Why are Electrical Cables and Wires insulated?

Electrical Technology

What is Purpose of Insulated Covers around Wires and Conductors?

Table of Contents

Insulator, Insulation & Insulation Materials?

An insulator is a material or substance that does not readily conduct heat or electricity. This is because insulators lack free-moving electrons that can carry charge. When a conductor is covered with an insulating material, such as PVC (Polyvinyl Chloride), XLPE (Cross-Linked Polyethylene), or Rubber, it is said to be insulated. The process of applying this protective covering on conductors is called insulation.

The insulating layer around a conductor serves several purposes: it prevents electrical energy and signals from escaping into the surroundings, protects against short circuits, and safeguards people from the risk of electric shock.

A key property of insulating materials is their dielectric strength, which refers to the maximum electric field that the material can withstand without breaking down or becoming conductive. Different insulators have varying dielectric strengths, making them suitable for different voltage levels and applications. For instance, XLPE has a higher dielectric strength compared to PVC, which is why it is often used in medium- and high-voltage cables.

Insulating materials are also categorized into insulation classes based on their thermal endurance and operating temperature limits. For example, according to IEC and NEMA standards, Class A insulation is rated for 105 °C, Class B for 130 °C, Class F for 155 °C, and Class H for 180 °C. These classifications ensure that the right insulation type is used depending on the application, whether in motors, transformers, or power cables.

Thus, proper insulation (considering both dielectric strength and insulation class) not only ensures the safe transmission of electricity but also enhances the durability and reliability of electrical systems.

Why are Cables Insulated?

Cables are insulated primarily for safety, to prevent electric shock and short circuits. In addition, insulation over conductors is provided for performance, and to protect against environmental damage and ensure efficient power delivery. The insulation acts as a non-conductive barrier, containing the electrical current within the cable.

With the exception of uninsulated power transmission lines mounted on electric poles and towers or bare grounding conductors, almost all cables and wires in use today are insulated. The level or degree of insulation resistance depends on the specific purpose for which the cable was designed. Besides minimizing energy loss or dissipation to the surroundings, one of the most important reasons cables are insulated is to protect people from the danger of electric shock.

Electricity is extremely dangerous! The first touch can be the last touch, as it rarely gives a second chance. Even a slight touch of a live cable can result in a fatal accident. The human body is a conductor of electricity, though not a perfect one. When it comes into contact with a live conductor, current flows from the conductor into the body. Since the body cannot safely carry away this current, exposure to more current than it can tolerate can be deadly.

To prevent such accidents in homes and workplaces, it is essential that cables are insulated. Insulation not only prevents current leakage but also stops direct contact with live conductors, thereby protecting us from electrocution.

Effect of Temperature on Insulating Materials

The behavior of electrical resistance under temperature variations differs between conductors, semiconductors, and insulators.

Conductors (e.g., copper, aluminum):

As temperature increases, the resistance of conductors also increases. This is because the vibration of metal atoms intensifies with heat, impeding the flow of electrons.

Semiconductors (e.g., silicon, germanium):

In semiconductors, resistance decreases as temperature increases. Higher temperatures provide energy to break covalent bonds, releasing more free charge carriers (electrons and holes). This increases conductivity, making semiconductors behave more like conductors at elevated temperatures.

Insulators (e.g., glass, mica, rubber):

At normal operating temperatures, insulators have very high resistance due to the absence of free charge carriers. However, when temperature rises significantly, some electrons may gain enough energy to cross the band gap, causing the insulator to behave like a semiconductor. If heating continues further, the material may eventually lose its insulating properties altogether, leading to dielectric breakdown

Insulation Resistance of the Cable

A cable conductor is provided with insulation of suitable thickness to prevent current leakage. The required thickness of the insulation depends on the purpose for which the cable is designed. In such cables, the path of current leakage is radial, meaning that leakage current flows outward through the insulation. The resistance offered by the insulation to the flow of leakage current is also radial along the length of the cable.

Cable conductor is provided with an insulation of suitable thickness to avoid the leakage of current. The thickness of any cable depends on the purpose of its design. The path of current leakage in such cable is radial. The resistance or opposition offered by the insulation to the flow of current is also radial throughout its length.

