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UNIT-III: ELECTRICAL SAFETY AND PROTECTIVE DEVICES

9.1 INTRODUCTION

• Switchgear: Apparatus for switching, controlling, and protecting electrical circuits and equipment.
• Domestic vs. Industrial Applications:
    - Domestic: Typically involves normal rated currents; less protective requirements.
    - Industrial: Requires more advanced switchgear for protections against various hazards and a higher standard of load handling.
• Components of Switchgear: Includes fuses, circuit breakers, isolators, relays, control panels, metering panels, lightning arresters, and transformers.
• Focus: The chapter discusses commonly used electrical protective devices including fuses, switch fuse units, miniature circuit breakers (MCB), moulded case circuit breakers (MCCB), and earth leakage circuit breakers (ELCB) or residual current breakers, and the use of multimeters and earthing.

9.2 FUSES

• Purpose: Protects electrical circuits from excessive currents caused by overloading or short circuits.
• Consequences of Excessive Currents:
    - Overloading results in overheating and potential damage to electrical motors.
    - Short circuits produce a direct connection between live and neutral conductors, allowing dangerous currents that can lead to fire.
• Function of a Fuse: Serves as an interrupting device that will melt when current exceeds its rated value, thus breaking the circuit and preventing damage.

General Construction of a Fuse

• Typically consists of a metal element between two terminals, mounted on an insulated base.

Function

• Normal Operation: Carries operating current without heating.
• Fault Conditions: Melts when current exceeds the predetermined value, consequently interrupting the circuit.
• Acts as a safety valve for the electrical circuit.

Fuse Element Material

• Commonly used metals: tin, lead, zinc, silver, antimony, copper, aluminum.
• Preferred Metals: Low melting point metals (tin, lead, alloys).
• Lead-tin alloy (37% lead, 63% tin) is favored due to its soft nature under pressure, unsuitable for higher currents beyond 10 A.
• For higher currents, copper wire is favored; silver is also used despite higher costs due to low oxidation.

9.3 TYPES OF FUSES

• The following fuses are commonly used:
    1. Semi-enclosed or rewirable type
    2. High rupturing capacity (H.R.C.) cartridge type

9.3.1 Semi-enclosed or Rewirable Type Fuses

• Construction: Semi-enclosed fuse element.
• Typical design: Fuse wire threaded through a porcelain bridge and connected to contact terminals.
• Operation: Wired in series; fuse element melts due to heat developed from high resistance, interrupting the circuit.
• Applications: Domestic installations where low fault currents exist.
• Advantages:
    - Cost-efficient.
    - Can be rewired after blowing off, enabling quick service restoration.
• Disadvantages:
    - Not suitable for high fault currents.
    - Less reliability due to inaccurate characteristics.
    - Wire deterioration from oxidation affects performance, leading to premature failure.

9.3.2 High Rupturing Capacity (H.R.C.) Cartridge Fuses

• Purpose: Withstand heavy stresses during fault clearing, such as short circuits.
• Construction: Fuse element sealed in a cartridge with ceramic or epoxy material, brass end caps, quartz powder for arc quenching.
• Operation: Similar to semi-enclosed types, but the reaction between vaporized fuse material and quartz produces high resistance material quenching the arc.
• Applications: Increasingly used in industrial settings over rewirable types.
• Advantages:
    - Enclosed design prevents deterioration.
    - Reliable protection.
    - Fast operation and can handle high fault currents without generating hazards.
• Disadvantages:
    - Higher cost than rewirable types.
    - Needs replacement after operation.

9.4 CURRENT RATING AND MINIMUM FUSING CURRENT

• Current Rating of Fusing Element: Continuous current value the fuse can carry without deterioration.
• Minimum Fusing Current: The threshold current that causes the fuse to melt.
• Fusing Factor: Ratio of minimum fusing current to the current rating of the fusing element, always greater than 1. Formula:
  $ext{Fusing Factor} = \frac{ ext{Minimum Fusing Current}}{ ext{Current Rating of Fusing Element}}$

9.5 SWITCHES

• Definition: Manually operated device for making/breaking electrical circuits.
• Capabilities: Can handle specified overload conditions but lacks inherent protection features.

9.6 COMPOSITE UNITS OF SWITCHES AND FUSES

• Used for both electrical isolation and circuit protection.
• Types of Units:
    1. Switch fuse unit (SFU)
    2. Fuse switch unit (FSU)

9.6.1 Switch Fuse Unit (SFU)

• Construction: Comprises a switch and fuse in series.
• Protection Provided: Electrical isolation and circuit protection against overloads and short circuits; typically rated up to 1000 A.
• Applications: Lighting circuits, distribution feeders, L.T. capacitors, motors, etc.

9.6.2 Fuse Switch Unit (FSU)

• Construction: A switch where the fuse link forms part of the moving contact.
• Protection Provided: Similar to SFU, suitable for applications with higher fault current levels.
• Applications: Distribution feeders, transformers, diesel generating sets.

