One of the important parts of designing or installing any electronic equipment is the selection of cables. This must be done carefully to ensure the proper functioning fo the equipment and the durability of the cable. There are many types of cables available in the market today — from multicore cables to submersible flat cables. To select the right cable, it is essential that we understand the various factors that influence performance.
Broadly speaking, there are four key factors that influence cable selection:
A key factor that must be considered to improve the longevity of a cable is installed. Where will it be installed? What is the environment in which it must perform? What are the hostile elements that can hamper performance or longevity? How can we ensure a safe layout and installation? The installation is important in not just the selection of the cable, but also to see that the procedure is carried out smoothly.
The environment of the installation will be a detrimental factor in selecting the material of the cable. Some environments limit our selection. For instance, only submersible flat cables can be used in submerged or semi-submerged conditions. Even after you have selected the cable, the installation process is just as important. The installation crew has to maintain the correct bending radius to avoid stress point that causes cracks.
The installation has to also consider future maintenance requirements, existing company or industry policies, practices, and legal requirements.
In certain conditions, we have to consider the cable construction. This will involve components like conductors, insulators and outer jackets. We also look at the conductor arrangement inside the cable. Copper and aluminum are the two most common conductors. They are proffered because of their workmanship, resistance to environmental corrosions, and ease of maintenance. Aluminum conductors are more common of the two.
The choice of insulation and outer jacket depends on many factors, including the installation, service conditions, load requirement, and ambient operating temperature. Some installations are very demanding and we have to choose the insulation carefully in such cases. For instance, conditions with highly corrosive atmosphere, presence of solvents and grease, insects, presence of ozone, and extreme temperatures.
Next comes the conductor’s arrangement. This can be a single-conductor, three-conductor cable or a multicore. Each has its advantages and disadvantages. Single conductors are easy to splice and easy to install. Three-conductor cable has less reactance. In addition, you may need a ground wire for certain applications and this is best in three-conductor cables. In some special conditions, you may even need multicore cables.
Hence, the arrangement of the cable can become very critical. Submersible flat cables, for instance, are chosen for their heavy insulation and linear arrangement of the conductors. The flat construction makes it more flexible and durable.
This is where we consider the cable load, specifically the voltage and current requirements. The three main factors here are current carrying capacity, regulation of voltage, short circuit rating. Some people make the mistake of ignoring the last two factors. But this can be a safety hazard. The current carrying capacity depends on thermal heating. The current capacity is designed to allow thermal dissipation. There are strict guidelines to make this easy and safe.
Voltage regulation is not normally a consideration unless there are frequent voltage drops or there are rotating loads. In addition, the cable size must be as per the short circuit rating. In other words, in case of a short circuit, the cable should be able to hold till the fault is checked.
One of the main considerations is whether one should go for shielded cables or unshielded cables. Unshielded cables cancel out the electromagnetic interference or EMI because of their twisted conductors. So, the first criteria are to check the prevalence of EMI in the area. But there are safety hazards to be considered when working with unshielded cables.
Shielded cables, on the other hand, are more effective in blocking interference. Shielding is considered when working within high voltage conditions, a damp environment, when moving from a conducting to the non-conducting environment, presence of contaminants, radio interference, and connection to aerial wires.
These are just some factors that we keep in mind when selecting cables —whether shielded, unshielded, single core, multicore or submersible flat cables.