[kcenicola]

In older houses, the Neutral and The ground wires are all tied to to the same terminal strip back at the main box. In newer houses the grounds are tied to a terminal that goes to a spike in the earth and the Neutral's go back to the telephone pole outside your house.

The problem with the older setup is that when people are wiring a 240V circuit and they only have 3 wires to connect to their device, they don't take the time to ground their device to the Neutral. So if there is ever a short circuit between one of the 120V hot legs and the device (oven) the oven becomes charged an becomes a shock hazard. If the oven is attached to the Neutral, if there is ever a short between the oven and one of the hot legs, the circuit breaker will trip.

When they changed 240V receptacles to have 4 wires, this forced people to attach the green ground wire to your device to protect against this.

KC