Basic Electrical Engineering

(b) To prevent overloading and short circuits

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

Types of Earthing

Earthing (grounding) is the connection of an electrical system or equipment to the general mass of earth through a low-resistance path. 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. A funnel arrangement at the top is used to pour water (especially in summer) to keep the soil moist and maintain low earth resistance. It is cheap and commonly used for domestic and small installations.

2. Plate earthing A copper plate (60 cm × 60 cm × 3 mm) or GI plate (60 cm × 60 cm × 6 mm) is buried vertically at a depth of about 3 m. Alternate layers of charcoal and salt are filled around the plate to reduce earth resistance. It is used where a very low earth resistance is required, such as in substations.

3. Rod earthing A copper or GI rod of about 12–25 mm diameter is driven vertically into the ground up to 3 m or more. It is simple and economical, and is used where the soil is soft and the water table is high.

4. Strip or wire earthing A copper strip or GI wire is laid horizontally in a trench about 0.5 m deep. It is used where a large area of land is available and a single electrode is not sufficient.

5. Mesh earthing A grid or mesh of interconnected conductors is buried over a large area. It is used in power stations and substations to keep step and touch voltages within safe limits.

6. Chemical earthing A maintenance-free electrode filled with a chemical compound (bentonite, graphite, etc.) is used to obtain a stable, very low earth resistance. It needs no watering and lasts long.

Earthing Used in Telecom Equipment

In telecom equipment, chemical earthing (maintenance-free earthing) is generally used, along with plate earthing where a very low earth resistance is needed. Telecom exchanges, BTS sites, and transmission equipment require a low and stable earth resistance (usually below 1–5 Ω) for protection against lightning surges, static charges, and fault currents, so chemical earthing is preferred because it is reliable, needs no regular watering, and gives a stable low resistance throughout the year.