is adequate.
There are special instruments which are designed to make earth-resistance measurements simple and straightforward. Most of these instruments adjust a potentiometer until no current exists in the potential electrode at balance and the resistance of the potential electrode and the connecting wiring does not affect the measurement value.