What are the measures to prevent or reduce electrical interference in control cables?
Dec 26,2022
The main measures to prevent or reduce electrical interference in control cables include the following three aspects:
1. Ground one spare core of the control cable.
Practice has shown that when the spare cores in control cables are grounded, the amplitude of interference voltage can be reduced to 25% to 50%. The increase in cable cost is not significant, and the operation is straightforward.
2. Control cables must not be used in circuits that could cause severe electrical interference.
This utility model comprises: (1) a weak-current signal control circuit and a strong-current signal control circuit; (2) low-level and high-level signal circuits; (3) under no circumstances shall the same control cable be used at each stage of the AC circuit breaker’s breaking process.
3. Metal shielding and grounding of the shielding layer
Metal shielding is an important measure for mitigating and preventing electrical interference, and it includes full shielding, sub-shielding, and double-layer full shielding. The selection of control cables with metal shielding should be based on the cable’s ability to resist interference and its electrical potential, as well as comprehensive measures to reduce interference, in order to meet the requirements for both interference reduction and voltage stabilization. High interference resistance demands higher investment. When steel tape armor and wire-braided shielding are fully utilized, the cable price can decrease by 10% to 20%.
For control interference in high-current circuits, since the signals involved are strong, control cables without metallic shielding can be selected—provided that they are located within ultra-high-voltage switchgear or run close to and parallel with high-voltage cables. However, when control cables used in low-level signal control circuits are situated in environments subject to interference and no effective anti-interference measures are in place, control cables equipped with metallic shielding should be chosen to prevent electrical interference from causing malfunctions or insulation breakdown in low-level signal circuits.
The selection principles for the shielding type of control cables in the signal circuits of computer monitoring systems are as follows:
(1) Digital signals, shieldable;
(2) For high-level analog signals, the overall shielding of the conductor cores should be used; if necessary, secondary shielding of the conductor cores may also be employed.
(3) For low-level analog signals or pulse signals, use the secondary shielding of the conductor core; if necessary, a composite overall shield with secondary shielding for the conductor core can also be used.
The following points should be noted regarding the grounding method of the shielding layer:
(1) The shielding layer of the control cable for the analog signal circuit in the computer’s temporary control system should be grounded at a single point. This is because, to ensure the normal operation of the computer monitoring system, even an interference voltage as low as about 1V can lead to errors in logical judgment. Centralized grounding helps prevent ground loops.
(2) Except for the control cables of computer monitoring systems, whose shielding layers are permitted to be grounded at only one point, the shielding layers of other control cables should be grounded at two points when subjected to significant electromagnetic induction interference, and at one point when subjected to significant electrostatic induction interference.
(3) The inner shielding layer of double-shielded or composite fully-shielded cables shall be grounded at one point, while the outer shielding layer may be grounded at two points.
(4) When selecting two-point grounding, it is also necessary to ensure that the shielding layer will not be burned out under the influence of transient currents.
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What are the measures to prevent or reduce electrical interference in control cables?
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