Control Cables: A Complete guide

Control Cables: A Complete Engineering and Technical Guide to the “Nervous System” of Automation

In the era of comprehensive automation and digitalization of industry, the reliability of technological process control becomes critically important. If a power cable is the “artery” supplying energy to equipment, then a control cable is the highly sensitive “nervous system”. It transmits not power, but low-voltage control, monitoring, and measurement signals, on the accuracy of which the operation of the entire system depends. The reliability of this “nervous system” determines the uninterrupted nature of production, the safety of facilities, and the correctness of data from APCS (Automated Process Control Systems).

1. Functional Purpose and Fundamental Differences from Power Cables

A control cable is a cable product intended for fixed connection to electrical instruments, apparatus, terminal assemblies of switchgears and control panels for the purpose of transmitting discrete (on/off) and analog (temperature, pressure, level) control, monitoring, and measurement signals.

Key functional and design differences from power cables:

2. Construction: Multi-Layer Protection of the Low-Current Signal

The construction of a control cable is optimized to protect the integrity of low-voltage signals in complex environments. Let’s analyze it layer by layer:

1. Conductor: Made of copper (KVVG grades) or aluminum (AKVVG grades). Copper provides better conductivity, contact reliability, and corrosion resistance, which is especially important for low-current circuits. Aluminum is used for budget solutions in less demanding conditions. Standard cross-sections: 0.75, 1.0, 1.5, 2.5, 4, 6, 10 mm².
2. Core Insulation: A layer of dielectric is applied to each core. Main materials:
   • PVC (V): Polyvinyl chloride compound. Most common. Has good dielectric properties, resistance to moisture, oils, acids. Modifications “ng” — reduced flammability.
   • Polyethylene (P): Has higher insulation resistance (not less than 300 MOhm*km vs. 6-10 MOhm*km for PVC).
   • Cross-linked polyethylene (Pv): For increased requirements for temperature resistance.
3. Twisting and Filling: Insulated cores are twisted into a single core. In cables of the KVVGz grade, the space between cores is filled (index “z”) with rubber compound or other material to impart a round shape, mechanical stability, and additional moisture protection.
4. Screen (if present): A critically important element for protection against electromagnetic interference. It is made in the form of a winding of aluminum or copper foil, often with a drain wire. It is marked with the letter “E” (e.g., KVVGE). Mandatory for installation near power lines, in conditions of strong industrial interference.
5. Belt Insulation and Armor: Belt insulation (tape) may be applied over the twisted cores or screen. For protection against mechanical damage during installation in the ground or tunnels, armor is used:
   • “Bb” — two steel galvanized tapes.
   • “K” — armor made of steel galvanized wires (for sections with tensile forces).
6. Outer Sheath: Protects the entire package from external influences. Most often made of PVC. It is the type of sheath that determines the fire safety properties of the cable during group installation:
   • ng(A) — does not propagate combustion according to category A.
   • ng(A)-LS — same + low smoke and gas emission (Low Smoke). Key requirement for modern facilities.
   • ng(A)-FRLS — fire resistant, maintains operability under flame conditions for up to 180 minutes.

3. Marking: Deciphering the Cable’s “Passport”

The marking of control cables according to GOST is a sequential code. Let’s look at the example KVVGE ng(A)-LS 14×1.5:

1. K — Control cable. Fundamental group designation.
2. Absence of “A” at the beginning — conductor material. If “A” is the first letter, the conductors are aluminum (AKVVG). Absence of “A” means copper conductors.
3. V — Core insulation made of PVC compound.
4. V — Sheath (outer) made of PVC compound.
5. G — Absence of protective cover (“bare”). In armored versions, this will be “B” or “K”.
6. E — Presence of a screen.
7. ng(A) — Sheath does not propagate combustion during group installation according to the highest category A.
8. -LS — Low Smoke, reduced smoke and gas emission.
9. 14×1.5 — Number of cores (14) and cross-section of each (1.5 mm²).

Other common indices: “z” — filling, “P” — flat design, “FR” — fire resistance.

4. Classification and Main Grades: From Standard to Specialized

Classification is based on key features: conductor material, type of protection, fire safety.

