Final answer:
The potential difference between the two bus bars in a 120/240 V three-wire system is 240 volts. Safety features such as grounding and circuit breakers provide protection against electrical hazards.
Step-by-step explanation:
The potential difference between the two bus bars in a three-wire system providing a 120/240 V service is 240 volts. This is because in the three-wire system, the neutral wire is grounded, effectively making it zero volts. The two hot wires each provide 120 volts relative to the neutral, but because they are 180 degrees out of phase with each other, the potential difference between the hot wires (the bus bars) is doubled to 240 volts. This allows for providing both 120 V for standard household appliances and 240 V for heavy-duty appliances that require more power. Three-wire systems have several safety features, including a circuit breaker or fuse to protect against thermal overload and grounding of the neutral wire and appliance cases to prevent electric shocks. The use of three-wire systems with grounding connections ensures a safer operating environment by providing a low-resistance path for excess current to dissipate into the earth.