Basic Electrical and Electronics Engineering (EE 103) – Chapter 4

4.1 Consumer Power Supply System

An electrical power system is a network of electrical components used to supply, transmit, and use electric power. The grid that provides power to homes and industries is a prime example of an electrical power system.

Power System Layout

The general layout of a power system includes:

• Generating Station: Where electrical power is produced.
• Generating Step-Up Transformer: Increases the voltage for efficient long-distance transmission.
• Transmission Lines: High voltage lines (e.g., 765, 500, 345, 230, and 138 kV) that carry power over long distances.
• Transmission Customer: Large industrial users who take power directly from the transmission system.
• Substation Step-Down Transformer: Decreases the voltage for local distribution.
• Subtransmission Customer: Customers who require power at intermediate voltages (e.g., 26kV and 69kV).
• Primary Customer: Customers who take power at the primary distribution voltage (e.g., 13kV and 4kV).
• Secondary Customer: Residential and commercial customers who receive power at low voltages (e.g., 120V and 240V).

Distribution System

The distribution system is the final stage in the delivery of electric power; it carries electricity from the transmission system to individual consumers.

Primary Distribution System:

• This part of the AC distribution system operates at voltages higher than general utilization levels.
• Commonly used primary distribution voltages are 33 kV, 11 kV, 6.6 kV, and 3.3 kV.
• For economic reasons, primary distribution is carried out by a 3-phase, 3-wire system.

Secondary Distribution System:

• This is the part of the system that delivers energy at the voltages that consumers use.
• The secondary distribution employs a 400/230 V, 3-phase, 4-wire system.

Distribution Substation Layout

A typical distribution substation steps down the voltage from an 11 kV overhead line to 415 V for consumer use and includes:

• Lightning Arrester (L.A): Protects the system from lightning strikes.
• DO Fuse (Dropout Fuse): A protective device to isolate the transformer.
• Isolator/Isolating Switch: Disconnects the transformer for maintenance.
• Transformer: Steps down the voltage from 11 kV to 415 V.
• LT OCB (Low Tension Oil Circuit Breaker): Provides protection on the low voltage side.

Equipment in a Substation

• Bus Bar: A conductor connecting multiple lines operating at the same voltage.
• Isolators: Used to disconnect parts of the system for maintenance (operated only under no-load conditions).
• Circuit Breaker: Automatically disconnects the circuit under fault conditions.
• Lightning Arrester: Diverts high-voltage surges from lightning to the earth.
• Current Transformer (CT): Measures and monitors current.
• Potential Transformer (PT) / Voltage Transformer (VT): Steps down voltage for measurement.
• Step-down Transformer: Converts high-voltage power to low-voltage power.

HT and LT Metering

• High Tension (HT) Metering: For large consumers on an 11 kV supply, CTs and PTs are used to measure power consumption on the high voltage side.
• Low Tension (LT) Metering: For low voltage consumers, the supply is connected directly to the energy meters.

Substation Earthing

Proper earthing is crucial for safety.

• A single transformer requires four earth electrodes: two for neutral earthing and two for the common body/equipment earthing.
• A two-transformer substation requires six earth electrodes.
• Lightning Arrestors must be earthed separately at two distinct locations for effective protection.

4.2 Overview of Electrical Wiring Components: Switches, Sockets, and Distribution Boards

Switches

A switch is a component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another.

• One Gang One Way Switch: Controls a single circuit from one location. It has two terminals.
• One Gang Two Way Switch: Controls a single circuit from two different locations. It has three terminals: COM (Common), L1, and L2.

Sockets

A socket outlet is a device installed in a fixed location to which a flexible cable from an appliance can be connected.

• 5A Socket: Used for light loads like lamps, TVs, and chargers.
• 15A Socket (Power Socket): Used for heavy loads like heaters, air conditioners, and refrigerators. These are wired on a separate sub-circuit.

Distribution Boards (DB)

A distribution board (also known as a panelboard or consumer unit) is a component of an electricity supply system that divides an electrical power feed into subsidiary circuits while providing a protective fuse or circuit breaker for each circuit in a common enclosure.

