Wires and Cables

Skin effect

When conducting an alternate current, the current tends to concentrate on the surface of a wire rather than through the center. This effect worsens at higher frequencies or larger wire gauges. Skin effect is caused by eddy currents originating from changes in the magnetic field due to the alternating current, which is why it worsens as frequency increases [8]. Litz wire is used to reduce the impact of skin effect since the strands are affected less than a solid wire of the same size. In situations where this effect is anticipated to be drastic, hollow conductors are sometimes used to save resources as very little current would flow through the center in the first place [9].

The skin depth is the depth at which the current density is equal to 1/e (approximately 37%) of the current density at the surface [8]. 

Types of wires

There are many types of wires which vary in capacity, flexibility, strength, protection against EMI and many other factors. The most simple type involves a solid wire made of a single strand of metal covered in some insulation. Conversely, stranded wires are made with multiple smaller strands replacing the single strand [18]. These are more flexible and more resistant to fatigue than solid wire. However, it's larger surface area leads to more dissipation and thus a lower current capacity. Solid wire is also cheaper to produce then stranded [19]. 

Tinsel wire is a variant of stranded wire made from wrapping thin strands of foil around a core. Tinsel wire is very flexible and more resilient to fatigue then solid wire or even regular stranded wire, though it doesn't carry a lot of current.

Litz wire is made up of several individually insulated wires twisted/braided together. This design helps to reduce losses in alternating current at high frequencies due to skin effect [20].

Coaxial cables are formed from 2 concentric wires (where one wire is contained within the cavity of another). These are typically used to carry signals in telecommunication [21].

Ribbon cables are formed from several, individually insulated wires aligned in parallel [21].

Twin-leads are flat cables composed of 2 conducting wires kept a short distance apart by strip of plastic. These are used to carry radio frequency (RF) signals. Sometimes there will be gaps in the plastic separating the 2 wires, referred to then as a ladder-line cable. This reduces the surface area on which water can accumulate and allows it to flow off [21].

A twisted pair consists of a pair of wires, twisted together. This arrangement mitigates the effect of crosstalk on both wires. Twisted pair cables consist of multiple twisted pairs and can be shielded (STP) or unshielded (UTP) [22].

Magnet wire, sometimes referred to as enameled wire or winding wire, refers to a solid copper or aluminum wire covered with a thin layer of insulation. The thin layer of insulation allows it to be wound into a coil without short circuiting [23]. Magnet wire is commonly used in transformers, motors and electromagnets among other things [24].

Fiber optic wire consists of a thin glass wire used to guide light along its path, thus carrying a signal. This signal is not affected by electrical noise, making the wire immune to electromagnetic interference. The wire is comprised of a core which is surrounded by cladding. The core has a much higher refractive index than the cladding, leading to total internal reflection. This keeps the light travelling along the wire even if it's bent [25]. Fiber optic wires can either be single or multi-mode fibers. Multi-mode fibers are wide enough to allow multiple light modes to travel through it whereas single mode fibers are much thinner and only allow one mode. Single mode fibers reduce attenuation, making them suitable for long distances unlike multi-mode fibers which are relegated to shorter distances [21].

Direct-burial cables are cables specifically designed to be buried outside without any other protection such as a conduit. They typically consist of cabling enclosed within a reinforced outer coating with a solid core, though the specifics vary. The cables boast strong protection against moisture, heat, abrasion, acidity and other hazards expected when buried underground [26].

Metal clad cable, sometimes referred to as BX cable, armored cable or metallic sheath cable, has an outer coating made of metal instead of an insulating material. This allows the outer layer to be grounded. The main benefit of metal sheathing is durability. Generally speaking, it is faster and cheaper to use metal clad cable than it is to deal with pipes or conduits [27]. When installing metal clad cable, make sure to use metal junction boxes as it isn't designed for use with other materials [28].

References

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