CFR > Title 7 - Agriculture > SUBTITLE B—Regulations of the Department of Agriculture > CHAPTER XVII—RURAL UTILITIES SERVICE, DEPARTMENT OF AGRICULTURE (parts 1700 to 1794) > PART 1755—TELECOMMUNICATIONS POLICIES ON SPECIFICATIONS, ACCEPTABLE MATERIALS, AND STANDARD CONTRACT FORMS > 1755.403—Copper cable telecommunications plant measurements.

1755.403—Copper cable telecommunications plant measurements.

(a) Shield or shield/armor continuity. (1) Tests and measurements shall be made to ensure that cable shields or shield/armors are electrically continuous. There are two areas of concern. The first is shield or shield/armor bonding within a pedestal or splice and the second is shield or shield/armor continuity between pedestals or splices.
(2) Measurement techniques outlined here for verification of shield or shield/armor continuity are applicable to buried cable plant. Measurements of shield continuity between splices in aerial cable plant should be made prior to completion of splicing. Conclusive results cannot be obtained on aerial plant after all bonds have been completed to the supporting strand, multigrounded neutral, etc.
(3) Method of measurement. (i) The shield or shield/armor resistance measurements shall be made between pedestals or splices using either a Wheatstone bridge or a volt-ohm meter. For loaded plant, measurements shall be made on cable lengths that do not exceed one load section. For nonloaded plant, measurements shall be made on cable lengths that do not exceed 5,000 feet (ft) (1,524 meters (m)). All bonding wires shall be removed from the bonding lugs at the far end of the cable section to be measured. The step-by-step measurement procedure shall be as shown in Figure 2.
(ii) Cable shield or shield/armor continuity within pedestals or splices shall be measured with a cable shield splice continuity test set. The step-by-step measurement procedure outlined in the manufacturer's operating instructions for the specific test equipment being used shall be followed.
(4) Test equipment. (i) The test equipment for measuring cable shield or shield/armor resistance between pedestals or splices is shown in Figure 2 as follows:

