Armour resistance in cables refers to the electrical resistance of the metal layer (armour) that is wrapped around a cable to provide mechanical protection, measuring how much the armour itself resists the flow of electricity. Armour resistance is a factor considered when designing armoured cables, especially in situations where the armour might carry unintended currents due to its metallic nature.
Purpose of armour
The primary purpose of cable armour is to shield the inner conductors from physical damage like crushing, pulling, or abrasion, not necessarily to block electrical signals.
Armour has become a key component in modern cables and electrical installations, offering greater durability and wiring protection.
Materials used
Armour is typically made of steel wires or steel tape, which have their own electrical resistance depending on their size and composition.
Importance of resistance
While the armour is meant to protect the cable, its resistance can become a factor in situations like high current applications where induced currents might flow through the armour, causing heat generation.
Types of Armoured Cables:
Steel Wire Armoured (SWA)
Steel wire armoured (SWA) cable is most commonly used in underground systems, power networks, and outdoor lighting installations. Its steel wire armour provides robust protection, making it suitable for direct burial.
Aluminum Wire Armoured (AWA)
AWA is similar to SWA but uses aluminium armour instead of steel, making it lighter. It’s often used where lower weight is beneficial, like in power transmission over long distances.
Steel Tape Armoured (STA)
STA cable is designed with steel tape armouring, offering a thinner protective layer. It’s typically used in indoor and buried applications where space is limited but protection is still needed.
Each type of armoured cable has a specific purpose, with suitability dependent on the environmental conditions, the type of installation, and the project electrical requirements.
Use of Armoured Cables
Armoured cable is designed for use in situations where electrical wiring faces a higher risk of mechanical damage, interference, or exposure to harsh environmental conditions. It’s a popular choice for ensuring safety and longevity in both commercial and residential settings.
Armoured Cable Applications
Outdoor installations
Its robust protection against weather elements means armoured cable is ideal for outdoor electrical wiring, including garden lighting, power supplies to outbuildings, etc.
Underground
When electrical cables need to be run underground, armoured cable is essential. Its protective armouring shields the internal wires from moisture, soil corrosion, and physical damage caused by digging or other disturbance.
Areas at risk of mechanical damage
In industrial environments or areas where cables might be exposed to potential mechanical damage (e.g. warehouses, factories, and construction sites), armoured cable offers the necessary protection to maintain circuit integrity.
High traffic areas
For safety and durability, armoured cable is recommended in places where there’s a high likelihood of interference or physical impact, such as in public buildings, schools, and hospitals.
Armoured Cable sizes
The size of armoured cable you choose is important for the safety, efficiency, and reliability of your electrical installation. Armoured cables come in a variety of sizes, which are determined by the cross-sectional area of the internal conductors measured in square millimetres (mm²).
Popular Armoured Cable sizes and uses
1.5 mm² & 2.5 mm² – For lighting circuits and small power tools in residential and commercial environments.
4 mm² to 6 mm² – For larger lighting installations, power in homes, and outdoor lighting needing a higher current.
10 mm² to 16 mm² – Often used for connections to heavy-duty electrical appliances, small industrial machinery, and large heating units.
25 mm² or more – These sizes are specific to high-load applications, such as main power supply lines, industrial installations, and large electrical machinery.
Armoured Cable thickness
The thickness of an armoured cable is important to make sure it can handle the intended electrical load without overheating or experiencing excessive voltage drop. Here’s how to select the right armoured cable thickness for the job:
Calculate the total current load: Add up the power ratings (in Watts) of all devices that will be connected to the cable. Convert this to amperes to understand the current requirement.
Length of cables: For longer runs, choose a thicker cable to mitigate voltage drop and ensure efficient power delivery.
Check Regulations: Always refer to national electrical codes or standards. They may specify a minimum thickness for certain scenarios.
Armoured Cable Glands
Glands play an important role in cable installations, acting as the interface between the armoured cable and what it connects to. Glands secure the cable in place as well as providing vital sealing and grounding (earthing).
Cable Glands and Armoured Cable installations
Cable glands are engineered to attach and secure the end of an armoured cable to the equipment. They deliver several benefits:
Securing cables – Glands firmly hold the cable, preventing it from being pulled out of the system.
Protection – Glands help maintain the ingress protection rating of the enclosure, keeping out dust, moisture, and other contaminants.
Grounding (earthing) and termination – Glands also ensure proper grounding of the armouring and create a safe electrical connection to the system.
Choosing the right gland – brass, aluminium or nylon – involves considering the type of armoured cable, whether it is a hazardous or non-hazardous environment and earthing (grounding) and sealing requirements.
