Regarding a cable installation issue?
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I would like to ask everyone: during cable installation, parallel return wires are installed, and it is said that this can reduce the intensity of electrical interference. But how does this function work?
In cable circuits where one end of the metal shield is grounded, in order to ensure that the induced voltage within the shield does not exceed the allowable limits, it is necessary to install a conductor that runs parallel to the cable circuit, with both ends of this conductor grounded; such a conductor is known as a return wire. When a single-phase ground fault occurs, the ground short-circuit current can flow back to the system’s center point through the return path. Since the magnetic flux generated by the ground current flowing through the return path cancels out part of the magnetic flux produced by the ground current in the cable conductors, this helps to reduce the induced voltage in the sheath during a short-circuit fault.
Reply to 2# zyz8438: I wonder if the statement that one end of the cable’s metal part should be grounded refers to a power supply system that is directly grounded. Additionally, the induced voltage mentioned here in relation to cable shielding should be generated when the cable is short-circuited. So, when the cable is operating normally, could there be an issue of excessive induced voltage on the cable? Thank you.
No, here it definitely refers to high-voltage (35 kV and above) single-core cables. Since it is a single-core cable, if the metal shielding is grounded in two sections, a large current will be induced throughout the shielding, causing the cable to heat up. One end of the cable’s metal layer being grounded does not mean that the power supply system is a directly grounded system; rather, it means that one end of the metal shielding layer (armoring and shielding layer) of a single-core cable is grounded.
Reply to 4# zyz8438: Thank you. May I ask why the current is much larger when both ends of the cable’s metal part are grounded, compared to when only one end is grounded?
With only one end grounded, a closed circuit is not formed, so there is no induced current; however, there is a relatively high induced voltage. Generally, it is required to be no more than 50-100V; if it is higher, an overvoltage protector must be installed.