Code of Federal Regulations

Code of Federal Regulations 503
[Please see PDF for image: ER02MY97.024 ]
(ii) A cable shield splice continuity tester shall be used to measure shield or shield/armor continuity within pedestals or splices.
(5) Applicable results. (i) The shield or shield/armor resistance per 1000 ft and per kilometer (km) for cable diameters and types of shielding materials are given in Table 1 (English Units) and Table 2 (Metric Units), respectively as follows:
Table 1—Shield Resistance @ 68 °F (20 °C) Cable Diameters Versus Shield Types
[English Units]
Outside diameter inches (in.) Nominal resistance ohm/1000 ft.
A B C D E F
0.40-0.49 0.77 1.54 1.65 1.96 2.30 5.51
0.50-0.59 0.64 1.28 1.37 1.63 1.91 4.58
0.60-0.69 0.51 1.03 1.10 1.31 1.53 3.67
0.70-0.79 0.44 0.88 0.94 1.31 3.14
0.80-0.89 0.38 0.77 0.82 1.14 2.74
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0.90-0.99 0.35 0.69 0.74 1.03 2.47
1.00-1.09 0.31 0.62 0.66 0.92 2.20
1.10-1.19 0.28 0.56 0.60 0.84 2.00
1.20-1.29 0.26 0.51 0.55 0.77 1.84
1.30-1.39 0.24 0.48 0.51 0.71 1.70
1.40-1.49 0.22 0.44 0.47 0.65 1.57
1.50-1.59 0.21 0.41 0.44 0.61 1.47
1.60-1.69 0.19 0.38 0.41 0.57 1.37
1.70-1.79 0.18 0.37 0.39 0.54 1.30
1.80-1.89 0.17 0.35 0.37 0.51 1.24
1.90-1.99 0.16 0.33 0.35 0.49 1.17
2.00-2.09 0.15 0.31 0.33 0.46 1.10
2.10-2.19 0.15 0.29 0.31 0.43 1.03
2.20-2.29 0.14 0.28 0.30 0.42 1.00
2.30-2.39 0.14 0.27 0.29 0.40 0.97
2.40-2.49 0.13 0.25 0.27 0.38 0.90
2.50-2.59 0.12 0.24 0.26 0.36 0.87
2.60-2.69 0.12 0.23 0.25 0.35 0.83
2.70-2.79 0.11 0.22 0.24 0.33 0.80
2.80-2.89 0.11 0.22 0.24 0.33 0.80
2.90-2.99 0.11 0.22 0.23 0.32 0.77
3.00-3.09 0.10 0.21 0.22 0.31 0.73
3.10-3.19 0.10 0.20 0.21 0.29 0.70
3.20-3.29 0.10 0.20 0.21 0.29 0.70
3.30-3.39 0.09 0.19 0.20 0.28 0.67
3.40-3.49 0.09 0.18 0.19 0.26 0.63
3.50-3.59 0.09 0.18 0.19 0.26 0.63
3.60-3.69 0.08 0.17 0.18 0.25 0.60
3.70-3.79 0.08 0.17 0.18 0.25 0.60
3.80-3.89 0.08 0.16 0.17 0.24 0.57
3.90-3.99 0.08 0.16 0.17 0.24 0.57
4.00-4.99 0.07 0.15 0.16 0.22 0.53
Where: Column A-10 mil Copper shield.
Column B—5 mil Copper shield.
Column C—8 mil Coated Aluminum and 8 mil Coated Aluminum/6 mil Coated Steel shields.
Column D—7 mil Alloy 194 shield.
Column E—6 mil Alloy 194 and 6 mil Copper Clad Stainless Steel shields.
Column F—5 mil Copper Clad Stainless Steel and 5 mil Copper Clad Alloy Steel shields.
Table 2—Shield Resistance @ 68 °F (20 °C) Cable Diameters Versus Shield Types
[Metric Units]
Outside diameter millimeters (mm) Nominal Resistance ohm/km
A B C D E F
10.2—12.5 2.53 5.05 5.41 6.43 7.55 18.08
12.7—15.0 2.10 4.20 4.49 5.35 6.27 15.03
15.2—17.5 1.67 3.38 3.61 4.30 5.02 12.04
17.8—20.1 1.44 2.89 3.08 4.30 10.30
20.3—22.6 1.25 2.53 2.69 3.74 8.99
22.9—25.1 1.15 2.26 2.43 3.38 8.10
25.4—27.7 1.02 2.03 2.16 3.02 7.22
27.9—30.2 0.92 1.84 1.97 2.76 6.56
30.5—32.8 0.85 1.67 1.80 2.53 6.04
33.0—35.3 0.79 1.57 1.67 2.33 5.58
35.6—37.8 0.72 1.44 1.54 2.13 5.15
38.1—40.4 0.69 1.34 1.44 2.00 4.82
40.6—42.9 0.62 1.25 1.34 1.87 4.49
43.2—45.5 0.59 1.21 1.28 1.77 4.26
45.7—48.0 0.56 1.15 1.21 1.67 4.07
48.3—50.5 0.52 1.08 1.15 1.61 3.84
50.8—53.1 0.49 1.02 1.08 1.51 3.61
53.3—55.6 0.49 0.95 1.02 1.41 3.38
55.9—58.2 0.46 0.92 0.98 1.38 3.28
58.4—60.7 0.46 0.89 0.95 1.31 3.18
61.0—63.2 0.43 0.82 0.89 1.25 2.95
63.5—65.8 0.39 0.79 0.85 1.18 2.85
Code of Federal Regulations 505
66.0—68.3 0.39 0.75 0.82 1.15 2.72
68.6—70.9 0.36 0.72 0.79 1.08 2.62
71.1—73.4 0.36 0.72 0.79 1.08 2.62
73.7—75.9 0.36 0.72 0.75 1.05 2.53
76.2—78.5 0.33 0.69 0.72 1.02 2.39
78.7—81.0 0.33 0.66 0.69 0.95 2.30
81.3—83.6 0.33 0.66 0.69 0.95 2.30
83.6—86.1 0.29 0.62 0.66 0.92 2.20
86.4—88.6 0.29 0.59 0.62 0.85 2.07
88.9—91.2 0.29 0.59 0.62 0.85 2.07
91.4—93.7 0.26 0.56 0.59 0.82 1.97
94.0—96.3 0.26 0.56 0.59 0.82 1.97
96.5—98.8 0.26 0.52 0.56 0.79 1.87
99.1—101.3 0.26 0.52 0.56 0.79 1.87
101.6—103.9 0.23 0.49 0.52 0.72 1.74
Where: Column A—10 mil Copper shield.
Column B—5 mil Copper shield.
Column C—8 mil Coated Aluminum and 8 mil Coated Aluminum/6 mil Coated Steel shields.
Column D—7 mil Alloy 194 shield.
Column E—6 mil Alloy 194 and 6 mil Copper Clad Stainless Steel shields.
Column F—5 mil Copper Clad Stainless Steel and 5 mil Copper Clad Alloy Steel shields.
(ii) All values of shield and shield/armor resistance provided in Tables 1 and 2 in (a)(5)(i) of this section are considered approximations. If the measured value corrected to 68 °F (20 °C) is within #30 percent (%) of the value shown in Table 1 or 2, the shield and shield/armor shall be assumed to be continuous.
(iii) To correct the measured shield resistance to the reference temperature of 68 °F (20 °C) use the following formulae:

Code of Federal Regulations

R68=Rt/[1 A(t−68)] for English Units

Code of Federal Regulations

R20=Rt/[1 A(t−20)] for Metric Units
Where:
R68=Shield resistance corrected to 68 °F in ohms.
R20=Shield resistance corrected to 20 °C in ohms.
Rt=Shield resistance at measurement temperature in ohms.
A=Temperature coefficient of the shield tape.
t=Measurement temperature in °F or ( °C).
(iv) The temperature coefficients (A) for the shield tapes to be used in the formulae referenced in paragraph (a)(5)(iii) of this section are as follows:
(A) 5 and 10 mil copper = 0.0021 for English units and 0.0039 for Metric units;
(B) 8 mil coated aluminum and 8 mil coated aluminum/6 mil coated steel = 0.0022 for English units and 0.0040 for Metric units;
(C) 5 mil copper clad stainless steel and 5 mil copper clad alloy steel = 0.0024 for English units and 0.0044 for Metric units;
(D) 6 mil copper clad stainless steel = 0.0019 for English units and 0.0035 for Metric units; and
(E) 6 and 7 mil alloy 194 = 0.0013 for English units and 0.0024 for Metric units.
(v) When utilizing shield continuity testers to measure shield and shield/armor continuity within pedestals or splices, refer to the manufacturer's published information covering the specific test equipment to be used and for anticipated results.
(6) Data record. Measurement data from shield continuity tests shall be recorded together with anticipated Table 1 or 2 values (see paragraph (a)(5)(i) of this section) in an appropriate format to permit comparison. The recorded data shall include specific location, cable size, cable type, type of shield or shield/armor, if known, etc.
(7) Probable causes for nonconformance. Among probable causes for nonconformance are broken or damaged shields or shield/armors, bad bonding harnesses, poorly connected bonding clamps, loose bonding lugs, etc.
(b) Conductor continuity. After placement of all cable and wire plant has been completed and joined together in continuous lengths, tests shall be made to ascertain that all pairs are free from grounds, shorts, crosses, and opens, except for those pairs indicated as being defective by the cable manufacturer. The tests for grounds, shorts, crosses, and opens are not separate tests, but are inherent in other acceptance tests discussed in this section. The test for grounds, shorts, and crosses is inherent when conductor insulation resistance measurements are conducted per paragraph (c) of this section, while tests for opens are inherent when tests are conducted for loop resistance, insertion loss, noise, or return loss measurements, per paragraphs (d), (e), or (f) of this section. The borrower shall make certain that all defective pairs are corrected, except those noted as defective by the cable manufacturer in accordance with the marking provisions of the applicable cable and wire specifications. All defective pairs that are not corrected shall be reported in writing with details of the corrective measures attempted.
(c) Dc insulation resistance (IR) measurement. (1) IR measurements shall be made on completed lengths of insulated cable and wire plant.
(2) Method of measurement. (i) The IR measurement shall be made between each conductor and all other conductors, sheath, shield and/or shield/armor, and/or support wire electrically connected together and to the main distributing frame (MDF) ground. The measurement shall be made from the central office with the entire length of the cable under test and, where used with all protectors and load coils connected. For COs containing solid state arresters, the solid state arresters shall be removed before making the IR measurements. Field mounted voice frequency repeaters, where used, may be left connected for the IR test but all carrier frequency equipment, including carrier repeaters and terminals, shall be disconnected. Pairs used to feed power remote from the CO shall have the power disconnected and the tip and ring conductors shall be opened before making IR tests. All conductors shall be opened at the far end of the cable being measured.
(ii) IR tests are normally made from the MDF with all CO equipment disconnected at the MDF, but this test may be made on new cables at field locations before they are spliced to existing cables. The method of measurement shall be as shown in Figure 3 as follows:

