Define earthing. State its types.
1 Answer
Definition of Earthing
Earthing (also known as grounding in North American English) is the process of connecting the non-current-carrying metallic parts of electrical equipment and the neutral point of a power supply system to the general mass of the Earth. This connection is made using a conductor with very low resistance.
The primary purpose of earthing is safety. It protects people from electric shock and prevents damage to electrical appliances and buildings by providing a safe path for fault current to flow to the ground.
How it Works:
If an insulation failure or a fault occurs, a live wire might touch the metal casing of an appliance. Without earthing, this casing becomes dangerously "live" at the supply voltage. If a person touches it, the current will flow through their body to the ground, causing a severe or fatal electric shock.
With a proper earthing system, the metal casing is already connected to the earth via a low-resistance path. The large fault current will immediately flow through the earth wire to the ground. This sudden surge in current will blow a fuse or trip a circuit breaker, instantly disconnecting the power supply and making the appliance safe.
Key Objectives of Earthing:
Human Safety: To protect individuals from electric shock.
Equipment Protection: To prevent damage to appliances by safely discharging fault currents.
Voltage Stabilization: To maintain the voltage of the system at a known, constant level with respect to the earth.
Overvoltage Protection: To safeguard the system from lightning strikes and voltage surges by providing a path for the high voltage to dissipate into the ground.
Types of Earthing
Earthing can be broadly classified into two main categories based on its purpose: System Earthing and Equipment Earthing. These are then implemented using various physical methods.
1. System Earthing
This involves connecting the neutral point of a power system (e.g., the star point of a transformer or alternator) to the earth. It is primarily concerned with the safety and protection of the electrical system itself.
Based on the International Electrotechnical Commission (IEC) standard, system earthing is categorized into three main types:
- TT System: The power source (transformer) has a direct connection to the earth. The consumer's installation also has its own independent connection to the earth. The "T" stands for Terra (Latin for Earth), so TT means the source is earthed and the load is earthed.
- TN System: The power source is directly earthed, and the consumer's earthing is connected to the system's neutral conductor. The "N" stands for Neutral. This is a very common system and has three sub-variants:
- TN-S: A separate protective earth (PE) conductor runs from the source to the consumer's installation. (S = Separate).
- TN-C: The neutral and protective functions are combined into a single conductor (PEN). (C = Combined).
- TN-C-S: A combination of the above, where the neutral and earth are combined in one part of the system (usually the supply side) and separate in another (the consumer side).
- IT System: The power source is either completely isolated from the earth or connected through a high impedance. The consumer's equipment is earthed locally. This system is used in critical applications like hospitals and industrial processes where continuity of power is essential even after a first fault. (I = Isolated).
2. Equipment Earthing
This is the most commonly understood form of earthing. It involves connecting the non-current-carrying metallic parts of electrical equipment (like the frame, casing, or enclosure) to the earth through a low-resistance conductor. Its sole purpose is to protect the user from electric shock if the casing accidentally becomes live.
Physical Methods of Earthing (How the Earth Connection is Made)
These are the practical techniques used to create the physical connection to the earth for both system and equipment earthing.
Pipe Earthing: A galvanized iron (GI) pipe of a standard diameter and length is buried vertically in the ground. The pit is filled with layers of charcoal and salt to maintain low resistance by keeping the surrounding soil moist. This is a very common and reliable method.
Plate Earthing: A copper or GI plate is buried vertically in a pit deep in the ground. The pit is also filled with alternate layers of charcoal and salt to increase conductivity and maintain moisture. This method is effective but more expensive than pipe earthing.
Rod Earthing: A solid copper rod or a hollow GI pipe with a pointed end is driven deep into the ground either manually or with a hammer. It is a simple and cost-effective method suitable for areas with sandy or rocky soil. Multiple rods can be connected in parallel to achieve the desired low resistance.
Strip or Wire Earthing: A copper strip or GI wire of a sufficient cross-section is buried in a horizontal trench in the ground. This method is used in rocky areas where excavation for deep pits is difficult.
In summary, system earthing protects the power grid, while equipment earthing protects the end-user. Both are crucial components of a safe and reliable electrical installation.