We know about household electrical wiring, but do you know about control cables? Control cables are cables that transmit signals or control operational functions from the control center to various systems. Early control cables had relatively simple functions, including indicator lights, instrument indications, relay and switchgear operation, and alarm interlock systems. In recent years, the widespread use of low-voltage power and computer networks has brought new capabilities and higher requirements to the selection and application of control cables. This article discusses some new issues that have arisen in the selection and use of control cables in recent years, which can be used as a reference.
How to Choose a Control Cable?
Today, the main control cable products are divided into three categories: polyvinyl chloride (PVC) insulated control cables, natural styrene-butadiene rubber (SBR) insulated control cables, and polyethylene (PE) insulated control cables. Cross-linked polyethylene (XLPE) and ethylene propylene rubber (EPR) insulated products are also available. The oil-impregnated paper-insulated lead-sheathed control cables, which were once produced, have been phased out.
The rated voltage of a control cable is denoted by U0/U. my country's national standard, issued in 1998, specifies 450/750V for plastic-insulated control cables. Countries like Germany have proposed standardizing 600/1000V control cables. Currently, my country can also produce 600/1000V plastic-insulated control cables. The rated voltage of rubber-insulated control cables is 300/500V. Control cables use copper cores with a nominal cross-section of 2.5mm² or less, with 2-61 cores; 4-6mm² with 2-14 cores; and 10mm² with 2-10 cores. The operating temperature of control cables is 65°C for rubber insulation and 70°C and 105°C for PVC insulation. Control cables used in computer systems generally use PVC, polyethylene, cross-linked polyethylene, or fluoroplastic insulation.
Precautions for Control Cable Use
1. Measures to Ensure the Proper Operation of Control Cables and Prevent Interference
To minimize the impact of insulation breakdown, mechanical damage, or fire in control cables, the national standard GB50217-91, "Design Specifications for Power Engineering Cables," stipulates that independent control cables should be used for dual protection systems, such as current and voltage protection, as well as DC power supply and trip control circuits, which require enhanced reliability. After the control cable is put into operation, there will be electrical interference between different cores of the same cable and between cables laid in parallel.
2. Measures to prevent or reduce electrical interference mainly include the following three aspects.
(1) Grounding a spare core of the control cable: Practice has shown that when a spare core in the control cable is grounded, the amplitude of the interference voltage can be reduced to 25%~50%, and the implementation is simple, while the cost of the cable is minimal.
(2) For circuits that will have serious consequences when electrical interference occurs, a single control cable should not be used. These include:
① Weak current signal control circuit and strong current signal control circuit.
② Low level signal and high level signal circuit.
③ The weak current control circuits of each phase of the AC circuit breaker for phase-by-phase operation should not use the same control cable. However, if each pair of return conductors in a weak-current circuit belongs to a different control cable, a loop may form during installation. Interlinking electromagnetic wires from nearby power sources can induce potentials, which can significantly interfere with low-level parameters in the weak-current circuit. Therefore, it is best to use a single control cable for both return conductors.
III. Metal Shielding and Shield Grounding
Metal shielding is an important measure for reducing and preventing electrical interference. It includes overall shielding, sub-shielding, and double-layer overall shielding. The type of metal shielding used in control cables should be selected based on the potential impact of electrical interference and factored into comprehensive interference suppression measures to meet interference and overvoltage reduction requirements. Higher interference prevention requirements require greater investment. Using steel tape armor or steel wire braided overall shielding increases cable prices by approximately 10% to 20%.
Control cables differ from general power cables in some respects. Functionally, control cables are used for control wiring of electrical equipment, while power cables are used to transmit and distribute high-performance electrical energy within the main power system. Some basic knowledge about control cables are explained above, I hope it can help everyone.