5. Application Areas: “Nerve Endings” of Industry and Infrastructure

• Industrial Automation (APCS): Connection between sensors on equipment (flow meters, thermocouples, level sensors) and programmable logic controllers (PLCs). Shielded versions (KVVGE ng-LS) are mandatory.
• Power Engineering and Switchgears: Control circuits for circuit breakers, disconnectors, position signaling of apparatus in switchgears 0.4-220 kV. Requires resistance to electromagnetic fields.
• Fire and Security Alarm Systems, ACS: Fire-resistant cables KVVG ng(A)-FRLS are used, capable of functioning under fire conditions to transmit signals to the control panel.
• Elevator and Crane Equipment: Control circuits, feedback, push-button stations. Flexible grades are often required (KGVV).
• Railway and Transport Infrastructure: Signaling, centralization and blocking systems, control of switches, traffic lights. Conditions: vibration, temperature fluctuations.
• Smart Buildings (BMS): Integration of heating, ventilation, lighting systems into a single control network. Cables with the LS index are used for indoor installation.

6. Practical Guide to Selection and Design

Algorithm for engineering selection of a control cable:

1. Determination of Electrical Parameters: Voltage in the circuit (usually 24V DC, 220V AC), type of current. Any standard cable (e.g., KVVG) is rated for 660/1000 V, which provides a multiple safety margin.
2. Calculation of the Number and Cross-section of Cores:
   • Number — based on the number of signals + reserve (usually 10-20%). Typical values: 4, 7, 10, 14, 19, 27 cores.
   • Cross-section — based on permissible voltage drop and mechanical strength. For relay current circuits (up to 5A) — 1.5 mm², for control circuits (buttons, signal lamps) — 0.75 or 1.0 mm². For long lines (>100m), calculation based on voltage drop is necessary.
3. Choice of Conductor Material: Copper — the standard of reliability. Aluminum — for budget projects with stationary installation and provided that special lugs are used.
4. Analysis of Installation Conditions and Selection of Construction:
   • Inside panels, trays: KVVG ng(A)-LS (for group installation).
   • Presence of electromagnetic interference: Mandatory KVVGE ng(A)-LS (shielded).
   • Installation in the ground: Armored KVBBShv.
   • Moving connection: Flexible KGVV ng-LS.
   • Fire hazard zones, evacuation systems: Fire-resistant KVVG ng(A)-FRLS.
5. Consideration of Fire Safety Requirements: For bundle installation in buildings — only cables with the “ng(A)” index. In places of mass gathering of people, subways, data centers — mandatory “ng(A)-LS” or “-LSLTx” (low toxicity).

7. Installation and Operation: Critical Nuances

• Minimum Bending Radius: For non-armored — at least 6 outer diameters, for armored — at least 10. Violation leads to deformation of insulation and screen.
• Joint Installation with Power Cables: Allowed, but when laying parallel in the same plane, the distance between them must be at least 100 mm. When crossing — an angle of 90°. In the absence of a screen, separate installation in different trays or with a metal partition is mandatory.
• Screen Termination: The screen must be grounded on one side (usually from the cabinet/PLC side) to prevent the formation of a ground loop and induced currents. Special shielded lugs or clamps are used.
• Core Marking: With a large number of cores, marking on both ends of the cable according to the connection diagram is mandatory. Tags, heat shrink tubing, or markers are used.

8. Trends and Future of Control Cables

• Increased Fire Safety: Trend towards a mass transition from conventional grades to LS control cables and fire-resistant FRLS even where not yet prescribed by norms, within the concept of “safety by default”.
• Intellectualization: Development of “smart” cables with built-in diagnostic functions for monitoring insulation integrity and polymer aging.
• Materials Science: Introduction of composite and halogen-free sheath materials combining high fire-resistant properties with environmental friendliness and increased resistance to oils and chemicals.
• Optimization for Data Centers and Telecom: Specialized thin cables with increased core packing density and improved characteristics for installation in server racks and telecommunication cabinets.

9. Conclusion: System Reliability Starts with Cable Selection

The control cable, being the link between the controlled object and the automation system, is the foundation of reliability. Its selection is not a secondary task but a comprehensive engineering solution, considering operating conditions, the electromagnetic environment, fire safety requirements, and cost-effectiveness throughout the entire life cycle (at least 15-25 years).

The use of high-quality cable products that comply with GOST 1508-78 and GOST 26411-85, such as control cables manufactured by JSC “Kazenergokabel”, combined with competent design and installation, is an investment in the stability, safety, and uninterrupted operation of any modern technological facility.