Code of Federal Regulations

Code of Federal Regulations 507
[Please see PDF for image: ER02MY97.025 ]
(iii) If the IR of the conductor cannot be measured because of breakdown of lightning arresters by the test voltage, the arrester units shall be removed and the conductor IR retested. If the IR then meets the minimum requirements, the conductor will be considered satisfactory. Immediately following the IR tests, all arrester units which have been removed shall be reinstalled.
(3) Test equipment. (i) IR measurements shall be made with either an insulation resistance test set or a direct current (dc) bridge type megohmmeter.
(ii) The IR test set shall have an output voltage not to exceed 500 volts dc and shall be of the hand cranked or battery operated type.
(iii) The dc bridge type megohmmeter, which may be alternating current (ac) powered, shall have scales and multiplier which make it possible to accurately read IR from 1 megohm to 1 gigohm. The voltage applied to the conductors under test shall not exceed “250 volts dc” when using an instrument having adjustable test voltage levels. This will help to prevent breakdown of lightning arresters.
(4) Applicable results. (i) For all new insulated cable or wire facilities, the expected IR levels are normally greater than 1,000 to 2,000 megohm-mile (1,609 to 3,218 megohm-km). A value of 500 megohm-mile (805 megohm-km) at 68 °F (20 °C) shall be the minimum acceptable value of IR. IR varies inversely with the length and the temperature.
(ii) The megohm-mile (megohm-km) value for a conductor may be computed by multiplying the actual scale reading in megohms on the test set by the length in miles (km) of the conductor under test.
(iii) The objective insulation resistance may be determined by dividing 500 by the length in miles (805 by the length in km) of the cable or wire conductor being tested. The resulting value shall be the minimum acceptable meter scale reading in megohms.
(iv) Due to the differences between various insulating materials and filling compounds used in manufacturing cable or wire, it is impractical to provide simple factors to predict the magnitude of variation in insulation resistance due to temperature. The variation can, however, be substantial for wide excursions in temperature from the ambient temperature of 68 °F (20 °C).
(v) Borrowers should be certain that tip and ring IR measurements of each pair are approximately the same. Borrowers should also be certain that IR measurements are similar for cable or wire sections of similar length and cable or wire type. If some pairs measure significantly lower, borrowers should attempt to improve these pairs in accordance with cable manufacturer's recommendations.

