DIY

Last week I installed a 3 pole fan switch for the bathroom extractor to over ride the dodgy humidity sensor it has. It gets turned it on and off manually now. I have a replacement fan ready to install, but I need to cut out 3mm of glass to get it to fit. I am going to wait for the summer in case the worst happens and the glass shatters. This is why I could answer what you needed.

This is similar to the switch I used with a 25mm box to mount it. You can buy a metal mounting box, if you want to bury the fitting.

I prefer a fixed placement box over the loose cable of a rocker switch. Some of the rocker switches have poor cable retention systems, which can result in the cable coming loose over time. Moreso when using thicker cabling. I don't mind rockers for lighting, but for fixed hardware, its not my first choice, and tbf a rocker switch doesn't look very nice if it is on permanent display.

I wouldn't wire up a dimmer switch to a motor. It can cause the motor to stall between poles, which can result in a component becoming hot.

Low-voltage lowdown. For an electric motor, torque changes as the square of the voltage applied. A 10% increase in voltage, for example, will boost torque 21% (1.1×1.1=1.21). Conversely, at 90% of rated voltage, the motor suffers a 19% reduction in torque (0.9×0.9=0.81). A more severe undervoltage condition, 20% below rated value, would reduce the motors torque to only 64% of rating (0.8×0.8 =.64). The effects are a 156% overload and catastrophic failure.

If torque decreases below the torque required by the load, the motor would stall. At that point, the only product of the motor is heat. However, a 100-hp motor, with locked rotor, becomes a 500kW resistive heater.

Fig. 2. Insulation life is halved for every 10°C rise in temperature. In this example, insulation life drops from four years to less than six weeks for a 50°C change in temperature.

Excess heat is a problem for motors because insulation life is halved for every 10°C increase in temperature (Fig. 2). At the same time, the temperature of the winding will rise 10°C to 15°C for each 10% drop in voltage. That means the insulation life of a motor that operates on 10% lower voltage will decrease to only 50% to 75% of its expected life.

Undervoltage events that last long enough to increase winding temperature cause irrevocable damage to winding insulation and unexpected failures later. A sustained low-voltage event can damage the insulation system of every electric motor running during the event. The only motor that's safe during a prolonged undervoltage event is one that's turned off.

https://www.ecmweb.com/

As for how to wire, breaker switches do not have a positive and negative connections. They only either connect or break the circuit. Obviously the wiring has to lead to each corresponding cable (positive to positive).

The earth is critical on devices that have metal shrouds. The earth should be continuous. You can do that by either wiring both earth connections into one side of the switch so it is not ever broken, or by adding a wire cable connection and tucking it safely inside the mounting box..