Electrical equipment should always be properly connected to an electrically neutral source. What is this source called

Earth or ground

Explanation:

Earth is the reference point in an electrical circuit from which voltages are measured, it connects the house's electrical system physically to the ground.  It functions in a house to ensures that exposed electrical parts do not have a dangerous voltage relative to the Earth, which could cause an electric shock if a grounded person touched them, and also to make sure that the failures of internal insulation triggers protective mechanisms such as fuses or circuit breakers in the circuit to remove power from the device.