Question

State significance of earthing. Draw and explain pipe earthing. State the values of earth resistances for : (i) Substation (ii) Residential wiring (iii) H.T. Line (iv) L.T. Line

Answer 1

1. Significance of Earthing

Earthing (or grounding) is a fundamental safety measure in any electrical installation. Its primary significance is to protect people, equipment, and property from electrical hazards. Here are the key reasons why earthing is crucial:

• Safety of Personnel: It provides a safe path for fault current to flow to the ground in case of an insulation failure or accidental contact between a live conductor and the metallic body of an appliance. This causes a high current to flow, which immediately trips the fuse or circuit breaker, disconnecting the power supply and preventing a dangerous electric shock.
• Protection of Equipment: By providing a low-resistance path for fault currents, earthing prevents these currents from flowing through and damaging sensitive electrical and electronic equipment.
• Voltage Stabilization: Earthing provides a common reference point (0 Volts) for the entire electrical system. It helps to stabilize the voltage of the system with respect to the ground, preventing overvoltages and ensuring the system operates reliably.
• Protection Against Lightning and Surges: Earthing systems are designed to safely discharge the massive energy from a lightning strike or a power surge into the earth, thereby protecting the entire installation from catastrophic damage.

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2. Pipe Earthing: Drawing and Explanation

The image provided is a detailed diagram of the Pipe Earthing method. This is one of the most common and reliable methods used for electrical earthing.

Explanation of Pipe Earthing

Objective: To create a low-resistance connection between the electrical installation's earth point and the general mass of the earth.

Construction and Components (as seen in the diagram):

1. Earth Electrode: The main component is a Galvanized Iron (G.I.) Pipe of a standard diameter (e.g., 38mm) and length (at least 2.5 meters). It is perforated with holes (e.g., 12mm diameter) to allow water to seep into the surrounding soil, keeping it moist. The pipe is placed vertically in a pit dug into the ground.

2. Pit and Filling:
   A pit is dug to a depth of at least 1.25 meters, with a deeper, narrower bore to accommodate the 2.5m pipe.
   The space around the G.I. pipe is filled with alternate layers of charcoal (or coke) and salt, each about 15 cm thick.

   *   **Salt** absorbs moisture from the surroundings and acts as an electrolyte, reducing the soil's resistivity.
   *   **Charcoal** is porous and retains the moisture for a long time. This combination creates a highly conductive medium around the pipe, significantly lowering the earth's resistance.
   

3. Watering Arrangement:
   A funnel with a wire mesh (to prevent debris from entering) is placed at the top.
   This funnel is connected to a smaller G.I. pipe (12.7mm in the diagram) that runs down into the main pit. This arrangement allows water to be poured into the pit, especially during dry seasons, to maintain moisture and ensure low earth resistance.

4. Inspection Chamber:
   A cement concrete chamber (30cm x 30cm) is built around the top of the pit.
   It is fitted with a Cast Iron (C.I.) cover hinged to a C.I. frame. This protects the connections, prevents accidents, and allows for easy inspection and watering.

5. Earth Connection:
   The earth wire from the main switchboard is brought to the pit through a G.I. pipe (19mm in the diagram) for mechanical protection.
   This wire is securely connected to the top of the main G.I. pipe (the electrode) using G.I. nuts and washers, ensuring a firm and reliable electrical connection.

Working Principle:
When a fault occurs (e.g., a live wire touches the metal casing of a motor), the fault current flows from the casing, through the earth wire, to the G.I. pipe. The charcoal and salt mixture provides a very low-resistance path for this large current to dissipate safely into the ground.

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3. Standard Values of Earth Resistance

The acceptable value of earth resistance depends on the type of installation and the soil conditions. Lower resistance is always better. Here are the generally accepted maximum values as per standard practices (like Indian Standards IS: 3043):

(i) Substation (Large):
* 0.5 Ω to 1.0 Ω. For large power substations, the earth resistance must be very low (ideally less than 1 Ω, often aiming for 0.5 Ω) to handle potentially massive fault currents safely.

(ii) Residential Wiring:
* 5.0 Ω. For domestic or small commercial installations, an earth resistance of up to 5 Ω is generally considered safe. In areas with very dry or rocky soil, a value up to 8 Ω may be permissible, but 5 Ω is the preferred standard.

(iii) H.T. (High Tension) Line:
* 10 Ω. This value typically applies to the earthing of individual transmission line towers or poles.

(iv) L.T. (Low Tension) Line:
* 5.0 Ω. This applies to the poles in the low-voltage distribution network that supplies power to consumers.