Specialty cables in mechanical engineering - Types of connection, control cables and their applications

In industry, cables and lines ensure safe and efficient signal and energy transmission. But which cable is suitable for which application? Whether it’s a power cable, data cable or hybrid solution, the right choice depends on factors such as flexibility, insulation material, shielding and compliance with (inter)national standards. In this article, we will introduce you to different types of cables and lines, discuss their structure and give you a decision-making aid for selecting cables.

Types of cables

Both cables and wires are used to transmit signals, data, and electrical energy. So why the two terms? In the English-speaking world, for example, the distinction is not made; everything falls under the term "cable". There is no clean technological separation. However, if one would like to make a characterization, cables are named as a sub-term of the lines according to VDE. They must meet specific requirements such as e.g., the requirement for "installation in the earth area". In general, three types of cables or lines are distinguished:

• Power lines transmit electrical energy.
• Signal and control cables transmit electrical signal states
• Data cables transmit electrical signal states, with a higher data rate than e.g. signal lines
• Hybrid cables combine the transmission of electrical energy and data.

Control cables in detail

The term control cable refers to lines that have been specialized for the transmission of control signals (both analog and digital) between a control unit and an installed component within a system or machine. A typical application for control cables is the automation industry, where they are used to transmit signals and power smaller applications. To transmit different control signals, the control line usually contains several wires. The wires are color-coded for easy installation and identification.

Construction of cables and lines

The basic construction of cables and lines consists of several insulated electrical conductors enclosed by an outer protective sheath. In cables and lines, there are often additional layers, e.g., metallic sheath layers for shielding. Jacketing is a critical factor for protecting against mechanical or chemical stresses and temperature influences. The choice of sheath material is critical to the longevity and reliability of the line and depends on the specific conditions of use. PVC (polyvinyl chloride) is a good choice, e.g., under chemical stress; PUR (polyurethane) is better when near mineral oils. For the laying of control cables in the ground, the sheath material may be reinforced with a closed-cell outer sheath, which offers a high resistance to pressure loads.

Now learn how to select the right (control) lines.

Selection of lines

Control cables and connection cables must withstand various challenges. Torsion, dynamic movement, or tensile loads are just a few of these challenges. The following environmental conditions and influencing factors must be taken into account during selection.

Installation type and the problem with bending and torsion

In principle, cables and lines are used either movably in flexible environments or fixed in a static environment. Particularly when used in areas with moving applications, there are increased requirements for the flexibility and breakage resistance of the cables and lines.

A torsional load on the cables and lines, i.e., the twisting about their own axis, must generally be avoided.  However, bends are often allowed to some degree depending on the cable type or line used. To learn more about torsion, check out our Torsion: How to understand torsion.

In automated applications and robotic systems, control cables must be able to pass through a variety of bending cycles without material fatigue. Control cables made of PUR are particularly flexible. These flexible control cables can be used both in fixed routing under high mechanical loads and in flexible applications with frequent movement. An example of use in a static environment would be the home installation, where the cables are laid once permanently and then usually remain in place.

Flexible control cables can also be used in so-called energy chains to protect against overuse. Energy chains, also called drag chains, serve the safe routing of flexible cables in moving system parts over a specified distance. They prevent crushing and abrasion of cables that are always in motion. They also prevent tangling of multiple lines side by side.

You can also read more about energy chains in our article Avoiding cable breakage - Selecting the right energy chain.

Jacket / Shielding

The cable jacket protects the inner conductors from mechanical influences, moisture, and chemical substances. Protection against corrosion and erosion is critical, especially in demanding environments. Depending on the application, materials such as polyvinyl chloride (PVC), polyurethane (PUR) or polyethylene (PE) are used.

Additional shielding, for example through a metallic braid, significantly reduces susceptibility to electromagnetic interference.

