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 gray screws in the switch.
Reattach the outlet plate for the switch. Turn the main circuit breaker on and test the immersion heater.