They are two small, cheap accessories, and they are wired the same way: in parallel with the load. That is where the confusion comes from. But they solve opposite problems, and fitting the wrong one fixes nothing.
The short rule: a bypass when the load is too small; an RC snubber when the load is inductive.
By symptom
| Bypass | RC snubber | |
|---|---|---|
| The problem | The load is very small | The load is inductive |
| What you see | The LED flickers or glows when off; the Shelly drops off Wi-Fi when switched off; the lamp never quite comes on | The Shelly restarts on switch-off; clicks or sparks at the relay; the relay sticks; interference with other equipment |
| Where it happens | No-neutral devices (1L, 2L, Dimmer) under 20 W | Motors, fans, contactors, transformers, blinds |
If your symptom is in neither column, neither of them is likely to fix it.
When you need a bypass
A Shelly that works without neutral has to draw its power from somewhere, and it does so by letting a small current through the load itself. With an incandescent bulb nobody noticed. With a 7 W LED, that current is enough to make it glow or flicker with the circuit switched off.
Shelly sets the threshold at 20 W: below it, on a no-neutral installation, you need the bypass. To see which side you are on, add up the bulbs on the circuit: four 6 W LEDs are 24 W, above; three 5 W ones are 15 W, below.
With neutral you never need one: the Shelly powers itself and does not depend on the load.
When you need an RC snubber
Here the load is not small: it is inductive. Motors, fans, transformers, contactor coils, some LED drivers. When the relay opens, the coil resists the interruption and sends back a voltage spike that can reach hundreds of volts in microseconds. That spike is what restarts the Shelly, makes the contacts spark, or ends up welding them together.
The snubber is a 100 Ω resistor and a 0.1 µF capacitor in series, rated 600 V AC. It gives that energy a momentary path to discharge through without damaging anything.
There is no need to fit one to every motor just in case: if the installation works normally, you do not need it. Check the manual for the specific device.
Why they are not interchangeable
- The bypass adds a permanent consumption, under 4 W, so the device can power itself.
- The snubber consumes nothing in normal operation: it acts only at the instant of switch-off.
A bypass does not damp spikes, and a snubber does not supply the current a no-neutral device needs. Needing both at once is possible — a no-neutral device on a very small extractor motor — but rare.
If it is a bypass: which one
It depends neither on the bulb nor on the installation, but on which Shelly you are fitting. All three share the 20 W threshold and install the same way, in parallel with the load, inside the light fitting.
| Bypass | Bypass 1L/2L | Bypass Dimmer | |
|---|---|---|---|
| For | 1L and Dimmer 2 | 1L and 2L Gen3 | Dimmer Gen3 |
| Generation | Previous | Gen3 | Gen3 |
- Bypass for 1L/2L Gen3: keeps the no-neutral relay powered and on Wi-Fi with the light off. A 2L Gen3 running two circuits that are both under 20 W needs two: one per circuit.
- Bypass for Dimmer Gen3: on a dimmer the problem is worse — besides the glow, flicker while dimming and jumps in brightness at the low end. It steadies the circuit and widens the usable dimming range.
- The original Shelly Bypass: for the previous generation’s 1L and Dimmer 2. If you have just bought the device, it is almost certainly not yours.
Check your Shelly’s generation before ordering the bypass, and do not mix them: each is specified for its own device, and fitted to another it can misbehave.
Frequently asked questions
Can they make a meter show consumption with everything off?
The bypass can: it draws under 4 W permanently, and a meter sees it. The snubber should not, as long as it is in parallel with the load and not across the relay contacts.
Can they go in the consumer unit?
The bypass goes in the light fitting, next to the load. The snubber goes as close as possible to the inductive load: in the consumer unit it loses much of its effect.