Lesson 5: Dry Contacts and Relays
Understand voltage-free contacts, N/O, N/C and relay-controlled devices.
Plain-English explanation
This lesson explains the idea in practical wiring terms, with a focus on CCTV, alarms, intercoms, access control, PoE and low-voltage systems.
What to look for on a diagram
- Which device starts the circuit or data path.
- Which cable type is shown.
- Whether the line is power, data, signal or relay control.
- Where the field/device side ends and the cabinet/control side begins.
- What needs to be recorded in the as-built notes.
Practice task
Open a related diagram and identify every power path, data path and control path. Then write down what cable type is used for each.