dry contact wiring diagram
This is a free printable dry contact wiring diagram: download the diagram as SVG or open it and print to paper or PDF.
A dry contact wiring diagram configuration establishes reliable control over complex electrical systems through strategic use of contactors and distributed protection. The circuit topology separates control signals from power paths, allowing low-voltage logic to manage high-power loads safely. Protective devices are rated and coordinated to ensure selective tripping during fault conditions, preventing cascade failures across unaffected branches. Component tolerance analysis ensures predictable operation across manufacturing variations and environmental temperature ranges. The ground distribution uses star topology to minimize return-path impedance and reduce voltage distortion.
How to wire dry contact wiring diagram
- Identify the dry contact source device Determine which device provides the dry contact — a relay, contactor auxiliary contact, thermostat, flow switch, or pressure switch. Check the device datasheet for contact voltage and current ratings before wiring.
- Identify the receiving device input type Check whether the BMS, PLC, or controller input is voltage-free (it supplies its own sense voltage) or requires an external voltage. Wiring a sense voltage into a voltage-free input shorts the input and damages the controller.
- Run signal cable between the two devices Use screened twisted-pair cable for signal runs over 5 m to reject electrical interference. Earth the screen at one end only to avoid ground loops. Label both cable ends with the circuit reference before pulling.
- Terminate at the dry contact device Connect the two signal wires to the COM and NO or NC terminals of the relay or switch as required by the control logic. Normally open triggers on close; normally closed triggers on open. Choose based on your fail-safe requirement.
- Terminate at the receiving device Connect the wires to the digital input terminal and the common (COM) or return terminal of the controller. Refer to the controller wiring diagram and confirm terminal numbers match your drawings before applying power.
- Test the contact operation Use a multimeter on continuity or a DVM to confirm the contact opens and closes as expected when the source device activates. Confirm the controller input registers the correct state change in the software or indicator LED.
Frequently asked questions
What is a dry contact in wiring?
A dry contact is a relay or switch contact that has no internal voltage source. It simply opens or closes a circuit — the connected equipment provides the voltage. This isolates two circuits electrically while still allowing one to trigger the other.
What is the difference between a dry contact and a wet contact?
A wet contact has a voltage applied to it by the device that owns the contact. A dry contact carries no voltage of its own — the external circuit provides it. Building automation, alarm panels, and PLCs almost always use dry contacts for inter-device signalling.
What voltage can I run through a dry contact?
Dry contacts are rated by their switching current and voltage capacity. Typical relay dry contacts handle 250 V AC at 10 A or 30 V DC at 5 A. Always check the relay or device datasheet. Running excessive voltage damages the contact and creates a fire risk.
How do I wire a dry contact to a building management system input?
Connect one wire from the BMS digital input terminal to one side of the dry contact, and wire the BMS common (COM) terminal to the other side. The BMS supplies a small sense voltage through the contact. When the contact closes, the BMS registers a change of state.
Can I use a dry contact output to switch a 220 V load directly?
Only if the dry contact is rated for 220 V at the required current. Most BMS and alarm dry contact outputs are rated for low-voltage signal levels only. To switch a mains load, use the dry contact to trigger an interposing relay rated for 220 V.
Related diagrams
- 0 10v led dimming wiring diagram
- 1 4 stereo jack wiring diagram
- 1 gang 1 way switch wiring diagram
- 1 gang 2 way switch wiring diagram
- 1 ohm sub wiring diagram
- 1 ohm wiring diagram