For a single core cable conductor of radius r₁, internal sheath radius r₂, length l and insulation material resistivity ρ, the perimeter of the conductor is 2πrl. The thickness of the insulation will be given as dr.

R_ins = ρdr/2πrl

When integrated, we will have:

R_ins = ρ/2πl[loge r₂ /r₂ ]

The insulation resistance of a cable (R_ins) is inversely proportional to its length (1/l), which contrasts with the typical conductor resistance formula R = ρl, where ρ (rho) is the constant and known as the resistivity of the material.

Some cables are designed with more than one insulating layer and, in certain cases, more than one core. The central wire serves as the main conductor, while an outer core or layer serves the purpose of grounding and shielding. This shielding prevents electromagnetic waves and radiation from escaping the cable and also reduces external electromagnetic interference (EMI) from affecting the signal.

A common example of such a cable is the coaxial cable, which consists of:

A central conductor (carries the signal),
An insulating dielectric layer,
A metallic shield (grounding and EMI protection), and
An outer insulating jacket for mechanical protection.

A coaxial cable conducts electrical signals using an inner conductor, which is typically made of copper due to its low resistivity. In some cases, the copper conductor may be plated to enhance performance. This inner conductor is surrounded by one or more layers of dielectric insulation, often made of materials such as polyethylene or Teflon.

Between the insulating layers, a shield is placed, usually in the form of an aluminum foil or a woven copper braid (sometimes both for better shielding). The shield serves as the outer conductor, which carries little to no voltage under normal operation, and it is typically grounded. The entire assembly is then enclosed in an outer PVC jacket, which provides mechanical protection from the external environment.

The key advantage of the coaxial design is that both the electric and magnetic fields generated by the signal are confined to the dielectric layer between the inner conductor and the shield, with minimal leakage outside the cable. This shielding also prevents external electromagnetic fields and radio frequency interference (RFI/EMI) from penetrating into the cable, ensuring reliable signal transmission.

Additionally, conductors with a larger diameter have lower resistance, which reduces signal loss and minimizes field leakage. Similarly, cables with greater dielectric thickness (better insulation) provide stronger protection against interference. Since weaker signals are more vulnerable to disruption, coaxial cables with multiple insulation and shielding layers are preferred for transmitting low-power or high-frequency signals, such as in television, internet, and radio frequency communication systems.

Features of an Insulated Cable

Since the insulation resistance of a cable depends on its intended purpose, engineers must carefully consider several factors when designing it. For example, coaxial cables require additional insulation, not only to prevent power leakage but also to confine electromagnetic radiation within the cable. Depending on the application, cables may have one, two, three, or even four layers of insulation. Each type is engineered for specific requirements in power, signal transmission, or environmental conditions.

Key features of insulated cables include:

Heat resistance – withstands elevated operating temperatures without degradation.
High insulation resistance – minimizes leakage current and ensures electrical safety.
Mechanical strength – resistant to cuts, tears, and abrasion.
Enhanced electrical and mechanical properties – provides durability and efficient performance.
Chemical resistance – resists oil, solvents, and various chemicals.
Environmental protection – resistant to ozone, UV radiation, and harsh weather conditions.

FAQs

Q1: What is the purpose of insulation on electrical wires?

A: The insulation on electrical wires prevents accidental contact, protects against electric shock, avoids short circuits, reduces power leakage, and shields the conductor from environmental damage such as moisture, heat, or chemicals.

Q2: Why do some conductors not have insulation outside them, like in house wiring, electric motors, etc.?

A: In certain applications, conductors may be left bare or only partially insulated because they are placed inside protective enclosures, conduit pipes, motor housings or overhead transmission lines. These barriers provide the necessary protection instead of insulation.

Q3: Is insulation necessary for all conductors?

A: No. Not all conductors require insulation. For example, bare grounding conductors and overhead transmission lines often remain bare, since they are either intentionally exposed for grounding or installed in safe, elevated positions where direct human contact is unlikely.

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