Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Armoured Cables
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Armoured Cables
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Subject: Electrical Installation And Maintenance Work
Class: Senior Secondary 2
Term: 2nd Term
Week: 2
Theme: Electrical Installation And Tools Accessories
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Identify types of armored cable. State parts of armored cable State how to join and terminate armored cables.
conductors.
5. Fitting the Gland: Ensure the armour is clean and free of burrs. Slide the armour cone and compression ring over the exposed armour wires, ensuring the armour is properly seated in the cone. Push the cable into the equipment entry point. Screw the main body of the gland onto the equipment, then tighten the locknut on the inside to secure it. Tighten the gland's compression nut firmly to clamp the armour wires between the cone and the compression ring, establishing good earth continuity and mechanical retention. Use a gland spanner for proper tightening.
6. Stripping and Connecting Conductors: Cut the insulated conductors to the appropriate lengths for connection to the terminals inside the equipment. Strip the insulation from the ends of the conductors. Terminate the conductors into the appropriate terminals, ensuring proper crimping or tightening. Ensure the armour earth connection is robustly made to the equipment's earth terminal or chassis.
7. Final Checks: Verify all connections are tight and secure. Ensure the gland is fully tightened, providing a proper seal and earth continuity. Perform insulation resistance and continuity tests as per safety standards. Tools required for Armoured Cable Termination: Hacksaw, measuring tape, sharp knife/cable stripper, files (for smoothing armour edges), gland spanner, screwdrivers, bolt cutters (for armour wires).
Definition of Armoured Cable: An armoured cable is an electrical cable designed with an additional layer of metallic armouring, typically steel wire or tape, beneath the outer sheath. This armouring provides enhanced mechanical protection against crushing, impact, and abrasion, making it suitable for harsh environments where regular unarmoured cables would be vulnerable to damage. The armour also often serves as an Earth Continuity Conductor (ECC) or protective conductor for the circuit. Types of Armoured Cable (Performance Objective 1): The primary types of armoured cables are distinguished by the material and form of their metallic protection.
1. Steel Wire Armoured (SWA)
Cable: Construction: This is the most common type. It consists of multiple steel wires helically wound around the inner cores of the cable. The steel wires provide excellent mechanical protection, making the cable highly resistant to crushing and impact.
Applications in Nigeria: Widely used for underground power distribution, industrial installations (e.g., factories, mills), outdoor lighting (street lights, compound lights), power supply to remote equipment (e.g., agricultural pumps, boreholes), and areas where severe mechanical stress is anticipated.
Characteristics: Robust, good bending radius (due to individual wires), often used in three-phase systems.
2. Steel Tape Armoured (STA)
Cable: Construction: Instead of wires, this cable uses two layers of steel tape, wrapped concentrically around the inner cores, often with an overlap. This provides good resistance against crushing and sharp objects but is generally less flexible than SW
A. Applications in Nigeria: Typically used for lighter mechanical protection requirements, sometimes in building installations where it might be run in ducts, or for control circuits. Less common for heavy-duty underground applications compared to SW
A. Characteristics: Offers good impact resistance, but generally stiffer than SWA.
3. Aluminium Wire Armoured (AWA)
Cable: Construction: Similar to SWA, but uses aluminium wires for armouring instead of steel.
Applications in Nigeria: Primarily used for single-core cables. The non-ferrous nature of aluminium means it does not interfere with the magnetic field generated by the alternating current in a single core, which can be an issue with steel armouring (causing eddy currents and heating).
Therefore, AWA is preferred for high-current single-core circuits where steel armour would lead to losses.
Characteristics: Lighter than SWA, good for single-core applications to mitigate eddy currents, offers good mechanical protection. Parts of Armoured Cable (Performance Objective 2): A typical multi-core armoured cable (e.g., SWA) consists of several distinct layers, each serving a specific purpose. (Teacher may describe or present a diagram to students).
1. Conductors: Purpose: The core elements responsible for carrying the electrical current.
Material: Typically stranded copper or aluminium. Copper offers excellent conductivity and flexibility, while aluminium is lighter and more cost-effective for larger cross-sections.
