Introduction
We have all, at some point, held a live electrical cable and not received an electrical shock. Magic, right? Well, this is because of the cable’s insulation.
Insulation is an important non-conductive material that surrounds and protects the individual wires or conductors.
It resists electrical leakage and preserves the material integrity of the conductor by shielding it against environmental threats like water and heat.
The longevity and effectiveness of an electric conductor depend on its insulation.
There are so many compounds and materials that can be used as cable insulators, each with its pros and cons, ranging from temperature ranges, cost, toxicity, ease of use, etc.
We will look at each of the different insulators in turn and discuss reasons why we should use (or not use) each one also making use of the table below
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Thermoplastic
We will first discuss insulators made by thermoplastics. Thermoplastics are a type of polymer that becomes pliable or moldable when heated and solidifies when cooled.
PVC (Polyvinyl Chloride) stands out as the most widely used thermoplastic insulator for cables, valued for its cost-effectiveness, durability, and wide availability.
However, it’s important to note that when PVC burns, it releases thick, toxic, black smoke due to its chlorine content.
This can pose a significant health risk in environments where low smoke and toxicity levels are crucial, such as confined spaces like tunnels.
Referring to the table above, common PVC cables are grouped under insulation types these being V-75, HFI-75-TP, TPE-75, V-90, HFI-90-TP, and V-90HT.
In these designations, the letters signify the variant of the PVC cable, while the number indicates the maximum operating temperature.
For example, the cable ‘HFI-90-TP’ can operate at a maximum temperature of 90 degrees Celsius.
It’s important to understand that these temperature limits refer to sustained temperatures under normal current-carrying conditions, not the maximum temperature during short circuit conditions.
While certain PVC cables can operate at temperatures of up to 105 degrees Celsius, this is dependent on the cable being installed in a manner that shields it from mechanical damage at temperatures higher than 75 degrees Celsius.
As a standard, the normal operating temperature for these cables, including flexible cords used as installation wiring, is based on a temperature of 75 degrees Celsius.
This is because there’s a significant risk of thermal deformation of the insulation if the cables are fixed or installed in a manner that exposes them to severe mechanical pressure at higher temperatures.
Thermosetting
The next insulator we will discuss is the thermosetting insulator. Thermosetting compounds are a type of polymer that, once cured or hardened through a chemical reaction, cannot be re-melted or reshaped.
This sets them apart from thermoplastic materials, which can be melted and reprocessed multiple times. In this group of insulators, we normally make use of XLPE and EPR types of thermosetting compounds.
Cross Linked Polyethylene (XLPE)
XLPE (Cross-Linked Polyethylene) forms a structure where different polyethylene chains are linked together. This prevents the polymer from melting or separating at elevated temperatures, making XLPE ideal for higher temperature applications.
The normal operating temperature range is typically between 90°C and 110°C, with a maximum temperature limit of 250°C. These conductors maintain their shape effectively at higher temperatures and do not deform under mechanical stress.
Common XLPE cable insulations include X-90, X-90UV, X-HF-90, and X-HF-110. The naming convection is the same as those of PVC cables and similar temperature conditions apply.
Cross Linked Elastomerics
Cross-Linked Elastomeric (EPR) is a copolymer of ethylene and propylene, commonly referred to as an ‘elastomer’. It is more flexible than XLPE but has higher dielectric losses.
Normal operating temperatures are typically between 90°C and 110°C, with a maximum temperature limit of 250°C. Referring to the table above, common Elastomer cables include R-EP-90, R-CPE-90, R-HF-90, R-CSP-90, R-HF-100, R-E-110, and R-S-150.
As with previous insulations already discussed, the naming convention is the same and similar temperaure conditions apply.
Mineral Insulated Metal Sheathed (MIMMS)
Metal Sheathed (MIMMS), also known as MI cables or MI wiring, are a type of electrical cable used in applications that require high levels of fire resistance, mechanical strength, and durability.
