7 CFR 1755.406 - Shield or armor ground resistance measurements.
(a) Shield or armor ground resistance measurements shall be made on completed lengths of copper cable and wire plant and fiber optic cable plant.
(1) The shield or armor ground resistance measurement shall be made between the copper cable and wire shield and ground and between the fiber optic cable armor and ground, respectively. The measurement shall be made either on cable and wire lengths before splicing and before any ground connections are made to the cable or wire shields or armors. Optionally, the measurement may be made on cable and wire lengths after splicing, but all ground connections must be removed from the section under test.
(2) The method of measurement using either an insulation resistance test set or a dc bridge type megohmmeter shall be as shown in Figure 18 as follows:
(1) The shield or armor ground resistance measurements may be made using an insulation resistance test set, a dc bridge type megohmmeter, or a commercially available fault locator.
(2) The insulation resistance test set should have an output voltage not to exceed 500 volts dc and may be hand cranked or battery operated.
(3) The dc bridge type megohmmeter, which may be ac powered, should havescales and multipliers which make it possible to accurately read resistance values of 50,000 ohms to 10 megohms. The voltage that is applied to the shield or armor during the test should not be less than “250 volts dc” nor greater than “1,000 volts dc” when using an instrument having adjustable test voltage levels.
(4) Commercially available fault locators may be used in lieu of the above equipment, if the devices are capable of detecting faults having resistance values of 50,000 ohms to 10 megohms. Operation of the devices and method of locating the faults should be in accordance with manufacturer's instructions.
(1) For all new copper cable and wire facilities and all new fiber optic cable facilities, the shield or armor ground resistance levels normally exceed 1 megohm-mile (1.6 megohm-km) at 68 °F (20 °C). A value of 100,000 ohm-mile (161,000 ohm-km) at 68 °F (20 °C) shall be the minimum acceptable value of the shield or armor ground resistance.
(2) Shield or armor ground resistance varies inversely with length and temperature. In addition other factors which may affect readings could be soil conditions, faulty test equipment and incorrect test procedures.
(3) For the resistance test method and dc bridge type megohmmeter, the ohm-mile (ohm-km) value for the shield or armor ground resistance shall be computed by multiplying the actual scale reading in ohms on the test set by the length in miles (km) of the cable or wire under test.
(i) The objective shield or armor ground resistance may be determined by dividing 100,000 by the length in miles (161,000 by the length in km) of the cable or wire under test. The resulting value is the minimum acceptable meter scale reading in ohms. Examples for paragraphs (d)(3) and (d)(4) of this section are as follows:
Equation 1. Test Set: Scale Reading * Length = Resistance-Length
75,000 ohms * 3 miles = 225,000 ohm-mile
(75,000 ohms * 4.9 km = 367,000 ohm-km)
Equation 2. 100,000 ohm-mile ÷ Length = Minimum Acceptable Meter Scale Reading
100,000 ohm-mile ÷ 3 miles = 33,333 ohms
(161,000 ohm-km ÷ 4.9 km = 32,857 ohms)
(ii) Since the 33,333 ohms (32,857 ohms) is the minimum acceptable meter scale reading and the meter scale reading was 75,000 ohms, the cable is considered to have met the 100,000 ohm-mile (161,000 ohm-km) requirement.
(5) Due to the differences between various jacketing materials used in manufacturing cable or wire and to varying soil conditions, it is impractical to provide simple factors to predict the magnitude of variation in shield or armor to ground resistance due to temperature. The variations can, however, be substantial for wide excursions in temperature from the ambient temperature of 68 °F (20 °C).
(e) Data record. The data shall be corrected to the length requirement of ohm-mile (ohm-km) and a temperature of 68 °F (20 °C) and shall be recorded on a form specified in the applicable construction contract.
(1) When results of resistance measurements are below the 100,000 ohm-mile (161,000 ohm-km) requirement at 68 °F (20 °C), the jacket temperature, soil conditions, test equipment and method shall be reviewed before the cable or wire is considered a failure. If the temperature is approximately 68 °F (20 °C) and soil conditions are acceptable, and a reading of less than 100,000 ohm-mile (161,000 ohm-km) is indicated, check the calibration of the equipment; as well as, the test method. If the equipment was found to be out of calibration, recalibrate the equipment and remeasure the cable or wire. If the temperature was 86 °F (30 °C) or higher, the cable or wire shall be remeasured at a time when the temperature is approximately 68 °F (20 °C). If the test was performed in unusually wet soil, the cable or wire shall be retested after the soil has reached normal conditions. If after completion of the above steps, the resistance value of 100,000 ohm-mile (161,000 ohm-km) or greater is obtained, the cable or wire shall be considered acceptable.
(2) When the resistance value of the cable or wire is still found to be below 100,000 ohm-mile (161,000 ohm-km) requirement after completion of the steps listed in paragraph (f)(1) of thissection, the fault shall be isolated by performing shield or armor ground resistance measurements on individual cable or wire sections.
(3) Once the fault or faults have been isolated, the cable or wire jacket shall be repaired in accordance with§ 1755.200, RUS Standard for Splicing Copper and Fiber Optic Cables or the entire cable or wire section may be replaced at the request of the borrower.
[62 FR 23998, May 2, 1997]
Title 7 published on 2015-01-01.
No entries appear in the Federal Register after this date, for 7 CFR Part 1755.