Numbering System & Symbols Library - Input/Output

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Fuse (fb)

• Protects downstream circuits/devices from overcurrent.

• Melts when current exceeds a rated value, breaking the circuit.

• Ensures faults don’t damage equipment or wiring.

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Fused Terminal Block (FTB)

• Combines a wire termination point with built-in fuse protection.

• Provides both connection and circuit protection in one device.

• Simplifies panel wiring while adding safety.

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Limit Switch (LS)

• Detects physical movement or position of a machine part (e.g., end of conveyor travel).

• Used for interlocks, safety stops, and machine sequencing.

• Often has mechanical actuators like levers, rollers, or plungers.

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Level Switch (LVS)

• Detects the presence or height of a material (liquid or bulk solid) at a certain level.

• Used for tank overfill protection, pump control, and material handling.

• Can be float-based, capacitance-based, or ultrasonic.

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Contact, Normally Open

• A switch/contact that is open by default and closes when actuated.

• Allows current only when the controlling device activates.

• Used for enabling circuits, start buttons, or PLC inputs.

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Contact, Normally Closed

• A switch/contact that is closed by default and opens when actuated.

• Provides current flow until the device changes state.

• Used for stop circuits, fail-safe conditions, and safety interlocks.

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Pushbutton (PB), 3-position

• Operator input device with three selectable states (e.g., Forward–Off–Reverse).

• Used where two actions need to be controlled from one button.

• May be maintained (stays in position) or momentary (returns to center).

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2-Wire Photoeye (ZS), Schematic

• Optical sensor that detects presence/absence using a light beam.

• Two-wire version acts like a series-connected switch (simpler wiring).

• Often used for detecting objects on conveyors or packaging lines.

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3-Wire Photoeye (ZS)

• Optical sensor with separate power, ground, and output wire.

• Allows sourcing/sinking logic and faster, more reliable detection.

• Most common photoeye wiring type in modern systems.

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Pilot Light (PL)

• Provides visual indication of machine or circuit status.

• Commonly used for “Power On,” “Fault,” “Running,” etc.

• Improves operator awareness and troubleshooting.

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2-Wire Proximity Switch (PRS), Schematic

• Non-contact sensor that detects presence of a target (metal for inductive, anything for capacitive).

• Two-wire version wired in series with the load, similar to a mechanical switch.

• Used for end stops, object detection, and position sensing.

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3-Wire Proximity Switch (PRS)

• Non-contact sensor with separate power, ground, and output.

• Offers faster response and compatibility with PLC input cards.

• More flexible than 2-wire for industrial automation.

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Pressure Switch (PS)

• Detects when fluid/air pressure reaches a setpoint.

• Provides an electrical signal for control (e.g., start a compressor at low pressure, stop at high).

• Common in pneumatics, hydraulics, and process control.

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Shielded Cable, 2-Wire

• Two-conductor cable with a conductive shield layer.

• Reduces electrical noise and interference (important for sensors and analog signals).

• Used where signal integrity is critical.

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Selector Switch (SS), 2-Position

• Operator control switch with two maintained positions (e.g., On–Off).

• Often used for manual/auto modes or start/stop functions.

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Selector Switch (SS), 3-Position

• Operator control switch with three maintained positions (e.g., Forward–Off–Reverse).

• Provides more control options from one device.

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Selector Switch (SS), 3-Position Spring Return to Center

• Similar to the 3-position selector, but automatically returns to the center (neutral) when released.
• Often used for jogging motors or momentary operations.

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Solenoid Valve (XCV)

• Electromagnetically actuated valve that controls fluid or air flow.
• Used to drive cylinders, actuators, or direct process flows.
• A critical “output” device in pneumatics/hydraulics.

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Temperature Switch (TS)

• Detects when temperature reaches a preset threshold.

• Provides on/off control for heaters, fans, or alarms.

• Common in HVAC, process heating, and safety shutdowns.

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