In North America at least, in the box where the service first enters the house, the equipment grounding conductors are connected to the neutral wire. In this case that box is the main breaker panel.

Remember that there will typically be at least four wires to deal with—line, load, neutral, and ground—and that there will also be a traveler wire if you’re dealing with a multi-pole circuit.

Neutral wire Copper wire coated with blue plastic that ... a path for current to flow from the case of the device to the ground if there is a fault A fuse provides a built-in fail-safe to the ...