Fundamentals of Electronics

(c) 320V

In a series circuit the source voltage equals the sum of the voltage drops, so the third drop is $V_3 = 900 - (300 + 280) = 320\ \text{V}$.

(b) To prevent overloading and short circuits

A circuit breaker is a protective device that automatically opens (trips) the circuit when excessive current flows due to overload or short circuit, thereby protecting equipment and wiring from damage.

(d) Both A and B

An antenna is a reciprocal (bidirectional) device. When transmitting, it converts the electrical signal from the transmitter into electromagnetic (radio) waves radiated into space; when receiving, it converts the incoming electromagnetic waves back into an electrical signal. Hence it performs both functions.

Types of Earthing

Earthing (grounding) is the connection of the non-current-carrying metallic parts of an electrical system or equipment to the general mass of earth through a low-resistance path. It protects personnel from electric shock and equipment from damage. The main types are:

1. Pipe earthing A galvanized iron (GI) pipe of about 38 mm diameter and 2–3 m length is buried vertically in the ground, with its top about 30 cm below ground level. A funnel or watering pipe is fitted at the top so that water can be poured in during dry seasons to keep the soil moist and the earth resistance low. It is cheap, simple and commonly used for domestic and small installations. The pipe may be surrounded with salt and charcoal to reduce resistance.

2. Plate earthing A copper plate (60 cm × 60 cm × 3 mm) or GI plate (60 cm × 60 cm × 6 mm) is buried vertically on its edge at a depth of about 3 m. Alternate layers of salt and charcoal are packed around the plate to reduce earth resistance. It gives lower resistance than pipe earthing and is used where a very low earth resistance is required, but it is costlier and needs more excavation.

3. Rod earthing A copper or GI rod of about 12–25 mm diameter is driven vertically into the ground to a depth of 3–5 m by hammering. It is cheap and quick to install and needs less space, so it is suitable where the soil is soft and the water table is high. Its resistance is comparatively higher, so it is used for small loads and lightning arrestors.

4. Strip or wire earthing A copper strip or GI wire is laid horizontally in a trench about 0.5 m deep, in the shape of a ring or a straight run. It is used where a number of earth connections are needed over a large area, such as in substations and transmission towers. It requires a large area of land.

5. Mesh earthing A grid or mesh of interconnected conductors is buried in the ground over the whole area of a substation. It equalizes the potential over the entire area and gives very low earth resistance. It is used in large substations and power stations to keep step and touch voltages within safe limits.

6. Chemical earthing A maintenance-free earthing system in which the electrode is surrounded by a chemical compound (bentonite, graphite or similar) that retains moisture and keeps the earth resistance low and stable throughout the year. It needs no watering and is used in modern installations, including telecom towers and equipment.

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Earthing used in telecom equipment (in brief)

In telecom equipment, chemical earthing (maintenance-free earthing) is generally used. Telecom exchanges, BTS towers and equipment rooms need a very low and stable earth resistance (usually below 1–5 Ω) throughout the year, because the equipment works at low voltage and is sensitive to surges and lightning. Chemical earthing provides a low, constant resistance without regular watering. It is combined with a mesh/ring earth (common bonding network) in which all equipment, the tower, cable trays and the lightning arrester are bonded to a single earth to avoid potential differences.