Switchgear Components: Contactors, Relays, and Circuit Breakers

Switchgear Classification and Components

Sensors

Sensors collect system status information. Interrupters often incorporate detectors.

Data Processing

Data processing units, often composed of relays and contactors, manage automatic cycles (combinatorial or sequential) based on system importance and logic requirements.

Command and Control

Control circuits, including elements like coils and starters, are activated based on available data and processing logic.

Man-Machine Interface (MMI)

The MMI enables operator interaction, allowing starting, stopping, and control of the system through devices such as buttons and switches.

Contactors Explained

A contactor is an electrically controlled switch used for making or breaking an electrical circuit. It typically has a defined rest position and de-energizes (opens or closes contacts) when its actuating force is removed.

Actuation Types

Contactors can be actuated using different methods:

• Electromagnetic: Driven by an electromagnet.
• Electromechanical: Involving mechanical linkages.
• Pneumatic: Actuated by compressed air pressure.
• Hydraulic: Actuated by liquid pressure.

Selection Criteria

Choosing a contactor requires considering several factors:

• Breaking Capacity: The maximum current the contactor can safely interrupt at a specified voltage.
• Making Capacity: The maximum current the contactor can handle when closing the circuit (often related to inrush current).
• Operating Voltage: The nominal voltage the contactor's coil and main circuit are designed for.
• Rated Current: The continuous current the contactor can carry through its main contacts without overheating.
• Contactor Lifespan: The expected operational endurance, often specified in the number of switching cycles.

Understanding Relays

A relay is an electrically operated switch that uses a change in one electrical circuit (input/coil) to control the state (open/closed) of another circuit (output/contacts).

Types of Relays

• Protection Relays: Their primary mission is to detect abnormal conditions like overloads or short circuits and initiate protective actions (e.g., tripping a circuit breaker).
• Control (Command) Relays: Used within control logic circuits to switch other devices or manage sequences.
• Measurement Relays: Designed to operate based on specific values or changes in measured electrical quantities (e.g., voltage, current, frequency, phase angle).

Thermal Relays

A common type of protection relay, the thermal relay, typically operates using a bimetallic strip composed of two metals with different thermal expansion coefficients. Heat generated by excessive current causes the strip to bend, eventually tripping a mechanism. The relay is chosen based on the load type and its current rating. For comprehensive protection, thermal relays are usually installed in conjunction with suitable circuit breakers or fuses.

Circuit Breakers

Circuit breakers are protective devices that combine the functions often found in separate relays and switches. They typically offer protection against both overloads (often using a thermal mechanism similar to thermal relays) and short circuits (usually using a faster electromagnetic mechanism). They provide automatic disconnection of the circuit during fault conditions.

Selection Criteria

Selecting the appropriate circuit breaker involves considering its electrical ratings (voltage, current, breaking capacity) and its specific tripping curve.

Common Tripping Curve Types

The tripping curve defines how quickly the breaker trips at different levels of overcurrent:

• Type B: Trips relatively quickly on small overcurrents; suitable for protecting cables and resistive loads with low starting currents.
• Type C: A common choice for general industrial applications, tolerating moderate starting currents (e.g., motors, fluorescent lighting).
• Type D: Designed for circuits with high inrush currents, such as transformers or large motors, providing delayed tripping for overloads but fast short-circuit protection.
• Type Z: Very sensitive, tripping quickly even on small, short-duration overcurrents; used for protecting sensitive electronic circuits.