They consist of a metallic sheath, usually made of copper or stainless steel, which encases one or more conductors. The conductors themselves are typically made of copper.
The key feature of MIMMS cables is the insulation material, which is composed of powdered magnesium oxide (MgO) packed around the conductors.
This insulation provides excellent thermal and electrical properties. Additionally, because of its inorganic nature, it is highly fire-resistant.
Paper Cable
The last insulation we will discuss is paper based. Paper-based insulation, considered the oldest form of power cable insulation, remains used mainly in high voltage cables.
To function effectively, the paper insulation must be impregnated with a dielectric fluid, such as oil resin or a synthetic alternative.
Often, a lead sheath is applied over the insulation to safeguard against water or moisture infiltration, as paper insulation is notably sensitive to moisture.
When properly impregnated, paper insulation demonstrates a good dielectric strength and thermal stability, while also affording a level of mechanical protection to the conductors.
Over time, paper insulation can degrade due to factors like moisture, temperature variations, and the breakdown of insulating oil. This can lead to a reduction in its effectiveness as an insulator.
In recent years, newer insulating materials like cross-linked polyethylene (XLPE) and ethylene propylene rubber (EPR) have largely replaced it in new cable installations.
Their advanced performance characteristics and prolonged lifespan have contributed to the shift away from paper insulation.
Core Insulation Colour
It’s important to clearly label conductors, indicating their specific functions as active, neutral, earthing, or equipotential bonding conductors.
To comply to safety standards, avoid using conductors with green, yellow, or green/yellow combinations for insulation or sheathing as active or neutral conductors in installation wiring.
The recommended color codes are: red, white (or uncolored), and blue for phase cores; black for neutral cores; and green/yellow for earth cores.
Additionally, for active conductors, red or brown is recommended for single-phase systems, while red, white, or blue are suitable for multiphase setups.
It’s important to note that internal wiring within equipment is not categorized as installation wiring, but it may still be subject to specific equipment standards.
To ensure proper identification, color coding through sleeving or other methods, aligning with the above colour codes, is allowed at each termination point and can be employed for various purposes.
It’s worth mentioning also that switchboard wiring falls outside the classification of installation wiring.
Insulation marking
In Australia, power cables are marked with various information to indicate their specifications and compliance with relevant standards. Some of the common markings found on power cables in Australia include:
- Manufacturer’s Name or Logo: This indicates the company that produced the cable.
- Cable Type or Category: This specifies the type of cable, such as single-core, multi-core, flexible, etc.
- Voltage Rating: Indicates the maximum voltage that the cable can handle safely in the form U0/U where Uo is the rms power frequency voltage to earth of the supply voltage and U is the rms power frequency voltage between phases of the system for which the cable is desgned. An example of such a marking is 0.6/ 1kV or 450/750V.
- Conductor Size: This denotes the cross-sectional area of the conductor, typically measured in square millimeters (mm²).
- Insulation Material: Indicates the material used for the cable’s insulation (V-75, HFI-75-TP).
- Number of Cores: Specifies how many conductors are within the cable.
- Standard Compliance Markings: Indicate that the cable meets specific Australian and international standards, such as AS/NZS 5000.1 for general wiring and AS/NZS 1125 for conductors.
- Date of Manufacture: Provides information about when the cable was produced.
- Length Marking: Indicates the length of the cable, typically in meters.
- Fire Resistance Rating: If applicable, this marking indicates the cable’s resistance to fire.
Rounding Up
In summary, cable insulation is a critical component in ensuring the safety, efficiency, and longevity of electrical systems.
From the commonly used PVC to the high-temperature resilience of XLPE and the unique properties of paper-based insulation, each type serves a specific purpose and comes with its own set of advantages and limitations.
Understanding these differences is key to making informed decisions about the right insulation for your specific needs.
If you’ve learned something new from this post, you might also be interested in other articles on related subjects available on our blog’s homepage. While you’re here, take a moment to explore our collection of electrical engineering calculators. Cheers!