An RCD is an electrical safety device which will immediately shut off the electricity if it detects electricity that is "leaking" to earth which could harm someone using electrical equipment. They offer a high level of protection from electric shock and they also reduce the risk of fire by detecting electrical leakage in wiring and accessories (especially relevant in older installations).
They work by comparing the current flow in the Active (supply) and Neutral (return) conductors within an electrical circuit. If the current flow becomes unbalanced, e.g. from a bad heater element, some is leaking to earth. The RCD is designed to disconnect (trip) the electricity supply within 10 to 50 milliseconds if it senses a harmful leakage in the circuit, typically at 30 milliamps.
Be aware of where all your system’s reset buttons are and what they do. They are rectangular in shape with a ‘test’ & ‘reset’ button. If the RCD is tripped it interrupts the power to everything. If it won’t reset, or if it immediately trips, it is doing what it is designed to do: protecting you from a faulty component somewhere.
Heaters, blowers and ozone generators can be troublesome as they all have parts which can wear or fail, allowing water in; this should make the RCD trip immediately. RCDs are sensitive and can also be affected by lightning, heavy humidity or condensation. Try pushing the reset button to restore your spa it its usual working order. If you find that the RCD is tripping frequently or when a particular component is being used, it’s time to have a thorough check to try to determine the cause.