Electrical switchboards and switchgear - Working Principle

Introduction

Article 240 of the National Electrical Code (NEC) covers overcurrent protection and notes that all electrical conductors shall be protected. Overcurrent protective devices (OCPDs) consist of fuses and circuit breakers. This article covers the basics of panelboards, switchboards, and switchgear, the three main options for organizing, housing, and utilizing the OCPDs. For simplicity, only breakers will be referenced in this article when discussing OCPDs.

Both were patented by Thomas Edison — the circuit breaker in 1879 and the fuse in 1890. Although fuses were the first OCPDs widely used in homes and commercial buildings, circuit breakers have also had a rich history of protecting electrical installations and are very common today.

Working Principle of Electrical Switchgear

Electrical switchgear and switchboards, often housed in metal cabinets with busbars and breakers, safely control, protect, and distribute electricity. Switchgear handles high/medium voltage (up to 350kV) with automatic, compartmentalized, and draw-out components for fault isolation. Switchboards typically handle low voltage (≤ 600V) for distribution with fixed-mounted components, often acting as the main service entrance. 

• Function: Designed for high-voltage and industrial environments to manage power flow and protect equipment from damage due to short circuits, overloads, and surges.
• Components: Utilizes circuit breakers (often vacuum or air-insulated), relays, and fuses housed in separate, secure compartments for safety.
• Operation: If a fault occurs, relays detect abnormal conditions (e.g., overcurrent) and trigger circuit breakers to immediately break the circuit. They feature draw-out breakers, allowing maintenance without shutting down the entire system.
• Protection Level: High, with advanced capabilities for fault isolation.

Switchboard Working Principle (Distribution & Monitoring)

• Function: Used primarily for low-voltage power distribution in commercial or residential buildings, dividing a main power source into smaller circuits.
• Components: Consists of a front-mounted panel holding buses, molded case circuit breakers (MCCBs), and meters.
• Operation: Takes incoming power and distributes it to various loads. It provides manual control over these loads, allowing for the isolation of specific circuits for maintenance.
• Safety Level: Lower than switchgear, usually without extensive compartmentalization.

In conclusion:

Electrical switchboards and switchgear are the primary "traffic controllers" and "bodyguards" of an electrical system. While they share similar components, their working principles differ based on their primary function: distribution (switchboards) versus protection (switchgear).

1. Electrical Switchboard Working Principle

• Power Intake: Electricity enters through a main breaker or lugs from a transformer or utility line.
• Division & Routing: The power flows onto horizontal and vertical busbars (conductive bars), which act as common connection points for multiple branch circuits.
• Protection: Each branch circuit has its own molded case circuit breaker (MCCB) or fuse. These protect the downstream equipment from overloads or short circuits by physically breaking the connection if the current exceeds safe levels.
• Control: Operators can manually toggle switches to de-energize specific parts of a building (e.g., lighting, HVAC) for maintenance.

Switchgear Working Principle

• Continuous Monitoring: Protective relays act as sensors, constantly measuring voltage and current. If they detect an abnormality (like a massive surge), they send a signal to the circuit breaker.
• Fault Interruption: Unlike a simple switch, switchgear uses power circuit breakers (often air, vacuum, or SF6 gas-insulated) that can safely quench the massive electrical "arc" created when high-voltage contacts pull apart.
• Isolation & Safety: Components are often compartmentalized (isolated in metal bins). This "draw-out" design allows a technician to pull out a single breaker for repair without shutting down the entire system, preventing widespread downtime.
• Coordination: Switchgear is built to withstand faults for a longer duration (up to 30 cycles) to allow downstream devices to trip first, ensuring only the faulty section is disconnected.

Core Differences Summary