The nature and classification of cable faults
1. Classification by fault material characteristics
It can be divided into three types: series fault, parallel fault and composite fault.
(1) series failure
A series failure (metal material defect) is a failure in which one or more conductors of the cable (including lead and aluminum sheaths) are disconnected. It is a generalized cable open circuit fault. Broken or incomplete disconnection due to damage to the continuity of the core. Incomplete disconnection is especially difficult to find. The series faults can be divided into: one-point breaking, multi-point breaking, one-phase breaking, multi-phase breaking and so on.
(2) Parallel failure
Parallel failure (insulation defect) is a short-circuit fault that occurs when the insulation level between the outer skin or the conductor is lowered and the normal operating voltage cannot be withstood. It is a generalized cable short circuit fault. Such faults cause short circuit, grounding, flashover breakdown, etc. due to insulation damage between the cores or between the cores, and the frequency of occurrence is high in the field. Parallel faults can be divided into one phase grounding, two phase grounding, two phase short circuit, three phase short circuit and the like.
(3) Compound failure
Composite failures (insulations in both insulating materials and metallic materials) mean that the insulation between the core and the core is faulty. It includes a phase disconnection and grounding, two-phase disconnection and grounding, two-phase short-circuit and grounding.
2. Classification by fault point insulation characteristics
In the case of the insulation fault Rf of the root cable fault point and the breakdown gap G, the cable fault can be divided into four categories: open circuit fault, low resistance fault, high impedance fault, and flashover fault. This classification is a basic classification method for field cable faults*, which is particularly advantageous for the selection of detection methods.
Wherein, the magnitude of the gap breakdown voltage UG depends on the distance G of the discharge channel of the fault point (ie, the breakdown gap), and the magnitude of the insulation resistance Rf depends on the degree of carbonization of the cable medium at the fault point, and the magnitude of the distributed capacitance Cf depends on the degree of moisture at the fault point. .
(1) Open circuit failure
The continuity of the metal part of the cable is broken, forming a broken wire, and the insulating material at the point of failure is also damaged to varying degrees. On-site measurement of the insulation resistance Rf is infinite (∞) with a megohmmeter, but in the DC withstand voltage test, electrical breakdown will occur; check the core wire conduction, there is a breakpoint. The field is usually in the form of one-phase or two-phase disconnection and grounding.
(2) Low resistance fault
The cable insulation is damaged and a ground fault occurs. The on-site ohmmeter is used to measure the insulation resistance Rf is less than 10Z0 (Z0 is the wave impedance of the cable, generally between 10 and 40 Ω). On-site low-voltage power cables and control cables have a high probability of low-resistance faults.
(3) High resistance fault
The cable insulation is damaged and a ground fault occurs. On-site measurement of the insulation resistance Rf is greater than 10Z0 with a megohmmeter, and electrical breakdown occurs during the DC high-voltage pulse test. High-resistance faults are high-voltage power cables (6KV or 10KV power cables) with a high probability* high cable fault, up to 80% of the total fault.
When the field is measured, the author generally takes Rf=3KΩ as the boundary between high-resistance and low-resistance faults. Since Rf = 3KΩ, it is just as good as the measurement current of 10 to 50 mA required for the loopback bridge* measurement.
(4) Flashover failure
The cable insulation is damaged and a flashover failure occurs. On-site measurement of the insulation resistance Rf is infinite (∞) with a megohmmeter, but flashover electrical breakdown occurs during DC withstand voltage or high voltage pulse test. Flashover faults are more difficult to measure, especially when a newly laid cable is subjected to a preventive test with a flashover failure. The field is generally detected by the DC flashover method.