What Is a Return Wall?

In order to prevent the magnetic field caused by the single-phase ground fault current from seriously affecting the outside world, an insulated wire with a low impedance can be laid in parallel along the cable line and grounded at both ends. This ground wire is called a return line, also known as a shielded conductor. ^[1]

In a cable line grounded at one end of a metal sheath, in order to ensure that the induced voltage in the sheath does not exceed the allowable standard, a conductor laid in parallel along the cable line must be installed, and both ends of the conductor must be grounded. This type of conductor is called backflow line. ^[2]

According to the relevant standards, the AC system is 110kV and above

In engineering design, for single-point grounding cable lines, designers usually arrange return lines along the line. However, it is rarely calculated whether it is necessary to suppress the induced voltage of the metal sheath or the interference to the nearby weak current lines. When multiple loop cables are parallel

After the single-phase ground fault current uses the return line as a loop, it not only reduces the inductive reactance of the metal sheath, but also eliminates the influence of the grounding network ground resistance. In order to minimize the value, the return line should be arranged as far away from the cable as possible. However, the three cables have the same probability of a single-phase ground fault, so they should be at the same distance from the three cables. In addition, during normal operation, it is required that no induced current be generated in the return line to avoid ohmic losses. Therefore, during the normal operation, the induced voltage on the return line must be zero.

In engineering applications, there are two ways to lay the return line. One is a parallel arrangement, which is easier to implement during laying, but an induced voltage will also be generated on the return line during normal operation. The other uses an arrangement as shown in FIG. During normal operation, it is required that no induced current be generated in the return line to avoid ohmic losses, so the induced voltage on the return line is required to be zero during normal operation. In order to achieve the above purpose, this arrangement is generally adopted for the return line.

(1) Single-ended grounding: single-phase grounding occurs when the cable line (sheathing grounding method: one end is grounded and the other end is with a sheath protector)

Reasons for stolen return lines

There are three main reasons for the theft of the return line:

First, in order to better protect the non-metallic outer sheath insulation of the cable, the material of the return line is mainly a copper core cable with an insulation layer. Many people who steal the return line just know that the recycling profit of copper is high, so they are salivating for the value of copper. Even if they risk the life of a livelihood, they have to find ways to remove the copper wire for recycling.

Secondly, under the normal operation of the cable line, the zero-sequence current flowing on the return line is very small, which does not pose a threat to human life, and adds criminal courage to those who steal power facilities.

Third, the location of the return line is not concealed enough, and it is often exposed to the outside, which makes people who steal power facilities more handy. ^[1]

Return line anti-theft protection

1. Anti-theft measures for running cable trench return lines:

(1) Add stainless steel expansion screws at every short distance, and fix the return line with special clips; (2) Add sandbag separation walls at each inspection well. (3) Block the top of the cable cover of the sandbag partition wall with cement mortar.

2. Requirements for new cable projects:

(1) For new cable projects with return lines, use sand filling as much as possible for protection. For cable trenches that cannot be filled with sand and prevent theft, it is recommended to embed the return lines on the surface of the cable trench body so that the return lines are not exposed. On the other hand, we can try to eliminate the criminal thoughts of theft from power facilities in the cable field by canceling the ground protection box, shortening the length of the ground cable, increasing the installation height of the ground cable, and improving the installation position of the ground cable; 2) For new cable projects with return lines, it is recommended to replace the return line material with galvanized round steel or aluminum stranded wire (extrusion PE layer), but the cross-sectional area of the replacement material should meet the requirements for thermal stability of short-circuit current; 3) When designing the cable project, fully consider that the cable line is arranged in a cross-interconnection regular triangle arrangement with equal lengths, or a combination of cross-connection and single-end grounding.

3. Increase inspection intensity to ensure inspection quality: (1) Since theft return lines often take places that are easy to get started, such as cable trench covers and manhole covers, cable operation and maintenance personnel should pay attention to relevant cable facilities and equipment during inspections. And change records in a timely manner, and the cover or manhole cover that has been moved or pried should be inside the cable trench to check the integrity and integrity of the auxiliary equipment of the cable; (2) After the new line is put into operation, Record and compare the circulating current values of the cable line under no-load operation and with load respectively. ^[1]