Guide to Designing A Custom Cable P. I
It’s often said that a system is only as good as its weakest component. Likewise, a high-quality cable is only as good as its weakest components. There are several components that comprise a custom cable assembly. Following is a list of critical factors to consider as you design a custom cable.
1. Cable Conductor:
The conductor is essentially the most important part of the cable; it’s responsible for moving energy or signal from one point to another. Conductors contribute greatly to the performance and reliability of electrical systems.
Cable Conductor Material:
The conductor can be made of different material. Silver, copper, nickel, aluminum, and tin are some of the most popular conductors. Conductors are often coated or covered to facilitate stripping of components applied over the conductor, to improve solderability or to provide electrostatic shielding for overlying insulation, etc. Electroplate and Mylar are two common applications.
Silver will be the most conductive metal; however its high price limits its usage to selective applications. Alternatively, silver plated wire is less expensive than using silver alone and is able to provide high conductivity. Nickel plated wire operates in extreme conditions and temperature ranges. Copper is a good conductor with a lower coefficient of thermal expansion. Tin plated copper is easy to solder, thus simplifying assembly. Aluminum is lightweight, more affordable, but less conductive than copper and oxidation is a particular problem with aluminum conductors.
Cable Conductor Configuration:
Conductors can be configured in a variety of ways. They can be constructed solid or stranded. Solid-core conductors are made of one single strand or core of wire while stranded conductors are made of several threads of wire. Typically a solid core wire is applied in situations where the wire is not designed to be continuously flexed. Stranded wires are used where the wire needs to be routed through tight space or experiences frequent flexing.
Solid conductors are less costly to produce and are more compact for the same current carrying capability as stranded conductors. Stranded conductors provide greater flexibility and withstand a greater amount of flexing and considerable movement.
2. Cable Insulation & Jacket:
The insulation of the cable acts as a buffer to avoid the conductor and the shield from coming in contact. It prevents current from coming in contact with one another, thus preventing interruption and electrical leakage. Electrical characteristics and physical considerations are two major considerations when deciding what type of insulation is needed for the application. We discuss some of the majors ones here:
Electrical Characteristics to Consider for Insulation Purposes:
Capacitance: a measure of the insulation’s ability to store electrical energy; generally lower capacitance equates to a higher performance cable
Attenuation: a measure of the cable’s loss of electrical energy
- Velocity of propagation: high velocity equates to lower electrical signal loss
- Dielectric strength: the material’s ability to withstand voltage breakdown
- Working voltage: maximum voltage allowable by UL to be applied to the cable
- Dielectric constant (Er): lower Er = lower capacitance, Higher impedance, and Lower attenuation
Physical Considerations to Consider for Insulation Purposes:
Elongation: how far the material will stretch before breaking
Tensile Strength: how much force it takes to break the insulation or jacket material
- Temperature rating: the range of temperatures at which the material can be used without degradation
- Flammability: ability of a cable to burn
- Flexibility: ability of a cable to bend
Types of Jackets for Cables:
The jacket is the outermost layer of a cable assembly. Its main responsibility is to protect the conductor, insulator and shield from mechanical, moisture and chemical damage. The jacket is also customizable in regards to colors and is a beneficial feature for cable identification, grouping and branding. There are two major types of insulation and jackets that are used today, Thermoplastic (TP) and Thermoset (TS).
Thermoplastic Cable Jackets:
Thermoplastic can readily be softened and re-softened by repeated heating, but remains in a safety usable solid form when operated within its rated temperature limits. TP jackets are lower in cost and lower in weight, making them very popular.
Polyvinylchloride (PVC), Polyethylene (PE), Polypropylene (PP), Polyurethane (PU), and Teflon are some of the most common TPs. Thermoplastic polyurethane (TPU) is a hybrid material and has seen an ever increasing application. It is a mixture between hard plastic and soft silicone. TPU is a very dense and smooth rubber with many properties, including elasticity, transparency, and resistance to oil, grease and abrasion.
TP materials have a diverse temperature range. For example, PVC has a normal temperature rating of 60-90 degrees Celsius, while Teflon has a normal temperature rating of 150-200 degrees Celsius.
Thermoset Cable Jackets:
Thermoset materials, when subject to heat and pressure, undergo a chemical change known as vulcanization or curing. The process sets the physical properties of the material so that if it is again exposed to heat, the material will not melt. TS is most flexible at room and lower temperature.
Natural rubber, Silicone rubber, Neoprene, and Cross-linked Polyethylene (XLPE) are some of the common TS materials.
In our next blog we will discuss other factors such as shielding, overmold and strain relief.
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