2024-10-23
Project Name: 400V low voltage cable fault at a university in Xi'an
Cable model: YJV 3*120+1*95, 400V armored low-voltage cable
Nature of the fault
1. First, use the insulation megohmmeter at 500V to test the insulation resistance. The resistance of phase A to zero and between phases is OMΩ, the resistance between phases B and C is about 2MΩ, the resistance between phases B and C is about 2MΩ to ground, and the resistance between phases BC to zero is 2MΩ. It is preliminarily determined that the resistance of phase A to ground and between phases is low resistance.
The resistance of phase A to zero and between phases is OMΩ
The resistance between phases B and C is about 2MΩ
The resistance between phases B and C is about 2MΩ to ground
2. Then use a multimeter to test A to ground, neutral line and BC phases, all of which are tens of Ω, which has confirmed that A has a short circuit fault to ground and between phases.
Cable length
One end of the cable is in the low-voltage cabinet and the other end is in the distribution room. The nominal total length is 370m and the actual measured length is 371.0 meters.
Waveform for cable length test
Cable fault test
Fault distance rough measurement method 1：
First, the low-voltage pulse method was used to measure the short-circuit fault distance of A phase to the neutral line, and it was found that the short-circuit fault distance was 43m. The total length of B phase and C phase to zero was about 371.0m. The short-circuit waveform of phase A to B and C was about 37m. Then the high-voltage flashover method was used to roughly measure the position to be 40m.
Waveform of rough measurement method 1
Analysis: Use the low-voltage pulse method to measure the entire length, between phases B and C, and between B and C to zero, the total length is 360m, phase A to B, C, and zero is a short-circuit waveform, the distance is 43m, and then use the high-voltage flashover method for re-measurement.
Fault distance rough measurement method 2：
The high-voltage flashover method was performed using an operating box and 40-6 capacitor, the voltage rose to 12kV, the cable fault point was broken down, and the standard near-end waveform was obtained.
Waveform of rough measurement method 2
Analysis:The waveform is a standard proximal fault waveform. Taking two adjacent peaks and troughs, the fault point is measured to be about 40m.
Excavate the fault point and find the fault point
Summary:
1. For low-voltage cables, especially in residential areas and places with many buildings, you must first find the path.
2. For preliminary judgment of faults with very low resistance to ground, give priority to using the step voltage method for precise positioning.
3. For precise positioning of low-voltage cables (especially those with armored layers), multiple methods must be used for precise positioning, because the armor may be grounded in multiple places, which may lead to misjudgment.
