You have a Shelly controlling an extractor fan. Every time you switch it off, the device restarts: it vanishes from the app for a few seconds and comes back. Or you start hearing a sharp click from the relay that was not there at first.
This is not a defect in the device or a fault in the installation. It is the physics of inductive loads, and there is an accessory designed for exactly this.
What happens when you cut a coil
A motor, a transformer or a contactor coil stores energy in a magnetic field while current flows through it. When the relay opens abruptly, that energy has to go somewhere.
The coil resists the interruption by generating a back electromotive force: a voltage spike that can reach hundreds of volts for a few microseconds. Very brief, but quite enough to cause four distinct problems.
The four symptoms, from annoying to serious
- The device restarts. The spike disturbs its supply and it reboots. The most common symptom and the most puzzling, because it happens always just as you switch off — never on switch-on.
- Arcing between the contacts. As they part, a spark jumps and gradually erodes the material.
- Welded contacts. Repeated arcing can eventually fuse the contacts together. The relay sticks and the load will not switch off, however firmly the app insists that it has.
- Electromagnetic interference in nearby electronics.
The third one deserves a pause: a welded relay is not an inconvenience, it is a safety failure. A motor you cannot stop from anywhere, and an interface lying to you about it being off.
Which loads cause it
- Fans and extractors
- Fridges and cold rooms
- Air conditioning
- Washing machines and dryers
- Transformers
- LED lighting drivers
- Anything with an electric motor
LED drivers surprise everybody. People associate snubbers with motors, but a driver has a coil too and causes exactly the same problems. If you get restarts when switching off a bank of downlights, do not rule this out just because «there is no motor there».
What an RC snubber is
A 100 Ω resistor and a 0.1 µF capacitor in series, rated at 600 V AC. The pair goes in parallel with the inductive load.
When the relay opens, the capacitor absorbs the spike and the resistor dissipates that energy in a controlled way, instead of it discharging as an arc across the contacts. In normal running it barely participates: it dissipates half a watt and only acts at the instant of switching.
Mistake 1: putting it next to the Shelly
The snubber goes as close as possible to the inductive load, not beside the device controlling it.
The reason is that the spike is generated at the coil. Damp it at the board and the spike has already travelled the whole cable run before it meets the snubber — and along the way it has done its work: interference and a disturbed supply. Damping it at source is far more effective than damping it at the end.
Mistake 2: wiring it across the relay contacts
This one is subtler, and is made with the best of intentions. There is another classic snubber topology: across the relay contacts rather than the load. It is valid in other contexts, and anyone who knows it tends to apply it from habit.
Here it has one concrete consequence: in that position the capacitor passes about 7 mA permanently at 230 V, even with the relay open.
And what do 7 mA flowing through a supposedly dead circuit produce? Precisely what the other article describes: LED bulbs with a phantom glow, and meters recording consumption with everything switched off. You will have traded one problem for another.
The correct position is in parallel with the load, between the relay output and neutral.
Catching it before the relay welds
The restarts are the early stage; welded contacts are the late one. A good deal of time can pass between the two, and that is your window.
A device that reboots every time a motor starts or stops leaves a recognisable trail in the console: its last seen resets over and over, in a pattern that follows how the equipment is used. Spot that on an installation you have handed over and you have a snubber outstanding — and you know it before the client rings to say the extractor will not switch off.
Common questions
Do I need one whenever I switch a motor? No. If the installation has run for months with none of the symptoms, there is no need to add one just in case. The symptoms above are the signal.
My relay is already stuck — will a snubber fix it? No. A welded contact is irreversible physical damage: the device has to be replaced. The snubber stops it happening to the new one.
Does it work on DC? No. The RC snubber is rated for 600 V AC. On DC circuits the usual protection against inductive spikes is a flyback diode, which is a different component.
How many do I need? One per inductive load you switch. A device governing two motors needs two.
What if what I am switching is a contactor? A contactor coil is a textbook inductive load. It is one of the clearest cases.