A Brief Discussion on Common Types of Cable Faults

1、 Low resistance fault, damage to cable insulation material, resulting in grounding fault. On site, use a megohmmeter to measure the insulation resistance Rf, which is less than 10Z0 (Z0 is the wave impedance of the cable, usually between 10-40 Ω). The probability of low resistance faults occurring in low-voltage power cables and control cables on site is generally high.

2、 Open circuit fault, the continuity of the metal part of the cable is disrupted, resulting in a broken wire, and the insulation material at the fault point is also damaged to varying degrees. The insulation resistance Rf measured on site with a megohmmeter is infinite (∞), but during the DC withstand voltage test, electrical breakdown may occur; Check the conductivity of the core wire, there is a breakpoint. On site, it usually appears in the form of one or two phase disconnection and grounding.
3、 High resistance fault, damage to cable insulation material, resulting in grounding fault. The insulation resistance Rf measured on site with a megohmmeter is greater than 10Z0. During the DC high-voltage pulse test, electrical breakdown may occur. High impedance faults are cable faults with a high probability of occurring in high-voltage power cables (6KV or 10KV power cables), accounting for over 80% of total faults.
4、 Flashover fault occurred due to damage to the insulation material of the cable. The insulation resistance Rf measured on site with a megohmmeter is infinite (∞), but flashover and electrical breakdown may occur during DC withstand voltage or high-voltage pulse tests. Flashover faults are difficult to detect, especially when newly laid cables undergo preventive testing and flashover faults occur. The DC flashover method is generally used for detection on site.
5、 Breakdown fault, in practical work, is a cable insulation damage event triggered by preventive testing, commonly referred to as cable breakdown. This type of fault occurs under direct current experimental voltage, and its insulation damage is electrical breakdown. The grounding point is generally intact with lead or copper foil, and there is no obvious deformation on the outside (except for mechanical trauma). Cable breakdown faults are mostly simple grounding faults, with high grounding faults. Upon dissection of the fault point, there are no carbonization points in the insulation material, but carbon pores and water tree aging structures can be detected through instruments.
6、 Operational fault refers to a cable fault that occurs during the operation of the factory power system, where voltage fluctuations or grounding signals are detected in the high-voltage secondary circuit of cable feeders, motors, and transformer cable leads (due to grounding tripping of power components with grounding protection), and the possibility of other power component faults is eliminated to determine the cable fault. The characteristic of this type of malfunction is ambiguity. The form of cable operation failure is cable blasting (such as phase to phase short circuit caused by two-point grounding); Another part of the operational faults may develop into cable breakdown faults (such as cable aging, insulation defects, etc.) due to the inability to withstand voltage during stop point inspections.

Phase to phase short circuit, ground short circuit, open circuit fault, and wire break fault are divided into two categories: high resistance fault and low resistance fault. That is to say, they either break through without grounding or directly fuse to ground.

Explain the methods for determining high resistance faults and ground faults separately.

High resistance fault

High resistance faults are more common in high-voltage cables because the insulation layer is thick and the voltage is high. After the cable fails, it will not directly face the ground. It is still a pure resistance of a certain size. When the resistance value is greater than 200 ohms, high-voltage flashover method is usually used to locate the fault point.

ground fault

Grounding fault, as the name suggests, refers to the connection between the fault point and the earth, forming a dead grounding state. The measurement method for this type of grounding fault is usually the step voltage method.

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