Immersion heaters are 3kW heaters that require ring main wiring. Ring main wiring gives the immersion heater the circuit capacity it requires. Immersion heaters don't shut off. The mains cable on an immersion heater has three wires: hot, neutral and ground. The connector cable runs from the immersion heater to a 20 amp double pole isolator switch with a neon red indicator light. It is important that heat resistance cable runs directly from the 20 amp switch to the immersion heater for safety purposes. Such cables have a code specified overheating safety cut out.
Shut off the main circuit breaker. Test with a voltmeter the breaker wires by touching the tip of the probe onto the wires of the circuit. If there is no OHM reading then the electricity is off.
Check to make sure the fuse in the switch is also off. The best way to make sure the switch is off is to pull the fuse in the fuse box. You can pull the fuse with your fingers.
Insert the thermostat rod into the immersion tank and attach the probe wire and the thermostat wire with tiny retaining screws to the rod and base of the tank. Use a Phillips screwdriver.
Squeeze the thermostat and the probe wire into a cable clamp and attach it to the base of the thermostat housing with two Phillips head screws. Replace the thermostat cover lid.
Run the flex cable from the thermostat to the 20 amp double pole isolator switch and attach the black, white and copper wires. The copper wire wraps around the green screw and the back and white each attach to the grey screws in the switch. Reattach the outlet plate for the switch. Turn the main circuit breaker on and test the immersion heater.
For insurance purposes it is best to have a licensed electrician inspect your wiring work.
The thermostat rod needs to be covered in water at all times. Overheating can cause a fire.