Underground power cable faults are complex and variable, causing power cable faults can be broadly categorized into the following categories.

1. mechanical damage

Cable failure caused by mechanical damage accounted for a large proportion of cable accidents. Some mechanical damage is very slight, did not cause failure, to months or even years after the damage to develop into a failure. The main causes of mechanical damage to the cable are:

(1) installation damage. Installation of careless injury to the cable; mechanical traction is too large strain on the cable; excessive bending injury to the cable.

(2) Directly affected by external damage. In the installation of the cable path or nearby civil construction, so that the cable directly by external damage.

(3) The vibration or impact load of moving vehicles can also cause cracks in the lead (aluminum) package of underground cables.

(4) Damage caused by natural phenomena. Such as the expansion of the intermediate joints or terminal head of the inner insulating rubber and expansion of the shell or cable sheath; mounted in the pipe mouth or bracket on the cable sheath abrasion; caused by land subsidence excessive tension, pulling off the intermediate joints or conductors.

2. Insulation moisture

Dampness of the insulation will cause the cable voltage resistance to drop and produce faults. The main reasons for cable moisture are:

(1) Water ingress due to unsealed or poorly installed joint box or terminal box structure.

(2) The cable is poorly manufactured with small holes or cracks in the metal sheath.

(3) Perforation of the metal sheath due to puncture or corrosion by foreign objects.

3. Aging and deterioration of insulation

Aging deterioration of insulation will cause the cable voltage resistance to drop and produce faults. The main reasons for cable aging are:

(1) slag quality or air gap within the cable medium, free and hydrolyzed under the action of electric field.

(2) Cable overload or poor ventilation of the cable trench, resulting in localized overheating.

(3) Loss of insulation in oil-impregnated paper-insulated cables.

(4) Overtime use of power cables.

4. Overvoltage

Overvoltage will make defective cable insulation layer electrical breakdown, causing cable failure. The main causes are: atmospheric overvoltage (such as lightning); internal overvoltage (such as operational overvoltage).

5. Poor design and production process

Cable head and middle design and production process is poor, will also cause cable failure. The main reasons are: electric field distribution design is not well thought out; improper material selection; poor workmanship, not according to the requirements of the regulations.

The nature and classification of cable faults

1. To fault material characteristics of classification

Can be divided into series faults, parallel faults and composite faults of three categories.

(1) series fault

Series fault (metal material defects) refers to the cable one or more conductors (including lead, aluminum sheath) disconnected fault. It is a cable open circuit fault in the broad sense. Because the continuity of the cable core is disrupted, the formation of broken or incomplete disconnection. Incomplete disconnection is particularly difficult to detect. Series faults can be specifically divided into: a point of open circuit, multi-point open circuit, one-phase disconnection, multi-phase disconnection, and so on.

(2) parallel faults

Parallel faults (insulation material defects) is to guide the body to the outer skin or between the conductor of the insulation level drops, can not withstand the normal operating voltage and the occurrence of short-circuit faults. It is a cable short-circuit fault in the broad sense. This type of fault due to the destruction of insulation between the cable core or between the cable core to the outer skin and the formation of short circuit, grounding, flashover breakdown and other phenomena, the frequency of occurrence in the field is high. Parallel faults can be specifically divided into: one-phase grounding, two-phase grounding, two-phase short circuit, three-phase short circuit.

(3) composite fault

Compound fault (insulation materials, metal materials are defective) refers to the cable core and the insulation between the core are faulty. It includes one-phase disconnection and grounding, two-phase disconnection and grounding, two-phase short-circuiting and grounding.

2. Classification of insulation characteristics at the point of failure

According to the cable fault point insulation resistance Rf and breakdown gap G, cable faults can be divided into open-circuit faults, low-resistance faults, high-resistance faults, flashover faults four categories. The classification method is the most basic classification method for on-site cable faults, which is especially beneficial for the selection of detection methods.

Among them, the gap breakdown voltage UG depends on the size of the fault point discharge channel (i.e., breakdown gap) distance G, the size of the insulation resistance Rf depends on the fault point of the cable dielectric carbonization, the size of the distribution capacitance Cf depends on the fault point of the degree of moisture.

(1) open-circuit fault

The continuity of the metal part of the cable is damaged, forming a broken line, and the fault point of the insulation material is also damaged to varying degrees. Field megohmmeter insulation resistance Rf for infinity (∞), but in the DC withstand voltage test, there will be an electrical breakdown; check the core wire conduction, there are broken points. The scene is generally in the form of a phase or two-phase disconnection and grounding.

(2) Low resistance fault

The cable insulation material is damaged and there is a ground fault. The scene with a megohmmeter to measure its insulation resistance Rf less than 10Z0 (Z0 for the wave impedance of the cable, generally taken between 10 ~ 40Ω). Field a