§1755.403. Copper cable telecommunications plant measurements.

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.

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.

The test equipment for measuring cable shield or shield/armor resistance between pedestals or splices is shown in Figure 2 as follows:

A cable shield splice continuity tester shall be used to measure shield or shield/armor continuity within pedestals or splices.

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:

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.

To correct the measured shield resistance to the reference temperature of 68 °F (20 °C) use the following formulae:

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:

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.

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.

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:

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.

IR measurements shall be made with either an insulation resistance test set or a direct current (dc) bridge type megohmmeter.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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.

The measured dc loop resistance shall be within ±5% of the calculated dc loop resistance when corrected for temperature.

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).

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.

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.

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.

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:

Noise. The step-by-step measurement procedure shall be as shown in Figure 7. Prior to leaving the CO for testing, dial the 900 ohm plus two microfarad quiet termination from a spare pair and measure the termination to determine that it actually is quiet. Circuit noise (noise-metallic) shall be measured at the customer access location across the tip and ring terminals of the pair under test. Power influence (direct reading with loop checking equipment) shall be measured at the customer access location from tip and ring conductors-to-ground (this connection is completed via the test unit). The power influence measurement includes the entire talking connection from the quiet termination to the customer. (That is, the power influence measurement includes all the CO equipment which normally makes up the connection.) Figure 7 is as follows:

Loop checking equipment which is available from several manufacturers may be used for these measurements. The equipment should have the capability of measuring loop current, insertion loss, circuit noise (NM) and power influence (PI). The test equipment manufacturer's operating instructions shall be followed.

Noise. The principal objective related to circuit noise (noise-metallic) and the acceptance of new plant is that circuit noise levels be 20 dBrnc or less (decibels above reference noise, C-message weighted (a weighting derived from listening tests, to indicate the relative annoyance or speech impairment by an interfering signal of frequency (f) as heard through a “500-type” telephone set)). For most new, properly installed, plant construction, circuit noise will usually be considerably less than 20 dBrnc unless there are unusually long sections of telephone plant in parallel with electric power facilities and/or power influence of paralleling electric facilities is abnormally high. When circuit noise is 20 dBrnc or less, the loop plant shall be considered acceptable. When measured circuit noise is greater than 20 dBrnc, loop plant shall still be considered acceptable providing circuit balance (power influence reading minus circuit noise readings) is 60 dB or greater and power influence readings are 85 dBrnc or greater. When circuit noise is greater than 20 dBrnc and circuit balance is less than 60 dB and/or power influence is less than 85 dBrnc, loop plant shall not be considered acceptable and the loop plant shall be remedied to make circuit balance equal to or greater than 60 dB.

Insertion loss. Some of the more common causes for failing to obtain the desired results may be due to reversed load coil windings, missing load coils, bridge taps between load coils, load coil spacing irregularities, excessive end sections, cables having high or low mutual capacitance, load coils having the wrong inductance, load coils inadvertently installed in nonloaded loops, moisture or water in cable, split pairs, and improperly spliced connections. The above factors can occur singularly or in combination. Experience to date indicates that the most common problems are missing load coils, reversed load coil windings or bridge taps.

Noise. Some of the common causes for failing to obtain the desired results may be due to high power influence from paralleling electrical power systems, poor telephone circuit balance, discontinuous cable shields, inadequate bonding and grounding of cable shields, high capacitance unbalance-to-ground of the cable pairs, high dc loop resistance unbalance, dc loop current less than 20 milliamperes, etc. The above factors can occur singularly or in combination. See TE&CM Section 451, Telephone Noise Measurement and Mitigation, for steps to be taken in reducing telecommunications line noise.

For loaded and nonloaded loops, the two traces in the pulse return pattern or impedance method (paragraph (f)(5)(i) of this section) shall be essentially identical. The degree of comparison required of the two traces is to be determined by experience.