Comparison between SFU and FSU

9.7 INTRODUCTION TO LOW-VOLTAGE CIRCUIT BREAKERS

• Definition: Automatic switching device able to make, carry, and break current safely.
• Operation Principle: When a fault occurs and current rises, it activates a relay via a current transformer leading to the opening of circuit breaker contacts to interrupt current flow and extinguish arcs.
• Difference from Fuse: Fuses require replacement while circuit breakers can be reset.

9.7.1 Miniature Circuit Breaker (MCB)

• Construction: Assembled in a moulded case using thermosetting powders; current carrying materials are copper/silver alloy.
• Protection Types: Overload (bimetallic strips) and short-circuit (magnetic attraction).
• Advantages: Easy for all users, can function as a switch, low aging issues, economical.
• Ratings: Available from 0.5 A to 100 A at varying voltage ratings with breaking capacity up to 3 kA.
• Applications: Lighting circuits, motor switching, etc.

9.7.2 Moulded Case Circuit Breaker (MCCB)

• Description: Compact air circuit breaker in a moulded insulating casing using atmospheric air for arc quenching.
• Applications: Used for heavy current circuits in both a.c. and d.c. systems.
• Advantages:
    - Minimal maintenance needed.
    - No recurring costs.
    - Helps prevent single-phasing, indicates fault tripping.
    - Available ratings from 16 A to 1600 A, breaking capacities up to 85 kA.

9.7.3 Earth Leakage Circuit Breaker (ELCB) or Residual Current Breaker (RCB)

• Definition: Protects against earth faults.
• Operation: Monitors balance of current through phase and neutral wires; detects leakage leading to tripping.
• Ratings: 30 mA (personal protection), 100 mA (installation protection), 300 mA (high-leakage installations).
  

9.8 CONNECTION DIAGRAM OF A TYPICAL ELECTRICAL INSTALLATION WITH PROTECTIVE DEVICES

• Importance of Wiring for Computers: Necessary for stable operations; requires provisions for air conditioning and UPS systems to maintain a constant supply and protect against voltage variations or loss during outages.

9.9 MULTIMETERS

• Definition: Instrument for measuring electrical quantities like voltage, current, and resistance.
• Types:
    1. Analog Multimeters: Readings by interpreting pointer positions; susceptible to reading errors.
    2. Digital Multimeters (DMM): Numeric displays, more accurate, may include additional measurement capabilities.

9.9.1 Analog Multimeters

• Components: Micro ammeter, shunts, multipliers, rectifiers within a single casing.
• Usage: Can measure d.c. and a.c. by appropriate selections of circuits and protections.

9.9.2 Digital Multimeters

• Sections:
    1. Signal conditioning
    2. Analog to digital conversion
    3. Display
• Advantages: Highly accurate, durable, with potential for measuring many parameters beyond voltage, current, and resistance.

9.10 IMPORTANCE OF EARTHING

• Purpose: Essential for safety; connects all metal casings to the earth to eliminate shock risks.
• Electrical Fault Scenarios: Describes the dangers of un-earthed vs. earthed appliances and the importance of blowing fuses during faults.

9.11 METHODS OF EARTHING

• Definition: Connection to earth to ensure safety.
• Types:
    1. Through water main
    2. Plate earthing
    3. Pipe earthing

9.11.1 Earthing through a Water Main

• Uses metal water mains for grounding in urban areas; inadequate for non-metallic mains.

9.11.2 Plate Earthing

• Involves burying a metal plate for grounding, often using layers of salt and coke for resistance reduction.

9.11.3 Pipe Earthing

• Uses a pipe buried in the ground with moisture enhancement methods to ensure effective earthing.

9.12 FACTORS AFFECTING EARTH RESISTANCE

• Components:
    1. Resistance of the earthing system to remote earth
    2. Resistance of local soil
• Factors: Include materials used, size, depth of electrodes, quality of connections, contact resistance, and soil type.

9.13 MAXIMUM PERMISSIBLE VALUE OF EARTH RESISTANCE

• Defined by type of installation,
• Representative values:
    - Large power stations: $0.5 \, ext{Ω}$
    - Major substations: $1.0 \, ext{Ω}$
    - Small substations: $2.0 \, ext{Ω}$
    - Other cases: $5.0 \, ext{Ω}$

9.14 METHODS OF REDUCING EARTH RESISTANCE

• Approaches:
    1. Use multiple electrodes in parallel
    2. Increase electrode size/depth
    3. Enhance the moisture content and chemical treatment of surrounding soil.

9.15 INDIAN ELECTRICITY RULES RELEVANT TO EARTHING

• Key Regulations:
    1. Neutral conductors must be earthed at multiple points.
    2. All metal casings and parts of electrical apparatus must be earthed.
    3. Testing of earthing systems must be carried out regularly.

9.16 POINTS TO REMEMBER

• Summary of Key Concepts:
    - Definition and components of switchgear.
    - Role and construction of the fuse.
    - Importance of protective devices in electrical installations, like MCB, MCCB, ELCB.

9.17 EXERCISES

• Review Questions: Covering topics like fuses, circuit breakers, and earthing methods.

• Theory Questions: Require explanations and diagrams for concepts discussed.

• This section provides practice that helps solidify knowledge and understanding of electrical safety and protective devices, preparing for examinations effectively.