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Numbering System & Symbols Library - Single Phase

Owen Davis

Created on October 17, 2025

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Circuit Breaker (CB) 1 Pole

  • Protects a single “hot” line in single-phase power from overloads/shorts.
  • Commonly used for control power circuits, lights, outlets, or small loads.
  • Trips automatically to interrupt the circuit when faults occur.
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Circuit Breaker (CB) 2 Pole

  • Protects both “hot” legs of a single-phase 240 V circuit.
  • Trips both poles together, ensuring the load is fully isolated.
  • Often used for motors, heaters, and higher-power single-phase equipment.
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Power Distribution Block (PDB) – 1 Pole

  • Splits one incoming line into multiple outgoing circuits.
  • Reduces wiring clutter by providing a central connection point.
  • Used to distribute control power (e.g., from a 24 VDC supply to multiple devices).
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Disconnect Switch (DISC) – 1 Pole

  • Provides a physical means of isolating a single power line.
  • Often used for maintenance safety on single-phase machines.
  • Can be lockable for lockout/tagout compliance.
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Fan (FN)

  • Provides forced air cooling inside control panels or equipment.
  • Keeps electronics like VFDs, PLCs, and power supplies within safe temperature limits.
  • Drawings show them so installers and maintenance know they must be powered/connected.
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Horn (HRN)

  • Audible alarm output device.
  • Used to alert operators of faults, process conditions, or safety warnings.
  • Often combined with stack lights for visual + audible signaling.
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Control Relay (CR)

  • Electrically operated switch used for logic/control.
  • Input coil energizes → contacts open/close to control other circuits.
  • Expands PLC outputs, provides isolation, or creates interlocking logic.
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Receptacle (RECP)

  • Standard outlet for plugging in auxiliary devices (tools, laptops, testing equipment).
  • Can be single-phase 120 V or 240 V depending on the system.
  • Included in panel drawings so installers wire them in properly.
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Pushbutton (PB) – Normally Open

  • Manual input device that only completes the circuit when pressed.
  • Used for “Start,” “On,” or similar enabling functions.
  • Common in motor starters, HMIs, and operator stations.
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Pushbutton (PB) – Normally Closed

  • Manual input device that is closed (conducting) until pressed.
  • Used for “Stop,” “Off,” or safety functions.
  • Provides fail-safe behavior: if wiring breaks, the circuit de-energizes.
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Transformer (T)

  • Steps voltage up or down between circuits (e.g., 480 V → 120 V → 24 V).
  • Used to supply control power (often 24 VDC or 120 VAC) from higher-voltage mains.
  • Provides electrical isolation and safety in control panels.
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