Essential knowledge for wire and cable inspection - Measurement of direct current Resistance of conductors

I. Inspection Purpose

The magnitude of the direct current resistance of a conductor is a fundamental performance index of wire and cable products and an important routine test and type test item. The purpose of the test is to check whether the resistance of the conductive core of the product exceeds the specified value of the standard; otherwise, it will affect the current-carrying capacity of the wire and cable products. At the same time, measuring the conductor resistance of the entire product can also detect certain defects in the production process, such as wire breakage or partial single wire breakage, and the conductor cross-section not meeting the standards, etc.

Ii. Scope of Application

This test method is applicable to measuring the direct current resistance of conductors in wires and cables.

Iii. Inspection Basis

Relevant product standards: GB/T 3956-2008, GB/T 3048.4-2007.

Iv. Test Equipment and Apparatus

1.QJ57 type, QJ44 type, QJ36 type DC Bridges, micro ohm meters, etc.

2. Universal conductor resistance measurement fixture.

V. Sampling and Testing Equipment

Cut a sample of at least 1 meter in length from the cable under test. When measuring with a conductor resistance measuring fixture, take at least 1.25 meters. Remove the coverings at both ends of the sample that connect to the measuring fixture, exposing approximately 100mm of the conductor (ensuring that the contact area between the sample and the fixture is a conductor). For large cross-section aluminum conductors (95-185mm²), take 3 meters; for 240mm² and above, take 5 meters. In case of dispute, 5 meters shall be taken for 185mm² and below, and 10 meters for 240mm² and above.

(1) When taking samples, it is necessary to avoid the specimens being stretched or the conductors being damaged.

(2) Remove the adhering substances, dirt and oil stains on the surface of the conductor, and the oxide layer on the surface of the connection point can be completely removed.

(3) Straighten the sample carefully by hand.

2. During type tests, the specimens should be placed in a test environment of (15-25) ℃ with a humidity not exceeding 85% for a sufficient period of time to reach temperature equilibrium. The ambient temperature variation shall not exceed ±1℃.

Vi. Test Procedures

1. A thermometer with a minimum scale of 0.1℃ should be used to measure the ambient temperature. The thermometer should be no less than 1 meter above the ground, no less than 10cm away from the wall, and no more than 1 meter away from the sample, and the two should be approximately at the same height.

2. Connect the sample to be tested to the universal conductor measuring fixture.

3. Measure the direct current resistance of a conductor with a length of 1 meter at t℃ in accordance with the operating procedures of the bridge.

4. The current introduction end of the aluminum stranded wire can adopt an aluminum crimping joint and be crimped in the conventional crimping method. The potential electrode can be made by tightly winding a 1mm diameter soft copper wire 1-2 times around the stranded wire and then knotting it to lead out.

Taking the QJ57 type bridge as an example, the test steps are as follows:

Connect the current terminals C1 and C2 of the bridge to the current terminals of the fixture, and the potential terminals P1 and P2 to the voltage terminals of the fixture. The sample should be clamped onto the fixture (Note: All single wires of the stranded conductor should be reliably connected to the fixture).

2. Turn on the power switch.

3. Adjust the zero position.

4. First, select the magnification dial and turn it to the desired position. If the resistance value to be measured is unknown, the 103 range should be chosen. Then adjust the measurement reading dial and place the first and second main reading dials at the approximate reading position of the resistance value to be measured. If the resistance value is unknown, the measurement reading dial can be turned to the maximum reading value (1.1111) first.

5. Adjustment of bridge balance: Before reaching the appropriate magnification and measurement reading dial, the sensitivity should be reduced as much as possible. As the magnification dial and measurement reading dial are adjusted, the sensitivity should be gradually increased.

(1) When measuring, first lock "G", then intermittently close and use the "B" button. At the same time, observe the direction of the pointer. If it deflects to the right, it indicates that the multiplier value is too low, and the multiplier dial should be increased; otherwise, it should be decreased.

(2) Adjust the measuring disk of the bridge so that the pointer points to zero. When adjusting, start from the reading disk with the high resistance value and continue until the last measuring disk.

(3) When the bridge approaches final equilibrium, adjust the sensitivity to the maximum.

(4) Zero again and measure once more.

(5) Read the measured resistance value: Read the reading record with at least four significant figures. The first reading dial should not be zero. The measured resistance value should be equal to the sum of the readings on each measurement reading dial and then multiplied by the multiple.

(6) After using the bridge, the multiplier should be turned to the "off" position and the "G" button should be released.

Vii. Evaluation of Test Results

Convert to the direct current resistance of a conductor at 20℃ with a length of 1000 meters according to the following formula: R20 = [Rt×254.5×1000] / ((234.5+ t) ×L)

In the formula: t - the sample temperature at the time of measurement, unit: ℃;

R20 - The DC resistance value of a conductor at 20 degrees, with the unit Ω / km;

Rt - The conductor resistance of a cable with a length of 1 meter at t degrees;

L - Cable sample length (referring to the length of the finished cable, not the length of a single insulated core or a single wire), unit: m. 2. For all products, the maximum resistance value of the conductor at 20℃ should not exceed the value specified in the standard to be considered qualified.