Code of Federal Regulations

Note: Only the megohm-mile (megohm-km) requirement shall be cause for rejection, not individual measurement differences.
(5) Data record. The measurement data shall be recorded. Suggested formats similar to Format I, Outside Plant Acceptance Tests—Subscriber Loops, or Format II, Outside Plant Acceptance Tests—Trunk Circuits, in § 1755.407 or formats specified in the applicable construction contract may be used.
(6) Probable causes for nonconformance. (i) When an IR measurement is below 500 megohm-mile (805 megohm-km), the cable or wire temperature at the time of testing must then be taken into consideration. If this temperature is well above 68 °F (20 °C), the measurement shall be disregarded and the cable or wire shall be remeasured at a time when the temperature is approximately 68 °F (20 °C). If the result is then 500 megohm-mile (805 megohm-km) or greater, the cable or wire shall be considered satisfactory.
(ii) Should the cable or wire fail to meet the 500 megohm-mile (805 megohm-km) requirement when the temperature is known to be approximately 68 °F (20 °C) there is not yet justification for rejection of the cable or wire. Protectors, lightning arresters, etc., may be a source of low insulation resistance. These devices shall be removed from the cable or wire and the cable or wire IR measurement shall be repeated. If the result is acceptable, the cable or wire shall be considered acceptable. The removed devices which caused the low insulation resistance value shall be identified and replaced, if found defective.
(iii) When the cable or wire alone is still found to be below the 500 megohm-mile (805 megohm-km) requirement after completing the steps in paragraph (c)(6)(i) and/or paragraph (c)(6)(ii) of this section, the test shall be repeated to measure the cable or wire in sections to isolate the piece(s) of cable or wire responsible. The cable or wire section(s) that is found to be below the 500 megohm-mile (805 megohm-km) requirement shall be either repaired in accordance with the cable or wire manufacturer's recommended procedure or shall be replaced as directed by the borrower.
(d) Dc loop resistance and dc resistance unbalance measurement. (1) When specified by the borrower, dc loop resistance and dc resistance unbalance measurements shall be made on all cable pairs used as trunk circuits. The dc loop resistance and dc resistance unbalance measurements shall be made between CO locations. Measurements shall include all components of the cable path.
(2) Dc loop resistance and dc resistance unbalance measurements shall be made on all cable pairs used as subscriber loop circuits when:
(i) Specified by the borrower;
(ii) A large number of long loops terminate at one location (similar to trunk circuits); or
(iii) Circuit balance is less than 60 dB when computed from noise measurements as described in paragraph (e) of this section.
(3) Dc resistance unbalance is controlled to the maximum possible degree by the cable specification. Allowable random unbalance is specified between tip and ring conductors within each reel. Further random patterns should occur when the cable conductor size changes. Cable meeting the unbalance requirements of the cable specification may under some conditions result in unacceptable noise levels as discussed in paragraph (d)(6)(iii) of this section.
(4) Method of measurement. The method of measurement shall be as detailed in Figures 4 and 5.
(5) Test equipment. The test equipment is shown in Figures 4 and 5 as follows:

Code of Federal Regulations

Code of Federal Regulations 510
[Please see PDF for image: ER02MY97.026 ]

Code of Federal Regulations

Code of Federal Regulations 511
[Please see PDF for image: ER02MY97.027 ]
(6) Applicable results. (i) The measured dc loop resistance shall be within ±5% of the calculated dc loop resistance when corrected for temperature.
(ii) The calculated dc loop resistance is computed as follows:
(A) Multiply the length of each different gauge by the applicable resistance per unit length as shown in Table 3 as follows:
Table 3—DC Loop Resistance @ 68 °F (20 °C)
American wire gauge (AWG) Loop resistance
ohms/1000 ft ohms/km
19 16.1 52.8
22 32.4 106.3
24 51.9 170.3
26 83.3 273.3
(B) Add the individual resistances for each gauge to give the total calculated dc loop resistance at a temperature of 68 °F (20 °C).
(C) Correct the total calculated dc loop resistance at the temperature of 68 °F (20 °C) to the measurement temperature by the following formulae:

