The nature and classification of common cable faults

Author: Noyafa–Cable Tester Manufacturer

Properties and classification of common cable faults (1) Series fault Series fault (metal material defect) refers to the nature and classification of common cable faults when one or more conductors (including lead and aluminum sheaths) of the cable are disconnected 1. According to the characteristics of fault materials, it can be divided into series fault, There are three types of parallel faults and composite faults. (1) Series fault Series fault (defect of metal material) refers to a fault in which one or more conductors (including lead and aluminum sheaths) of the cable are disconnected.

It is a generalized open cable fault. The continuity of the cable core is damaged, resulting in disconnection or incomplete disconnection. Incomplete disconnections are especially hard to spot.

The series fault can be divided into: one-point disconnection, multi-point disconnection, one-phase disconnection, multi-phase disconnection, etc. (2) Parallel fault Parallel fault (insulating material defect) is a short-circuit fault that occurs when the insulation level between the conductor and the outer skin or between the conductors drops and cannot withstand the normal operating voltage. It is a generalized cable short-circuit fault.

Such faults occur frequently in the field due to short-circuit, grounding, flashover breakdown and other phenomena due to the insulation damage between the cable cores or between the cable cores and the outer skin. Parallel faults can be divided into: one-phase grounding, two-phase grounding, two-phase short circuit, three-phase short circuit, etc. (3) Composite failure Composite failure (defects in insulating materials and metal materials) refers to the failure of the insulation between the cable core and the cable core.

It includes one-phase disconnection and grounding, two-phase disconnection and grounding, two-phase short-circuiting and grounding, etc. 2. Classification according to the insulation characteristics of the fault point According to the insulation resistance Rf and the breakdown gap G of the fault point of the cable, the cable fault can be divided into four categories: open circuit fault, low resistance fault, high resistance fault and flashover fault. This classification method is the most basic classification method for on-site cable faults, and is especially beneficial to the selection of detection methods.

Among them, the size of the gap breakdown voltage UG depends on the distance G of the discharge channel (ie the breakdown gap) at the fault point, the size of the insulation resistance Rf depends on the carbonization degree of the cable medium at the fault point, and the size of the distributed capacitance Cf depends on the moisture level of the fault point. . (1) Open circuit fault The continuity of the metal part of the cable is damaged, resulting in disconnection, and the insulating material at the fault point is also damaged to varying degrees. The insulation resistance Rf measured with a megohmmeter on site is infinite (∞), but during the DC withstand voltage test, there will be electrical breakdown; check the continuity of the core wire, there is a breakpoint.

The scene generally occurs in the form of one-phase or two-phase disconnection and grounding. (2) Low resistance fault The cable insulation material is damaged and a ground fault occurs. Use a megohmmeter on site to measure the insulation resistance Rf less than 10Z0 (Z0 is the wave impedance of the cable, generally 10 to 40Ωbetween).

On-site low-voltage power cables and control cables have a high probability of low-resistance faults. (3) High resistance fault The cable insulation material is damaged and a ground fault occurs. The insulation resistance Rf is more than 10Z0 measured with a megohmmeter on site, and electrical breakdown will occur during the DC high voltage pulse test.

High-resistance fault is the cable fault with the highest probability of high-voltage power cables (6KV or 10KV power cables), which can reach more than 80% of the total faults. In the field measurement, the author generally takes Rf = 3KΩIt is the boundary between high-resistance and low-resistance faults. because Rf =3KΩ, the measurement current of 10-50 mA necessary for accurate measurement of the loop method bridge can be obtained.

(4) Flashover failure The cable insulation material is damaged and flashover failure occurs. The insulation resistance Rf measured with a megohmmeter on site is infinite (∞), but in the DC withstand voltage or high voltage pulse test, there will be flashover electrical breakdown. Flashover failures are difficult to detect, especially when newly laid cables are subjected to preventive tests and flashover failures occur.

The field generally uses the DC flashover method for detection. 3. Classification by fault triggering causes and fault point characteristics According to the insulation damage of cables, cable heads and intermediate boxes with different characteristics during operation or preventive tests of power cables, it can also be divided into three types: blasting faults, breakdown faults and operating faults. kind. (1) Shooting failure In industrial and mining enterprises, the power cables in operation are seriously damaged due to various reasons, resulting in tripping accidents.

Called a cable blaster. The characteristics of this type of fault are: most of the cable fault points have lead wrap or copper skin rupture, and there are different degrees of external deformation; the nature of the cable fault is often manifested as two-phase short-circuit grounding or two-phase disconnection and grounding, and its grounding resistance is generally small. , Analyse the fault point, and find the carbonization point of arc breakdown or dendritic discharge carbon channel and crack. The cable shooting failure has obvious fault characteristics. In most cases, the operating personnel on duty can provide the approximate position of the shooting.

Therefore, except for a few more complicated cases of such faults, which require distance measurement, generally only need to use a multimeter to determine the specific nature of the fault (single-phase grounding, short-circuit grounding, disconnection grounding, etc.), and the acoustic measurement method can be used to directly determine the point, which is simple and clear. (2) Breakdown fault In actual work, the cable insulation damage event triggered by the preventive test is habitually called cable breakdown. Such faults all occur under the DC test voltage, and the insulation damage is electrical breakdown, the grounding point is generally intact with lead wrap or copper skin, and there is no obvious external deformation (except mechanical trauma).

The cable breakdown faults are mostly simple ground faults, the ground faults are relatively high, the fault point is dissected, and the insulating material has no carbonization point, but the aging structure of carbon holes and water branches can be found through the instrument. For cable breakdown faults, especially some high-resistance grounding cable breakdown faults, the difficulty of testing is ranging. Because this type of fault is relatively hidden, the test parameters are complex and changeable, and there is a lack of regularity, the distance measurement is the key to whether the fault point of the cable can be found quickly.

“High voltage loop method”,“Hammer method”Both have the most effective methods for detecting such faults. (3) Operation fault It refers to the fact that when the power system of the factory is in operation, the cable feeder of the cable feeder, the motor and the transformer, the voltage fluctuation of the high-voltage secondary circuit or the grounding signal is found (the grounding trip of the power element with grounding protection occurs), Cable faults determined by excluding the possibility of other electrical components failing. The biggest feature of this type of failure is that it is not clear.

The extreme form of cable operation failure is cable blasting (such as phase-to-phase short circuit caused by two-point grounding); another part of the operation fault develops into cable breakdown fault (such as cable aging, insulation defect) due to the failure of withstand voltage during the stop point inspection. etc.); there are also some cable operating failures due to improper installation of cable lead wires (such as insufficient distance between cables or ground, dirty cable heads, or water ingress of the motor foundation, etc.), these failures are mainly caused by some simple treatment; most Ambiguous are those cable operating faults that are momentarily grounded, producing unstable flashovers. After the power failure of the cable, a considerable part of the insulation resistance measurement and DC withstand voltage test can pass, and after the cable is put into the system, it can also operate normally for a period of time; the rest is the single-phase grounding cable fault, which accounts for about 40% of the operating faults, such ground faults generally have no obvious external deformation, and the grounding resistance is not too high (usually tens to hundreds of ohms). Anatomical failure points have subtle carbonation spots.