You have a Shelly 1PM Gen4 controlling an extractor fan. Every time you switch it off, the device resets: it disappears from the app for a few seconds and comes back. Or you hear a sharp click in the relay that wasn't there at first.
It's not a fault of the Shelly or the installation. It's the physics of inductive loads, and there's an accessory designed for exactly that.
What happens when you cut a coil
A motor, a transformer or a contactor coil store energy in the form of a magnetic field while current flows. When the relay opens suddenly, that energy has to go somewhere.
The coil resists the cut by generating a back-EMF (counter-electromotive force): a voltage spike that can reach hundreds of volts for microseconds. That spike is brief, but enough to cause four different problems.
The four symptoms
- Device resets. The spike momentarily disturbs the Shelly's power supply and it resets. It's the most frequent and most baffling symptom, because it always happens right when you switch off.
- Electric arc between contacts. As they separate, a spark jumps. Over time it erodes the relay material.
- Welded contacts. The repeated arc can end up fusing the contacts together. The relay stays stuck and the load doesn't switch off even though the app says it's off.
- Electromagnetic interference. The spike generates noise that can affect nearby electronic equipment.
The third is the most serious: a welded relay is a safety fault, not a nuisance.
Which loads cause it
Shelly lists the RC Snubber's usual applications in its documentation:
- Fans and extractors
- Fridges and cold rooms
- Air conditioning
- Washing machines and dryers
- Transformers
- LED lighting drivers
- Any equipment with an electric motor
LED drivers surprise a lot of people. The snubber is associated with motors, but a driver also has a coil and can cause the same problems.
The solution: the RC Snubber
It's a 100 Ω resistor and a 0.1 µF capacitor connected in series, with a nominal voltage of 600 V AC. The assembly is fitted 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 between the contacts.
In normal operation it barely intervenes: its dissipation is half a watt and it only acts at the instant of the cut.
The mistake that ruins its effectiveness
Shelly states it expressly: the snubber must go as close as possible to the inductive load, not next to the Shelly.
The reason is that the spike is generated in the coil. If the snubber is in the panel, the spike travels the whole cabling before being dampened, and along the way it has already done its damage: interference and disturbance of the power supply. Dampening it at the source is much more effective.
The second mistake: connecting it to the relay
There's another way to fit a snubber in electronics: in parallel with the relay contacts instead of with the load. It's a valid topology in other contexts, and that's why many people apply it here out of habit.
In that position, the capacitor lets through about 7 mA permanently at 230 V, even with the relay open. The result is LED bulbs with a ghost glow and meters that record consumption with the circuit off.
Follow Shelly's guidance: in parallel with the load, between the relay output and the neutral.
Frequently asked questions
Do I need it whenever I control a motor?
No. Shelly recommends checking each device's manual. If your installation has been running for months without symptoms, there's no need to add it preventively. The symptoms in this article are the sign.
My relay has stuck, does the snubber fix it?
No. A welded contact is irreversible physical damage: the device has to be replaced. The snubber prevents it from happening again on the new unit.
Does it work for DC loads?
The RC Snubber is rated for 600 V AC. In DC circuits the usual protection against inductive spikes is a flyback diode, which is a different component.
How many do I need?
One for each inductive load you switch. If a Shelly 2PM Gen3 controls two motors, you'll need two.
Does it affect consumption measurement?
Installed correctly, in parallel with the load, it shouldn't. If you notice odd readings, check the causes in why a Shelly meter reads more than expected.
What if what I'm controlling is a contactor?
A contactor coil is a textbook inductive load, and Shelly recommends it especially for that case.
Related products and guides
- RC Snubber: 0.1 µF, 100 Ω, 600 V AC.
- How to install the RC Snubber step by step
- Snubber or bypass: which one you need
Not sure whether your case needs it? Tell us which Shelly you have and what it controls, and we'll confirm it. Technical support in English and shipping from Spain.
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