Number of Cores: Can be single-core, two-core, three-core, four-core, or multi-core depending on the circuit requirements (e.g., phase, neutral, earth).
2. Insulation: Purpose: Electrically isolates the conductors from each other and from the surrounding environment to prevent short circuits and electric shocks.
Material: Commonly Polyvinyl Chloride (PVC) or Cross-Linked Polyethylene (XLPE).
PVC: Good general-purpose insulation, suitable for most applications, relatively cost-effective.
XLPE: Superior thermal performance, higher current carrying capacity, excellent resistance to moisture and chemicals, preferred for higher voltage and more demanding applications.
3. Bedding: Purpose: Provides a cushion and separation layer between the insulated cores and the metallic armouring. It protects the insulation from potential damage by the sharp edges or unevenness of the armour. It also fills the interstices between the cores, giving the cable a circular shape.
Material: Usually PVC or Low Smoke Halogen Free (LSZH) compound.
4. Armour: Purpose: The primary mechanical protection layer, guarding against impact, crushing, and abrasion. It also often serves as an Earth Continuity Conductor (ECC) or protective conductor for the circuit, providing a low-resistance path for fault currents.
Material: Steel Wire (SWA), Steel Tape (STA), or Aluminium Wire (AWA).
5. Outer Sheath: Purpose: The outermost protective layer, providing environmental protection against moisture, UV radiation, chemicals, and general wear and tear.
Material: Typically PVC, Polyethylene (PE) for outdoor/underground (better water resistance), or LSZH (Low Smoke Halogen Free) for public buildings where fire safety Purpose: The primary mechanical protection layer, guarding against impact, crushing, and abrasion. It also often serves as an Earth Continuity Conductor (ECC) or protective conductor for the circuit, providing a low-resistance path for fault currents.
Material: Steel Wire (SWA), Steel Tape (STA), or Aluminium Wire (AWA).
5. Outer Sheath: Purpose: The outermost protective layer, providing environmental protection against moisture, UV radiation, chemicals, and general wear and tear.
Material: Typically PVC, Polyethylene (PE) for outdoor/underground (better water resistance), or LSZH (Low Smoke Halogen Free) for public buildings where fire safety is paramount (produces less toxic smoke in a fire). How to Join and Terminate Armoured Cables (Performance Objective 3):
A. Joining Armoured Cables: Joining armoured cables is necessary when extending a run or repairing a damaged section. The joint must maintain both electrical continuity for the conductors and the armour, and mechanical strength and environmental protection.
Methods: Resin Joint Kits: Involves enclosing the joined conductors and armour within a mould, which is then filled with a two-part resin that hardens to form a robust, waterproof, and mechanically strong joint. Often used for underground or outdoor applications.
Heat Shrink Joint Kits: Utilizes heat-shrinkable tubes and components to insulate and seal the joint. When heated, the tubes shrink tightly around the cable, providing insulation, sealing, and mechanical protection.
Mechanical Connectors (for armour): Special connectors that clamp onto the armour to ensure earth continuity and mechanical integrity, especially when using resin or heat-shrink for the cores.
B. Terminating Armoured Cables (Using a Cable Gland): Termination refers to connecting the cable to an electrical apparatus (e.g., distribution board, motor, switchgear). A cable gland is crucial for safe and secure termination.
A cable gland serves multiple purposes: Mechanical retention: Secures the cable firmly to the equipment, preventing it from being pulled out.
Earth continuity: Connects the cable armour to the metallic enclosure of the equipment, ensuring a continuous earth path.
Environmental seal: Provides ingress protection (IP rating) against dust, moisture, and liquids, maintaining the integrity of the enclosure.
Strain relief: Prevents stress on the individual conductor terminations. Procedures for Terminating Armoured Cable with a Gland (e.g., SWA Brass Gland):
1. Preparation of the Equipment/Gland: Ensure the equipment entry point is clean and the correct size hole is drilled for the gland. Disassemble the gland into its component parts: locknut, main body, armour cone, compression ring, nipple/shroud (if applicable), and outer seal.
