Earthing and Protective Devices
Overview
Earthing and Protective Devices is a core chapter in ITI Electrician Semester 3 (NCVT). It covers the principles of earthing, types of earthing systems, protective devices like fuses, MCBs, ELCBs, and their working. Understanding earth resistance values, IS codes, and device ratings is essential. This chapter carries significant marks in theory and practical exams and is frequently tested in NCVT question papers.
Typed Notes
Purpose and Types of Earthing
Earthing connects non-current-carrying metal parts of electrical equipment to the earth to ensure safety. It limits voltage rise on equipment body during faults and provides a low-resistance path for fault current to flow, enabling protective devices to operate. Main types: (1) Plate Earthing – most common, uses GI or copper plate. (2) Pipe Earthing – uses a GI pipe of 38mm diameter, 2.5m long. (3) Rod Earthing – uses copper or GI rod. (4) Strip or Wire Earthing – uses GI strip buried in ground. Pipe earthing is preferred in rocky soil areas.
Plate Earthing – Construction Details
A GI plate of size 600mm × 600mm × 6mm (or copper plate 600mm × 600mm × 3mm) is buried vertically at a minimum depth of 3 metres below ground level. The plate is surrounded by alternate layers of charcoal and salt to reduce earth resistance and retain moisture. A GI pipe of 19mm diameter is connected to the plate for watering. The earth wire is connected to the plate via a GI bolt. Earth resistance is tested using an Earth Tester (Megger). IS 3043 governs earthing installations in India.
Fuses – Types and Characteristics
A fuse is the simplest overcurrent protective device. It works on the heating effect of current. Key types: (1) Rewirable (Kit-Kat) Fuse – uses tinned copper wire, breaking capacity up to 3 kA, used in domestic wiring. (2) HRC (High Rupturing Capacity) Fuse – filled with quartz powder, breaking capacity up to 80 kA, operates in 0.002 sec, used in industrial panels. (3) Cartridge Fuse – enclosed type for medium applications. Fuse rating = 1.5 × full load current. Fusing factor = Minimum fusing current ÷ Current rating (always greater than 1). HRC fuse has fusing factor close to 1.
MCB, ELCB and RCCB – Working and Ratings
MCB (Miniature Circuit Breaker): Protects against overload (bimetallic strip – thermal action) and short circuit (electromagnet – magnetic action). Rated up to 100A, breaking capacity 10 kA. Types: B-curve (domestic), C-curve (commercial), D-curve (industrial). ELCB (Voltage type) detects voltage on earth wire; now replaced by RCCB. RCCB (Residual Current Circuit Breaker) / Current-operated ELCB: Compares current in phase and neutral; if difference exceeds 30 mA, it trips within 30 milliseconds. Protects humans from electrocution. RCCB does NOT protect against overload – always use with MCB.
Topics & Sub-topics
Har topic ke saath sub-topics aur example — chapter ka complete coverage.
01Purpose of Earthing
- Metal body ko zero potential
- Fault current → earth
- Shock protection
Geyser ka body earth na ho to leakage par touch karte hi shock lagta hai.
02Earth Electrode Types
- Pipe earthing (GI pipe)
- Plate earthing (cu/GI plate)
- Strip/rod earthing
Ghar mein pipe earthing common — 2-3 m deep, charcoal + salt layers ke saath.
03Earth Resistance Values
- Domestic < 5 Ω (target 1-2 Ω)
- Substation < 1 Ω
- Measured by earth tester
Earth resistance zyada aaye to electrode aur paani/glycose se improve karo.
04Protective Devices
- Fuse — oldest, rewirable
- MCB — trip on overload/short
- ELCB/RCCB — 30 mA human protection
RCCB 30 mA — isliye aajkal mandatory hai, yahi 'jyada se jyada' bachaata hai.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01Earth resistance should NOT exceed 1 ohm for power systems and 5 ohms for general installations as per IS 3043.
- 02Plate earthing uses a 600mm × 600mm × 6mm GI plate or 600mm × 600mm × 3mm copper plate buried at least 3 metres deep.
- 03MCB (Miniature Circuit Breaker) operates on both thermal (overload) and magnetic (short-circuit) principles and has breaking capacity up to 10 kA.
- 04ELCB (Earth Leakage Circuit Breaker) / RCCB trips when leakage current exceeds 30 mA, protecting humans from electric shock.
- 05HRC (High Rupturing Capacity) fuse can interrupt fault currents up to 80 kA and operates in less than 0.002 seconds.
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
What is the minimum depth at which a plate earth electrode should be buried as per IS 3043?
Explain the difference between a rewirable fuse and an HRC fuse.
Why is RCCB always used in combination with an MCB and not alone?
What is fusing factor? Calculate the fusing factor if minimum fusing current is 13A and current rating is 10A.
State the working principle of an MCB and name its three tripping curves used for different applications.
Earthing provides a safe fault-current path; earth resistance must be below 1–5 ohms (IS 3043). Plate earthing (GI 600×600×6mm, 3m deep) is most common. Fuses (rewirable and HRC) protect against overcurrent. MCBs provide dual protection (thermal + magnetic). RCCB/ELCB protects against earth leakage above 30 mA. These devices together ensure electrical safety in domestic and industrial installations.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.What is the standard size of a GI plate used in plate earthing as per IS 3043?
Q2.An RCCB trips when the leakage current exceeds which value?
Q3.Which type of fuse has the highest breaking capacity?
Q4.Which MCB tripping curve is recommended for domestic/residential electrical installations?
Mark karo — dashboard par progress dikhega.