Shielding minimizes external interference and reduces the emission of interference to other devices. MISUMI offers shielded control cables for various applications. These can be used in adverse environmental conditions, e.g., in a temperature range of -40 °C to +80 °C (PUR). For example, when used in cold areas, PUR control cables have greater flexibility compared to many other materials.

Lines: What is an electrical conductor?

An electrical conductor is a material in which electrical current can flow. Most often, the highly electrically conductive material copper is used.

When this drawn material is insulated as a single wire, it is called a single-wire conductor. If multiple wires are used and provided with common insulation, it is called a multi-wire conductor. According to the possible use of the conductor, a distinction is also made between rigid and flexible conductors. Rigid conductors are typically single-wire. Unlike rigid conductors, flexible conductors are typically multi-wire. The copper wires used in the flexible conductor are usually also twisted together.

An electrical cable usually consists of several individually insulated electrical conductors, in this context also called cores, which are collectively enclosed by a common insulation (sheath).

Structure of a control cable

A copper wire is usually installed in control cables, which is used due to its good electrical conductivity. For better solderability and corrosion resistance, it is additionally tin-plated. The individual wires are usually stranded in layers, whereby the lay length – i.e. the length of a complete rotation of the stranding – affects the flexibility and the mechanical load capacity of the cable. A shorter lay length results in greater flexibility, while a longer lay length increases stability.

Halogen content

Halogens, such as fluorine, chlorine, or iodine (Jod), have traditionally been used in cable manufacturing to increase the flame retardancy of the jacket. However, if there is actually a fire, they have a major disadvantage: They release highly toxic gases. For this reason, halogen-free cables are becoming increasingly important, especially in safety-critical areas such as industrial plants. When selecting cables and lines, the halogen content must therefore also be taken into account.

Additional selection criteria

Additional selection criteria for selecting leads and cables may include:

• Regional approval: The regional standards and approvals must be complied with. See the section below on the global standards.
• UV resistance: In direct sunlight, the sheath material should be UV resistant to prevent premature aging or embrittlement.
• Oil resistance: If oil contact is expected, the material should be oil resistant.
• Tested components: The cables should be selected according to the required nominal voltage, test voltage and current load capacity.
• Transmission distance: The maximum transmission distance must be taken into account to prevent signal losses.
• Clean room qualification: In the clean room, special requirements for particle emission and chemical resistance apply.

Global standards

Cables and wires are subject to different standards around the world. In addition to regional standards, there are also international standards. Compliance with these standards is also essential for the approval of machines and systems in which the lines are installed.

In Europe, the HAR (Harmonized Approval Mark) certification standard applies. Products with the HAR mark have been subjected to uniform quality assurance. Safety, fire behavior and mechanical strength are considered and evaluated. Once a cable is HAR marked, it can be easily distributed within the EU. Certain VDE standards continue to apply in Germany. Under this, cables and lines are not only tested for safety, but also to what extent they can be used in already existing electrical systems. VDE standards also apply beyond Germany.

Here is a selection of valid VDE standards:

• DIN VDE 0100-510: Selection and installation of electrical equipment - General provisions
• DIN VDE 0100-520: Selection and installation of electrical equipment – cable and line installations
• DIN VDE 0298-565-1: Guide to using cables and insulated lines rated at no more than 450/750 V
• DIN VDE 0298 Part 4: Use of cables and insulated lines for high-voltage power installations

For example, VDE 0100-510 determines in which colors conductors are marked.

• Protective conductor = green-yellow
• Neutral = blue
• Outer conductor = brown, gray or black (multiple outer conductors cannot be the same color)

In the United States, the Underwriters Laboratories (UL) organization confirms compliance with its own safety standards. First and foremost, the safety and fire behavior of cables and lines are tested. If the cables pass the test, they will receive UL certification, which simplifies the distribution of cables in Canada and the USA. In Canada, the CSA standard also regulates the Canadian standard. Safety aspects of electrical products are also the primary focus here.

For future-oriented production, it is important that cables meet several standard requirements so that they can be brought to market internationally.