NOYAFA Electronic specializing in testing instruments and cable testers for 16 years
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NOYAFA Electronic specializing in testing instruments and cable testers for 16 years
Author: Noyafa–CCTV Tester
Question: The working principle of the low voltage pulse reflection method Answer: The low voltage pulse reflection method (hereinafter referred to as the low voltage pulse method) is used to measure the low resistance, short circuit and open circuit faults of cables. According to statistics, such faults account for about 10% of cable faults. The low-voltage pulse method can also be used to measure cables. 1111 Question: The working principle of the low-voltage pulse reflection method Answer: The low-voltage pulse reflection method (hereinafter referred to as the low-voltage pulse method) is used to measure the low resistance, short circuit and open circuit faults of the cable.
According to statistics, such faults account for about 10% of cable faults. The low-voltage pulse method can also be used to measure the length of the cable, the propagation speed of electromagnetic waves in the cable, and can also be used to distinguish the intermediate head, T-joint and terminal head of the cable. Working principle During the test, a low-voltage pulse is injected into the cable, and the pulse propagates along the cable to the impedance mismatch point, such as short-circuit point, fault point, intermediate joint, etc., the pulse is reflected, and returns to the measurement point to be recorded by the instrument (Figure 3.1).
The time difference Δt between the transmitted pulse and the reflected pulse on the waveform corresponds to the time it takes for the pulse to travel back and forth between the measurement point and the impedance mismatch point. If the wave velocity V of the pulse in the cable is known, the distance from the impedance mismatch point can be calculated by the following formula. Low Voltage Pulse Reflection Schematic By identifying the polarity of the reflected pulse, the nature of the fault can be determined. The reflected pulse of the open fault has the same polarity as the transmitted pulse, while the reflected pulse of the short fault has the opposite polarity to the transmitted pulse.
It is known from Equation 3.1 that the wave velocity of the pulse in the cable is critical to accurately calculate the fault distance. When the wave velocity value of the cable is not known, the following method can be used to measure it. If the length of the cable under test is known, the wave velocity in the cable can be calculated according to the time Δt between the transmitted pulse and the reflected pulse at the cable terminal: Selection of the transmitted pulse 1) The shape of the pulse The voltage pulse used by the cable fault measuring instrument Generally, there are rectangles, exponentials, bells (also called raised cosines) and so on.
Since the formation of rectangular pulses is relatively easy, it is widely used. 2) Pulse width The pulse always has a certain time width. Assuming that it is τ, the reflected pulse that arrives within the time τ overlaps the transmitted pulse and cannot be distinguished. Therefore, the distance to the fault point cannot be measured, and a blind spot appears. Assuming that the pulse emission width is 0.5s and the cable wave speed is 160m/s, the measurement blind area is 40 meters. The wider the pulse sent by the instrument, the larger the measurement blind area.
From the point of view of reducing the blind spot, it is better to send the pulse with a narrower width, but the narrower the pulse, the richer the high-frequency components it contains, while the high-frequency loss of the line is large, so that the reflected pulse amplitude is too small, the distortion is serious, and the impact is far. Measurement effect of distance faults. In order to solve this problem, the pulse reflection instrument divides the pulse width into several ranges, and selects the pulse width according to the distance of the measurement. The longer the measurement distance, the wider the pulse.
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