Symbols and Wiring Diagrams: Introduction to Switches

 Switch Basics:

• Function: Switches open or close contacts to control the load.
• Key Terms:
  • Contact: Conducting part of the switch.
  • Pole: Number of sets of contacts in the switch.
  • Throw: Number of closed contact positions per pole.
• Types: Manual and automated switches (controlled by external forces like pressure or temperature).

Switch Types and Symbols:

• Single Throw Switch:
  • Example: Manual switch with line voltage on the line side, open position shown.
  • Operation: Closed switch allows power to reach the load.
• Double Pole Single Throw Switch:
  • Description: Two poles but only one throw.
  • Operation: Closed switch connects two sets of contacts.
• Single Pole Double Throw Switch:
  • Operation: Closes contacts in one direction (e.g., between 1 and 2) and opens in the other (e.g., between 1 and 3).
• Double Pole Double Throw Switch:
  • Description: Two poles with two sets of switches.
  • Operation: Switches both make and break circuits simultaneously.

Automated Temperature Switches (Thermostats):

• Bimetal Mechanism:
  • Composition: Two dissimilar metals with different expansion rates.
  • Operation: Warps or bends with temperature changes to open or close contacts.
  • Example: Bimetal strip with electrical contacts.
• Thermostat Symbols:
  • Cooling Thermostat: Switch closes on rising temperature.
  • Heating Thermostat: Switch closes on falling temperature.

Modern Thermostats:

• Solid State Technology: Programmable electronic thermostats use similar symbols despite the absence of bimetals or mercury.

Pressure Controls:

• Pressure Control Symbol:
  • Depicts bellows that open/close contacts based on pressure changes.
  • Safety Controls: Shown in closed position under normal conditions.
• Types of Pressure Controls:
  • Low Pressure Control: Opens on falling pressure.
  • High Pressure Control: Opens on rising pressure.
  • Adjustable Controls: Set by technicians to specific cut-in and cut-out values.

Safety Devices:

• Symbols and Examples:
  • Fuses: Shown as cartridge or control fuses.
  • Bimetal Overload: Opens contacts when heated.
  • Thermal Overload: Used in motor starters to prevent overheating.
  • Magnetic Overload: Uses electromagnetism to open contacts during high current.