For loaded loops, results for return loss measurements using a level tracer, with artificial line, in lieu of a PBN (paragraph (f)(5)(ii) of this section) shall meet the following requirements:

For loaded loops, the results of open circuit structural return loss measurements using a level tracer (paragraph (f)(5)(iii) of this section) shall meet the following requirements. For D66 and H88 loaded cables with uniform or mixed gauges, the worst value allowed for measured open circuit structural return loss between 1,000-3,500 Hz and 1,000-3,000 Hz, respectively, shall be approximately 0.9 dB (round trip) for each 100 ohms outside plant dc loop resistance including the resistance of the load coils. The value of 0.9 dB per 100 ohms for the round trip loss remains reasonably accurate as long as:

For loaded loops, the measured value of open circuit structural return loss can only be as accurate as the degree to which the dc loop resistance of the loaded pair under test is known. Most accurate results shall be obtained when the dc loop resistance is known by actual measurements as described in paragraph (d) of this section. Furthermore, where the dc loop resistance is measured at the same time as the open circuit structural return loss, no correction for temperature is needed because the loss is directly proportional to the loop resistance. Where it is not practical to measure the dc loop resistance, it shall be calculated and corrected for temperature as specified in paragraph (d)(6)(ii) of this section. When measuring existing plant, care shall be taken to verify the accuracy of the records, if they are used for the calculation of the dc loop resistance. For buried plant, the temperature correction shall be based at the normal depth of the cable in the ground. (Temperature can be measured by boring a hole to cable depth with a ground rod, placing a thermometer in the ground at the cable depth, and taking and averaging several readings during the course of the resistance measurements.) For aerial cable it shall be based on the temperature inside the cable sheath.

For loaded loops, the best correlation between the measured and the expected results shall be obtained when the cable is of one gauge, one size, and the far end section is approximately 2,250 ft (685 m) for D66 loading or 3,000 ft (914 m) for H88 loading. Mixing gauges and cable sizes will result in undesirable small reflections whose frequency characteristics and magnitude cannot be accurately predicted. In subscriber loop applications, cable gauge may be somewhat uniform but the cable pair size most likely will not be uniform as cable pair sizes taper off toward the customer access location and a downward adjustment of 1 dB of the allowed value shall be acceptable. “Long” end sections (as defined in TE&CM Section 424, “Guideline for Telecommunications Subscriber Loop Plant”) lower the expected value, a further downward adjustment of 3 dB in the allowed value shall be acceptable.

For loaded loops, the limiting factor when making open circuit structural return loss measurements is when the 1,000 Hz one-way loss of the loaded cable pair under test becomes 10 dB or greater; it becomes difficult to detect the presence of irregularities beyond the 10 dB point on the loop. To overcome this difficulty, loaded loops having a one-way loss at 1,000 Hz greater than 10 dB shall be opened at some convenient point (such as a pedestal or ready access enclosure) and loss measurements at the individual portions measuring less than 10 dB one-way shall be made separately. When field mounted voice frequency repeaters are used, the measurement shall be made at the repeater location in both directions.

When performing a pulse return pattern or impedance open circuit measurement on loaded and nonloaded loops, a “check mark” indicating that the pair tests good or an “X” indicating that the pair does not test good shall be recorded in the SRL column. A suggested format similar to Format I for subscriber loops in § 1755.407 or a format specified in the applicable construction contract may be used.

When performing open circuit return loss measurements using the return loss balanced to an artificial line or return loss using a level tracer on loaded loops, the value of the poorest (lowest numerical value) SRL and its frequency in the proper column between 1,000 and 3,500 Hz for D66 loading or between 1,000 and 3,000 Hz for H88 loading shall be recorded. A suggested format similar to Format I for subscriber loops in § 1755.407 or a format specified in the applicable construction contract may be used.

The highest frequency to be measured is determined by the type of carrier system. For T1 type carrier, the highest frequency is normally 772 kHz. For T1C type carrier, the highest frequency is normally 1576 kHz. The highest frequency to be measured for station carrier is 140 kHz.

The measured insertion loss of the cable shall be within ±10% of the calculated loss in dB when the loss is corrected for temperature.

If the measured value exceeds the ±10% allowable variation, the cause shall be determined and corrective action shall be taken to remedy the problem.