Cable shielding, especially for signal cables, can be divided into the following types:
Individual shielding—independent shielding of each core;
Pair shielding—independent shielding of each twisted pair;
Group shielding—independent shielding of a multi-core cable group;
Overall shielding—outermost shielding of the entire cable.
These shielding methods can be combined.
For detailed information on cable selection, see "GB50217-2007 Specification for Design of Power Engineering Cables":
3.6 Control Cables and Their Metallic Shielding
3.6.1 Dual protection systems for current and voltage, as well as DC power supply and trip control circuits, requiring enhanced reliability, should utilize separate control cables.
3.6.2 The following circuits should not share the same control cable:
1 Weak current signal and control circuits and strong current signal and control circuits.
2 Low-level signal and high-level signal circuits.
3 Weak current control circuits for each phase of an AC circuit breaker for split-phase operation. 3.6.3 Each pair of return conductors in a weak-current circuit should belong to the same control cable.
3.6.4 The phase and neutral conductors of each secondary winding of a current transformer and voltage transformer should be located in the same cable.
3.6.5 Control cables for strong-current circuits may not have a metal shield, except when located in high-voltage distribution equipment or when running adjacent to high-voltage cables for a long period, where interference suppression is required.
3.6.6 Control cables for weak-current signal and control circuits should be metal shielded when located in an environment subject to interference and where effective anti-interference measures are unavailable.
3.6.7 The type of metal shielding for control cables should be selected based on the potential impact of electrical interference, taking into account comprehensive interference suppression measures, and meeting requirements for reducing interference or overvoltage. Furthermore, the following provisions must be met:
1 Weak-current control cables located in distribution equipment above 110 kV should have an overall shield or double-layer overall shield. 2. Control cables for current, voltage, and signal contacts used for integrated circuit and microcomputer protection should be shielded.
3. Shielding for signal loop control cables in computer monitoring systems should comply with the following regulations:
1) For switching signals, an overall shield can be used.
2) For high-level analog signals, an overall shield for twisted pairs is recommended; if necessary, separate shielding can also be used.
3) For low-level analog signals or pulse signals, separate shielding for twisted pairs is recommended; if necessary, a combined overall shielding can be used.
4. For other situations, the appropriate shielding type should be selected based on factors such as electromagnetic induction, electrostatic induction, and ground potential rise.
5. When cables have steel armor or metal sheaths, their shielding function should be fully utilized.
3.6.8. For control cables requiring reduced electrical interference, a grounded spare core may be added, and the grounding should be performed at a single point on the control room side. 3.6.9 The grounding method for the metal shield of control cables shall comply with the following regulations:
1. The shield layer of the analog signal loop control cable of a computer monitoring system shall not be grounded at two or more points; instead, it shall be grounded at a centralized single point.
2. The shield layers of the current, voltage, and signal cables used for integrated circuit and microcomputer protection shall be grounded simultaneously at the switch installation location and in the control room.
3. For control cable shields other than those mentioned above, two-point grounding is recommended when electromagnetic induction interference is significant; single-point grounding may be used when electrostatic induction interference is significant.
For double shielding or composite overall shielding, single-point or two-point grounding is recommended for the inner and outer shields, respectively.
4. When choosing two-point grounding, it is also important to ensure that the shield layer will not melt under the influence of transient currents.