2. Measuring and Cutting the Cable: Measure the required length of the cable. Cut the cable squarely using a hacksaw or large cable cutters.
3. Stripping the Outer Sheath: Carefully measure and mark the length of the outer sheath to be removed (typically enough to expose the armour for the gland, plus conductor lengths). Using a sharp knife, score around the circumference of the outer sheath without cutting into the armour. Slit the outer sheath lengthwise and peel it back. Remove the cut piece.
4. Preparing the Armour for Glanding: Slide the gland locknut and main body over the exposed outer sheath. Measure and mark the length of armour to be removed (enough to allow the armour to sit correctly within the gland's compression ring, usually about 25-50mm beyond the outer sheath). Carefully cut the individual steel wires (or tape) of the armour using hacksaw, file, or small bolt cutters. It is crucial to cut all wires evenly. File off any sharp edges. Bend back the cut armour wires and trim the bedding layer beneath the armour, leaving enough to cushion the inner conductors.
5. Fitting the Gland: Ensure the armour is clean and free of burrs. Slide the armour cone and compression ring over the exposed armour wires, ensuring the armour is properly seated in the cone. Push the cable into the equipment entry point. Screw the main body of the gland onto the equipment, then tighten the locknut on the inside to secure it. * Tighten the gland's compression nut firmly to clamp the armour wires between the cone and the compression ring, establishing good earth continuity and mechanical Teacher Activities: Introduction and Engagement (5 min): Display a sample of armoured cable (if available) or a detailed diagram. Ask students about situations where they think a normal cable might not be strong enough. Introduce armoured cables as the solution.
Concept Explanation (15 min): Explain the definition and purpose of armoured cables. Present and discuss the different types of armoured cables (SWA, STA, AWA), highlighting their construction, characteristics, and typical applications with Nigerian examples (e.g., SWA for street lights, industrial use). Break down and explain each component part of an armoured cable using the sample or a detailed cross-sectional diagram. Emphasize the function of each layer.
Demonstration of Termination (20 min): Verbally or by using a detailed step-by-step diagram/chart, demonstrate the process of terminating an armoured cable using a cable gland. If resources permit, conduct a live demonstration on a scrap piece of armoured cable, clearly showing each step from stripping the outer sheath to fitting the gland and preparing the conductors. Emphasize safety precautions. Explain the importance of cable glands, earth continuity, and proper tightening.
Facilitate Discussion and Q&A (5 min): Encourage students to ask questions and clarify any uncertainties regarding the types, parts, or termination procedures.
Student Activities: Observation and Participation: Actively observe the teacher's explanation and demonstration.
Identification Practice: Work in small groups to identify and list the different parts of an armoured cable from a provided diagram or actual sample.
Discussion: Participate in class discussions, asking questions and contributing to understanding.
Note-Taking: Take detailed notes on the key concepts, types, parts, and termination procedures.
Conceptual Application: Discuss where they might have seen armoured cables used in their local communities (e.g., at the school, in a nearby factory, for borehole pump wiring).
Rural Electrification and Agricultural Projects: Armoured cables are extensively used in Nigeria for powering boreholes, irrigation pumps, and processing units in rural agricultural settings. Their robust nature protects against accidental damage from farm machinery or animal interference, ensuring reliable power supply for food production and water access. For instance, the cable feeding power from a generator or transformer to a water pump in a community farm in Oyo state would likely be an SWA cable laid underground.
Industrial and Commercial Installations: In Nigerian factories, manufacturing plants (e.g., cement factories in Ogun, textile mills in Kano), and large commercial buildings (shopping malls, office blocks in Lagos), armoured cables are crucial for distributing power to heavy machinery, production lines, and main distribution boards. They provide the necessary mechanical protection in harsh industrial environments and ensure continuity of power supply, minimizing downtime and increasing productivity.
Public Infrastructure and Street Lighting: Armoured cables are fundamental for street lighting projects, traffic light installations, and power supply to public utilities like water treatment plants or communication towers across Nigeria. They are often buried directly in the ground, benefiting from their mechanical protection against excavation accidents or vandalism, contributing to safer roads and public spaces.