Jawaharlal Nehru Technological University Hyderabad
Jawaharlal Nehru Technological University Hyderabad B.Tech III DESIGN ESTIMATION AND COSTING OF ELECTRICAL SYSTEMS Previous Question Paper (November/December - 2020 - R16)
Answer any five questions. All questions carry equal marks.
Main
What are the different types of electrical service connections? List advantages and disadvantages.
Answer
Electrical service connections are the physical links between the utility distribution network and the consumer's premises. They are primarily classified based on the installation method and the voltage level required.
The two main types of service connections are:
- Overhead Service Connection: Conductors are run overhead from the distribution pole to the consumer's building using insulators and brackets.
- Underground Service Connection: Insulated cables are buried underground, typically protected by conduits or armored cables, connecting the distribution point to the consumer's main switchboard.
The advantages and disadvantages are summarized below:
| Type | Advantages | Disadvantages |
|---|---|---|
| Overhead | Low initial installation cost; faults are easily located and repaired; modifications are simple. | Vulnerable to weather damage (storms, trees); poses safety risks; aesthetically unpleasing; higher maintenance due to environmental exposure. |
| Underground | High reliability; immune to weather conditions; aesthetically superior; safer as there are no exposed live parts. | High initial excavation and installation cost; fault location is difficult and time-consuming; modifications are complex and expensive. |
Additionally, connections are categorized by voltage level: Low Tension (LT) services for domestic and small commercial loads (typically 230V single-phase or 415V three-phase) and High Tension (HT) services for large industrial consumers requiring 11kV or higher, necessitating dedicated substations or transformers.
Prepare material required for overhead service connection to the home of 2 kW load at 240V, 50Hz supply. The supply is to be taken from 20 meter away from the home. Assume diversity factor as 1.66 and future load as 100%.
Answer
To estimate the material for a 2 kW service connection, we first calculate the total connected load and the resulting current. Given a load of 2 kW, a diversity factor of 1.66, and 100% future load, the total design load is 4 kW. At 240V, the current is approximately 16.67 A. A 2-core 6 sq. mm PVC insulated aluminum cable is suitable for this service connection.
The following material list is required for a 20-meter overhead service connection:
- Service Cable: 2-core 6 sq. mm PVC insulated aluminum cable (approx. 25 meters, including sag and connections).
- Insulators: 2 numbers of shackle insulators with D-brackets for mounting at the pole and the house.
- Supports: 1 number of 40mm diameter GI pipe (approx. 3 meters long) for the house service mast.
- Clamps and Hardware: GI stay wire, stay rod, and turnbuckle for anchoring; cable cleats and saddles for securing the cable to the wall.
- Protection: 1 number of 20A double-pole MCB inside a distribution box for service protection.
- Earthing: 1 number of GI earth wire and pipe/plate earthing kit for the service mast.
- Miscellaneous: Insulation tape, binding wire, and screws.
The service mast must be installed at a height ensuring a minimum ground clearance of 4.5 meters for the overhead line across the path.
Explain how the electrical material cost is estimated for residential installation.
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List out the points to be checked while carrying out the inspection of wiring installation.
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What are the factors to be considered in the process of the mechanical design of overhead lines?
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Estimate the quantity of material required for running 80 km, a single circuit of 66 kV transmission line using four-legged fabricated steel structures. The conductor used No.2 ACSR. Assume a span of 500 meters and every 10th tower is an anchor tower.
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Estimate the quantity of materials required for adding 132KV bay at 132KV grid substation.
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Define the following:
Luminous flux
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Brightness
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glare
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Luminous intensity
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candela
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