JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year DESIGN ESTIMATION AND COSTING OF ELECTRICAL SYSTEMS Previous Question Paper (January/February - 2021 - R16)

Earthing is essential in domestic electrical installations to ensure safety and prevent electrical hazards. Its primary purposes include:

• Protection against electric shock: By providing a low-resistance path to the ground, earthing ensures that in the event of insulation failure, the fault current flows to the earth rather than through a person touching the appliance.
• Voltage stabilization: It maintains the potential of the non-current-carrying metallic parts of equipment at zero volts relative to the earth.
• Overvoltage protection: It provides a discharge path for lightning surges and switching transients, protecting sensitive equipment and preventing fires.

The common types of earthing systems used are:

1. Pipe Earthing: This is the most common method for domestic use. A galvanized steel pipe of approximately 38 mm diameter and 2.5 meters length is placed vertically in a pit. The space around the pipe is filled with alternate layers of charcoal and salt to maintain moisture and lower soil resistance.
2. Plate Earthing: A copper plate (60 cm x 60 cm x 3.18 mm) or galvanized iron plate (60 cm x 60 cm x 6.35 mm) is buried vertically in a pit at a depth of at least 3 meters. The pit is filled with charcoal and salt, and a GI pipe is provided for pouring water to maintain earth conductivity.
3. Rod Earthing: A copper rod or galvanized steel rod is driven vertically into the ground. This method is suitable for sandy or rocky soil where digging deep pits is difficult.
4. Strip or Wire Earthing: Used in areas with rocky soil where excavation is difficult. Copper strips or GI wires are buried in horizontal trenches at a shallow depth, providing a large surface area for contact with the soil.

Wiring systems are methods of installing electrical conductors to distribute power safely. The choice depends on cost, durability, safety, and accessibility.

1. Cleat Wiring: Conductors are supported on porcelain cleats. It is a temporary system, easy to install and inspect, but visually unappealing and prone to mechanical damage. It is typically used in temporary sheds or construction sites.

2. Casing and Capping Wiring: Conductors are placed in wooden or PVC casings and covered with a matching cap. It provides good mechanical protection and is aesthetically pleasing, but it is a fire hazard if wooden casing is used and is not suitable for damp environments.

3. Batten Wiring (CTS/TRS): Conductors are laid on a wooden batten and held by brass link clips. It is cheap, easy to install, and durable, but it offers poor protection against mechanical damage and is unsuitable for outdoor use.

4. Conduit Wiring: Conductors are enclosed in metallic or PVC pipes. This is the most reliable and safest system. It provides excellent mechanical protection and fire resistance. It is categorized into:

• Surface Conduit: Pipes are mounted on the surface of walls using saddles.
• Concealed Conduit: Pipes are embedded within the wall plaster, providing a clean, aesthetic finish and high safety, commonly used in modern residential and commercial buildings.

5. Lead Sheathed Wiring: Conductors are insulated with vulcanized rubber and covered with a lead-aluminum alloy sheath to protect against moisture and mechanical injury. It is expensive and requires skilled labor for installation.