Code of Federal Regulations

Rt=R68×[1 0.0022×t—68)] for English Units

Code of Federal Regulations

Rt=R20×[1 0.0040×(t—20)] for Metric Units
Where:
Rt = Loop resistance at the measurement temperature in ohms.
R68 = Loop resistance at a temperature of 68 °F in ohms.
R20 = Loop resistance at a temperature of 20 °C in ohms.
t = Measurement temperature in °F or ( °C).
(D) Compare the calculated dc loop resistance at the measurement temperature to the measured dc loop resistance to determine compliance with the requirement specified in paragraph (d)(6)(i) of this section.
(iii) Resistance varies directly with temperature change. For copper conductor cables, the dc resistance changes by ±1% for every ±5 °F (2.8 °C) change in temperature from 68 °F (20 °C).
(iv) The dc resistance unbalance between the individual conductors of a pair shall not exceed that value which will result in a circuit balance of less than 60 dB when computed from noise measurements as described in paragraph (e) of this section. It is impractical to establish a precise limit for overall circuit dc resistance unbalance due to the factors controlling its contribution to circuit noise. These factors include location of the resistance unbalance in relation to a low impedance path to ground (close to the central office) and the magnitude of unbalance in short lengths of cable making up the total circuit length. The objective is to obtain the minimum unbalance throughout the entire circuit when it is ascertained through noise measurements that dc resistance unbalance may be contributing to poor cable balance.
(v) Pairs with poor noise balance may be improved by reversing tip and ring conductors of pairs at cable splices. Where dc resistance unbalances are systematic over the total trunk circuit or loop circuit length, tip and ring reversals may be made at frequent intervals. Where the unbalances are concentrated in a shorter section of cable, only one tip and ring reversal should be required. Concentrated dc resistance unbalance produces maximum circuit noise when located adjacent to the central office. Concentrated dc resistance unbalance will contribute to overall circuit noise at a point approximately two-thirds ( 2/3) of the distance to the subscriber. All deliberate tip and ring reversals shall be tagged and identified to prevent plant personnel from removing the reversals when resplicing these connections in the future. The number of tip and ring reversals shall be held to a minimum.
(vi) A systematic dc resistance unbalance can sometimes be accompanied by other cable parameters that are marginal. Among these are pair-to-pair capacitance unbalance, capacitance unbalance-to-ground, and 150 kilohertz (kHz) crosstalk loss. Engineering judgment has to be applied in each case. Rejection of cable for excessive dc resistance unbalance shall only apply to a single reel length, or shorter.
(7) Data record. The measurement data for dc loop resistance and dc resistance unbalance shall be recorded. Suggested formats similar to Format I for subscriber loops and Format II for trunk circuits in § 1755.407 or formats specified in the applicable construction contract may be used.
(8) Probable causes for nonconformance. Dc loop resistance and dc resistance unbalance are usually the result of the resistance of individual conductors used in the manufacture of the cable. Resistance unbalance can be worsened by defective splicing of the conductors (splicing connectors, improper crimping tool, etc.).
(e) Subscriber loop measurement (loop checking). (1) When specified by the borrower, insertion loss and noise measurements shall be performed on subscriber loops after connection of a line circuit to the loop by the one person method using loop checking equipment from the customer access location. For this method, the central office should be equipped with a 900 ohm plus two microfarad quiet termination and a milliwatt generator having the required test frequencies; or a portable milliwatt generator having the desired frequencies may be used, especially, where several small offices are involved.
(2) At a minimum, insertion loss and frequency response of subscriber loop plant shall be measured at 1,000, 1,700, 2,300, and 2,800 Hertz (Hz). When additional testing frequencies are desired, the additional frequencies shall be specified in the applicable construction contract.
(3) Measurements of insertion loss and noise shall be made on five percent or more of the pairs. A minimum of five pairs shall be tested on each route. Pairs shall be selected on a random basis with greater consideration in the selection given to the longer loops. Consideration shall be given to measuring a large percentage, up to 100 percent, of all loops.
(4) Method of measurement— (i) Insertion loss. The step-by-step measurement procedure shall be as shown in Figure 6. The output level of the milliwatt generator tones shall be determined prior to leaving the CO. This shall be accomplished by dialing the milliwatt generator number from a spare line at the MDF and measuring with the same equipment to be used in the tests at customer access locations. The output levels shall be recorded for reference later. Insertion loss measurements shall be made across the tip and ring terminals of the pair under test. Figure 6 is as follows:

Code of Federal Regulations

Code of Federal Regulations 514
[Please see PDF for image: ER02MY97.028 ]
(ii) Noise. The step-by-step measurement procedure shall be as shown in Figure 7. Prior to leaving the CO for testing,