Low Tension (LT) switchgear represents a critical assembly of devices designed for the control, protection, and isolation of electrical circuits, typically operating at voltage levels below 1000 Volts AC or 1500 Volts DC. These components are indispensable for ensuring the safety of personnel and the longevity of electrical equipment within residential, commercial, and industrial settings.

1.1 Switch Fuse Unit (SFU):

• Detailed Function: An SFU serves as a foundational component that integrates the functionality of a manual isolation switch with overcurrent protection provided by fuses. It allows an operator to physically make or break the electrical circuit, thereby isolating a section for maintenance or operation, while simultaneously offering passive protection against excessive currents.
• Operating Principle:
• Switching: The "switch" part consists of robust contacts that can be manually opened or closed, physically disconnecting or connecting the circuit. This provides a clear visible break for safety.
• Protection: The "fuse unit" incorporates one or more fuses in series with the main circuit. These fuses contain a calibrated metallic element designed to melt and open the circuit if the current flowing through it exceeds a safe limit (due to an overload or a short circuit) for a specified duration. The fuse acts as a sacrificial device, protecting the downstream circuit from damage.
• Applications: Commonly found in older industrial power distribution boards, as main incomers for small buildings, or for individual motor starters where frequent switching is not required and fuse replacement is acceptable.
• Limitations: The primary drawback is that once a fuse blows, it must be manually replaced, leading to downtime. They lack the automatic resetting convenience of circuit breakers and do not offer advanced protection features like earth leakage detection.

1.2 Miniature Circuit Breaker (MCB):

• Detailed Function: An MCB is a highly sophisticated, automatically operated electromagnetic device that serves as a reusable switch designed to protect an electrical circuit from damage caused by overcurrent. This overcurrent can manifest as either an overload or a short circuit.
• Operating Principle:
• Thermal Tripping (for Overload Protection): Uses a bimetallic strip that bends when heated due to overload current, causing it to trip and open the circuit after a delay for temporary surges.
• Magnetic Tripping (for Short Circuit Protection): Utilizes an electromagnet that trips the circuit almost instantly when high fault current flows due to a short circuit.
• Advantages over Fuses: Resettable, provides quicker response, indicates ON/OFF status visually, and available in various configurations.
• Types (based on Tripping Characteristics / Curves):
• Type B Curve: 3 to 5 times rated current, ideal for resistive loads.
• Type C Curve: 5 to 10 times rated current, common for residential and commercial use.
• Type D Curve: 10 to 20 times rated current, designed for inductive loads that produce large inrush currents.

1.3 Earth Leakage Circuit Breaker (ELCB) / Residual Current Device (RCD):

• Detailed Function: These are crucial safety devices designed specifically to protect against electric shock and fire hazards caused by earth faults. They achieve this by rapidly disconnecting the power supply when a small leakage current flows to earth.
• Operating Principle (RCD): Based on the current balance principle, where the live and neutral conductors pass through a toroidal transformer. If a fault occurs, the RCD detects the imbalance and disconnects the circuit within milliseconds.
• Distinction between ELCB and RCD:
• Voltage-Operated ELCB: Older technology that measures voltage on the earthing conductor.
• Current-Operated RCD: Modern technology relied upon for better effectiveness without depending on the local earth electrode’s quality.
• Application: Mandated for new residential and commercial installations for safety.

1.4 Moulded Case Circuit Breaker (MCCB):

• Detailed Function: An MCCB is a robust electrical protection device designed for higher current ratings and fault levels than MCBs. It offers comprehensive protection against overload, short circuit, and provides manual switching and isolation capabilities.
• Operating Principle: Incorporates both thermal and magnetic tripping mechanisms but often features adjustable trip settings for fine-tuning based on system requirements.
• Application: Essential in main distribution boards, sub-distribution panels, and motor control centers.

1.5 Fuses (Types and Ratings):

• Detailed Function: Fuses are fundamental, single-use, overcurrent protective devices that melt to break the circuit when the current exceeds a threshold.
• Operating Principle: Composed of a metallic element encased in a non-combustible material, it operates based on the rapid heating effect due to electric current.
• Advantages: Simple, low cost, and fast operation but requires replacement after operation.
• Types of Fuses:
• Rewirable Fuses: Old design requiring manual replacement.
• Cartridge Fuses: Sealed and filled with materials for effective interruption.
• HRC (High Rupturing Capacity) Fuses: Designed to interrupt very high fault currents safely.
• Miniature Fuses: Small design for protection in electronic circuits.