Current through Register Vol. 54, No. 45, November 9, 2024
Section 39.69 - Ground resistance(a)Limits. It is recommended that the combined resistance of the ground wires and connections of any grounded circuit, equipment or lightning arrester should not exceed the values given below, if grounded connections made according to § 39.66 (relating to nature of ground connection) will sufficiently limit the resistance. It will frequently be impracticable with artificial grounds to obtain resistances in dry or other high resistance soils as low as the values given below for ordinary soils. In such cases two grounds shall be used as defined in § 39.68 (relating to methods of ground connection), and no requirement shall be made as to resistance. Reference should also be made to § 39.66. The current stated opposite the different resistances in the table is either the current capacity of a circuit from which leakage may occur to the ground circuit, or the continuous current capacity to which the grounded equipment or arrester is limited by design or by automatic cutouts. If the secondary is exposed only through transformer windings, this current capacity shall be that of the primary fuse of the transformer. If the secondary is exposed by the conductors of conflicting or crossing high voltage circuits, the current capacities shall be those of the automatic cutouts in such circuits. The product of the corresponding numbers in the first and second columns shall never be greater than 150, that is, the potential difference due to the stated current is never greater than 150 volts, if connections are made to water pipes. If more than one ground is made on the same circuit, equipment or arrester, in the same vicinity, all such grounds shall be considered collectively in respect to meeting the requirements of this section: Amperes | Water pipe grounds (in ohms) | Artificial grounds ordinary soils (in ohms) |
less than 10 | 15 | 1025 |
10 to 25 | 6 | 25 |
25 and above | less than 6 | 25 |
(b)Checking. The resistance of station grounds shall be checked when made. With artificial grounds this check may be made by measuring the voltage between the grounded point of the circuit, or the grounded frame of the equipment or the grounded point of the lightning arrester and an auxiliary metal reference rod or pipe driven into the ground, while a measured current is flowing through the ground connection and any exposed metal piping or other artificial ground in the vicinity, but not within 20 feet. If the station ground is to water piping, the check may be made with current flowing through the water piping and some independent piping system or artificial ground in the vicinity, but not within 20 feet. The auxiliary road or pipe shall be at least ten feet from any artificial ground or piping systems through which the measured current is made to flow. All ground connections shall be inspected periodically. Ground connections on distribution circuits shall, when installed, be tested for resistance unless multiple grounding to water piping systems is used. This section cited in 34 Pa. Code § 39.61 (relating to applicability); 34 Pa. Code § 39.62 (relating to scope); 34 Pa. Code § 39.63 (relating to modification or waiver of provisions); 34 Pa. Code § 39.65 (relating to ground conductor); 34 Pa. Code § 39.66 (relating to nature of ground connection); 34 Pa. Code § 39.67 (relating to attachment of ground conductor); 34 Pa. Code § 39.103 (relating to protective grounding); 34 Pa. Code § 39.183 (relating to ground wires); 34 Pa. Code § 39.197 (relating to grounding); 34 Pa. Code § 39.219 (relating to grounding or isolating service conduits); 34 Pa. Code § 39.292 (relating to grounding noncurrent-carrying parts); and 34 Pa. Code § 39.304 (relating to grounding of arresters for signaling systems).