4.3 Protective Devices, Their Construction, and Sizing

Protective devices are essential for interrupting fault currents to prevent damage to equipment and ensure personnel safety.

4.4 Wires and Power Cable

• Wire: A single solid or stranded conductor, either bare or insulated.
• Cable: Two or more insulated wires bundled together in a common outer sheath for protection.
• Conductor Materials: Copper and Aluminum are the most common. Copper has better conductivity and is more flexible, while Aluminum is lighter and cheaper.
• Insulation Materials: Polyvinyl Chloride (PVC) is widely used for its good insulating properties and low cost. Cross-linked Polyethylene (XLPE) is used for higher voltage applications as it can withstand higher temperatures.

4.5 Types of Wiring System

• Cleat Wiring: Insulated wires are supported on porcelain or plastic cleats. It is a temporary, cheap system, not suitable for domestic premises due to poor appearance and risk of damage.
• Batten Wiring (CTS/TRS): Wires are run on well-seasoned, straight teakwood battens. The wires are held by brass clips. It is neat in appearance and simple to install.
• Casing and Capping Wiring: PVC insulated wires are placed in grooves of a wooden or plastic casing and covered by a cap. It provides good protection from mechanical damage but is costly and has a risk of fire in the case of wooden casings.
• Conduit Wiring: Wires are drawn through pipes called conduits. This system provides the best protection against mechanical damage, fire, and moisture.
  • Surface Conduit Wiring: Conduits are mounted on the surface of walls or ceilings.
  • Concealed Conduit Wiring: Conduits are embedded inside the walls, ceilings, or floors during construction, providing the best aesthetics.

4.6 Determination of Size of Conductor

The size of a conductor is determined by three main factors:

• Current Carrying Capacity: The conductor must be able to carry the normal load current continuously without overheating. Standard tables (e.g., from NEC or IS codes) provide the capacity for different wire sizes and installation conditions.
• Voltage Drop: The conductor size must be large enough to ensure that the voltage drop from the source to the load is within permissible limits (typically 3-5%). Excessive voltage drop can cause poor performance of appliances.

  Voltage Drop (V) = Current (I) × Resistance (R)

• Short Circuit Rating: The conductor must be able to withstand the high thermal and mechanical stresses caused by a short-circuit current for the time it takes for the protective device to operate.

4.7 Earthing System and Its Importance

Earthing (or grounding) is the process of connecting the metallic non-current-carrying parts of electrical equipment (like the body of a motor or refrigerator) to the general mass of the earth.

Importance of Earthing:

• Personnel Safety: It protects humans from electric shock. If an insulation failure causes a live wire to touch the metal casing, the fault current flows safely to the earth, blowing the fuse or tripping the breaker, and isolating the supply.
• Equipment Safety: It protects equipment from damage due to fault currents and lightning strikes by providing a safe path for the discharge of current.
• Stable Voltage: It ensures that the voltage of the system remains stable and does not rise to dangerous levels.

Types of Earthing:

• Pipe Earthing: A galvanized iron (GI) pipe is buried vertically into the ground. It is a common and effective method.
• Plate Earthing: A copper or GI plate is buried vertically in an earth pit. This method is used where high fault currents are expected.

4.8 Electrical Safety Rules

• De-energize Circuits: Always turn off the power supply and use a tester to confirm there is no voltage before starting any electrical work. Use Lockout/Tagout (LOTO) procedures.
• Use Insulated Tools: Use tools with non-conductive handles designed for electrical work.
• Wear Personal Protective Equipment (PPE): This includes insulated gloves, safety glasses, and non-conductive footwear.
• Avoid Water: Never work on electrical devices in wet conditions or with wet hands.
• Use Correct Wire Size and Fuse/MCB: Ensure conductors and protective devices are correctly sized for the load to prevent overheating and fire.
• Maintain a Safe Distance: Keep a safe distance from overhead power lines and exposed electrical parts.
• Do Not Overload Sockets: Avoid plugging too many high-power appliances into a single outlet or extension cord.
• In Case of Electric Shock: Do not touch the victim. Switch off the main power source immediately. If not possible, use a non-conductive object (like dry wood) to separate the victim from the live source. Administer first aid and call for medical help.