What is a dry contact?

A normal smart relay turns power on and off to whatever is wired into it. A light, a fan, a heater. The electricity runs through the relay and out to the appliance.

A dry contact does not do that. It sends no power anywhere. It closes a gap in a circuit that something else is powering, then stays out of the way. That sounds like a small distinction. It is the difference between a relay that can open your garage door and one that cannot.

The easiest way to picture it

Think of the doorbell button by your front door. Your finger supplies no electricity. The doorbell has its own transformer doing that. Pressing the button just joins two contacts together and lets the doorbell's own circuit complete.

A dry contact is a remote-controlled finger. It presses a button that is already there, on a circuit that already has its own power supply. That is the whole idea.

What the terminals tell you

A Shelly 1 keeps its power terminals and its switching terminals apart, which shows the idea better than any description.

L and N are the mains supply. They power the module itself, so it can run its radio and remember its settings.

I and O are the dry contact. They are a separate pair with no internal connection to L and N. The circuit you wire into I comes back out of O whenever the relay closes, and it runs on its own supply the whole time. The module contributes nothing to it but the closure. There is a third terminal, SW, where a physical wall switch connects, which is a different job again.

That separation is the feature. Shelly's spec sheets call it potential-free. Other manufacturers call it volt-free, or a dry contact. Three names, one pair of terminals.

Why the other kind exists

Plenty of smart relays have no dry contact, and that is a trade rather than a cheaper build.

If a relay measures how much power an appliance is drawing, the current has to pass through the relay to be measured. That generally ties the output to the incoming live wire, so what you get is a switched 240 V output instead of a free pair of contacts. You gain the energy readings and lose the separation.

Which Shelly relays have one

Across the in-wall range the naming does the work for you: a model with PM after the number measures power and switches live. The DIN rail Pro models are the exception, because there is room in a switchboard enclosure to do both. That pattern holds across Shelly's Gen 3, Gen 4 and Pro lines, so an unfamiliar model number usually tells you what you are getting before you open the spec sheet. If you would rather not decode it, the Shelly picker asks what the job is and names the model.

What people actually use it for

Anything that already has its own controller and its own power supply, and simply wants a button pressed or a pair of contacts held shut.

• Garage door openers and gate motors, which expect a brief press
• Electric door strikes on a gate or an intercom
• Ducted or hydronic heating with a call-for-heat input
• A contactor coil for a hot water system or pool pump, where the load itself is too big for the relay

Irrigation is the common exception. Standard garden solenoids want 24 V AC held on for the whole watering period, supplied by a transformer, and a purpose-built irrigation controller does that job better than a relay and a transformer wired together.

The garage door, in practice

This is the job most people buy a dry contact for. The wiring is simple. The automation around it is where the care goes.

A single-button opener cycles through open, stop and close. Press it mid-travel and the door stops. Press again and it reverses. The relay has no idea which of those it just did, because all it ever sent was a pulse. Counting commands is not the same as knowing where the door is, which is why that sensor matters.

Closing a garage door from your phone when you cannot see it is a different question from opening one. The opener needs its own obstruction detection for that to be safe. AS/NZS 60335.2.95, the standard covering garage door drives, was amended to clarify that monitored safety beams are needed where an opener is smart-controlled or runs unattended, and that amendment took effect on 30 November 2020. Ask your installer what your opener has before you automate it to close on its own.

Three things people get wrong

Confusing the input with the output. A dry contact output hands another device a switch to read. A dry contact input is the reverse: the relay reads a switch that something else provides, such as a reed sensor or a thermostat. Same words, opposite direction, different terminals.

Assuming low voltage makes it a DIY job. The circuit you are switching might only run at 12 V, but the module is powered from the mains and sits in an enclosure with mains terminals in it. Running extra-low-voltage and mains wiring into the same enclosure has rules of its own about insulation and separation.

Feeding voltage into something that asked for a dry contact. If a heater or controller wants a volt-free input, it already has its own sensing voltage on those terminals, and sending 12 V or 240 V into it can destroy the board. The reverse bites too: a contact rated for 16 A of mains can go intermittent when all it has to switch is a few milliamps of signal, because there is not enough current to keep the contact surfaces clean.

Who can install it

If the work involves fixed 240 V wiring, a switchboard, a hard-wired appliance, or opening an enclosure with mains terminals inside it, it is licensed electrical work in every Australian state and territory. Nearly every dry contact install starts by putting a module somewhere that mains is present, so budget for an electrician as part of the job rather than as a fallback.