Like a bypass, an RC snubber has nothing to configure and no polarity. Unlike a bypass, its physical position is everything: the same component fitted well solves the problem, and fitted badly creates a new one.
If you are still diagnosing, read why the device restarts when a motor switches off first. This one is the fitting.
The rule: as close to the inductive load as you can get
Not at the consumer unit. Not next to the device. Next to the motor, the fan, the pump or the contactor coil that causes the spike.
The reason is straightforward: the voltage spike is born in the coil at the instant the current is broken. Damp it right there and it never propagates through the installation. Every metre of cable you leave between the coil and the snubber is a stretch the transient has already travelled — and along which it has had the chance to couple into whatever runs beside it.
This is the point most often overlooked, and it is the one that decides whether the fix works.
One snubber per load, not per device
What you are damping is the motor, not the relay. If a two-channel device drives two motors, you need two snubbers, one beside each motor.
Step by step
- Cut the power at the breaker and confirm it is dead with a multimeter.
- Find the load terminals. Usually the terminal box on the motor, the fan or the contactor itself.
- Wire it in parallel with the load, between the relay output and the neutral that feed the motor. No polarity — the order does not matter.
- Insulate the connections with heat-shrink or good-quality insulating tape. The terminals sit at mains voltage.
- Secure it. It must not dangle or sit within reach of moving parts. A motor terminal box has more than enough vibration to work a loose wire free over time.
- Restore power and test. Operate the load several times in a row. A single cycle proves nothing: the fault was intermittent by nature.
The mistake to avoid
Do not wire it across the relay contacts. It looks reasonable and it turns up on forums, but it puts the snubber in series with the load: it passes a permanent leakage current of around 7 mA even with the relay open.
With a motor you will not notice. With an LED fitting on the same circuit you will: those are exactly the milliamps that produce phantom glow or flicker. You will have traded one fault for another.
In parallel with the load. Always.
Typical specifications
- Capacitor 0.1 µF
- Resistor 100 Ω
- Dissipation ½ W
- Working voltage 600 V AC
One very common concrete case: if the device drives a contactor coil, that coil is the inductive load and the A1-A2 terminals are the spot. The full arrangement is in when to replace the relay with a contactor.
That 600 V margin on a 230 V supply is not overkill: the transient you are damping is precisely a short-lived overvoltage well above nominal.
If you cannot reach the motor
It happens: built-in motors, pumps in a chamber, loads with no accessible terminal box. Fit it as close as you can get to the load end of the circuit.
It works partially. It may be enough for the restarts to disappear, or it may reduce them without eliminating them. Worth trying before writing the problem off as unfixable — but worth knowing in advance that effectiveness falls off with distance.
Checking it without standing there watching
The symptom you are eliminating is a brief dropout. Confirming it by eye means being present, operating the load and watching whether the device responds.
The console makes it easier: the device’s connection history records the dropouts. Operate the load a few times, look for new gaps, and you have your answer without timing anything by hand. It is the difference between “I think it has stopped restarting” and knowing.
Maintenance
It is a passive component, but it lives bolted to a motor: heat, vibration and years. Worth inspecting whenever you are in that installation for any other reason — that it is still properly secured, that the insulation is intact, and that the terminals have not worked loose.
Installation by a qualified person. Working inside a motor terminal box is not the same as changing a bulb; if in doubt, have a registered electrician do it.