Code of Federal Regulations Title 7. Agriculture § 7.1755.870—Qualification Test Methods

(I) The test procedures described in this appendix are for qualification of initial designs and major modifications of accepted designs. Included in paragraph (V) of this appendix are suggested formats that may be used in submitting test results to RUS.

(II) Sample Selection and Preparation. (1) All testing shall be performed on lengths removed sequentially from the same 25 pair, 22 gauge jacketed cable. This cable shall not have been exposed to temperatures in excess of 38 °C since its initial cool down after sheathing. The lengths specified are minimum lengths and if desirable from a laboratory testing standpoint longer lengths may be used.

(a) Length A shall be 12±0.2 meters (40±0.5 feet) long. Prepare the test sample by removing the jacket, shield, and core wrap for a sufficient distance on both ends to allow the insulated conductors to be flared out. Remove sufficient conductor insulation so that appropriate electrical test connections can be made at both ends. Coil the sample with a diameter of 15 to 20 times its sheath diameter. Two lengths are required.

(b) Length B shall be 300 millimeters (1 foot) long. Three lengths are required.

(c) Length C shall be 3 meters (10 feet) long and shall be maintained at 23± 3 °C for the duration of the test. Two lengths are required.

(2) Data Reference Temperature. Unless otherwise specified, all measurements shall be made at 23± 3 °C.

(III) Environmental Tests—(1) Heat Aging Test—(a) Test Samples. Place one sample each of lengths A and B in an oven or environmental chamber. The ends of sample A shall exit from the chamber or oven for electrical tests. Securely seal the oven exit holes.

(b) Sequence of Tests. Sample B referenced in paragraph (III)(1)(a) of this appendix shall be subjected to the insulation compression test outlined in paragraph (III)(2) of this appendix.

(c) Initial Measurements. (i) For sample A, measure the open circuit capacitance and conductance for each odd pair at 1, 150, and 772 kilohertz after conditioning the sample at the data reference temperature for 24 hours. Calculate the average and standard deviation for the data of the 13 pairs on a per kilometer (per mile) basis.

(ii) Record on suggested formats in paragraph (V) of this appendix or on other easily readable formats.

(d) Heat Conditioning. (i) Immediately after completing the initial measurements, condition the sample for 14 days at a temperature of 65± 2 °C.

(ii) At the end of this period. Measure and calculate the parameters given in paragraph (III)(1)(c) of this appendix. Record on suggested formats in paragraph (V) of this appendix or on other easily readable formats.

(e) Overall Electrical Deviation. (i) Calculate the percent change in all average parameters between the final parameters after conditioning with the initial parameters in paragraph (III)(1)(c) of this appendix.

(ii) The stability of the electrical parameters after completion of this test shall be within the following prescribed limits:

(A) Capacitance. The average mutual capacitance shall be within 10 percent of its original value;

(B) The change in average mutual capacitance shall be less than 10 percent over the frequency range of 1 to 150 kilohertz;  and

(C) Conductance. The average mutual conductance shall not exceed 3.7 micromhos/kilometer (6 micromhos/mile) at a frequency of 1 kilohertz.

(2) Insulation Compression Test—(a) Test Sample B. Remove jacket, shield, and core wrap being careful not to damage the conductor insulation. Remove one pair from the core and carefully separate and straighten the insulated conductors. Retwist the two insulated conductors together under sufficient tension to form 10 evenly spaced 360 degree twists in a length of 100 millimeters (4 inches).

(b) Sample Testing. Center the mid 50 millimeters (2 inches) of the twisted pair between two smooth rigid parallel metal plates measuring 50 millimeters (2 inches) in length or diameter. Apply a 1.5 volt direct current potential between the conductors, using a light or buzzer to indicate electrical contact between the conductors. Apply a constant load of 67 newtons (15 pound-force) on the sample for one minute and monitor for evidence of contact between the conductors. Record results on suggested formats in paragraph (V) of this appendix or on other easily readable formats.

(3) Temperature Cycling. (a) Repeat paragraphs (III)(1)(a) through (III)(1)(c)(ii) of this appendix for a separate set of samples A and B which have not been subjected to prior environmental conditioning.

(b) Immediately after completing the measurements, subject the test samples to 10 cycles of temperature between −40 °C and +60 °C. The test samples shall be held at each temperature extreme for a minimum of 1.5 hours during each cycle of temperature. The air within the temperature cycling chamber shall be circulated throughout the duration of the cycling.

(c) Repeat paragraphs (III)(1)(d)(ii) through (III)(2)(b) of this appendix.

(IV) Control Sample—(1) Test Samples. One length of sample B shall have been maintained at 23± 3 °C for at least 48 hours before the testing.

(2) Repeat paragraphs (III)(2) through (III)(2)(b) of this appendix.

(3) Surge Test. (a) One length of sample C shall be used to measure the breakdown between conductors while the other length of C shall be used to measure core to shield breakdown.

(b) The samples shall be capable of withstanding, without damage, a single surge voltage of 20 kilovolts peak between conductors, and 35 kilovolts peak between conductors and the shield as hereinafter described. The surge voltage shall be developed from a capacitor discharge through a forming resistor connected in parallel with the dielectric of the test sample. The surge generator constants shall be such as to produce a surge of 1.5 x 40 microseconds wave shape.

(c) The shape of the generated wave shall be determined at a reduced voltage by connecting an oscilloscope across the forming resistor with the cable sample connected in parallel with the forming resistor. The capacitor bank is charged to the test voltage and then discharged through the forming resistor and test sample. The test sample shall be considered to have passed the test if there is no distinct change in the wave shape obtained with the initial reduced voltage compared to that obtained after the application of the test voltage.

(V) The following suggested formats may be used in submitting the test results to RUS:

Environmental Conditioning __________

Environmental Conditioning __________

Environmental Conditioning __________