What safety equipment should always be included in home-built equipment that is powered from 120V AC power circuits?

Including a fuse or circuit breaker in series with the AC "hot" conductor is essential for safety when building equipment powered from 120V AC circuits. This safety measure protects both the equipment and the user by interrupting the power supply in case of overcurrent conditions, such as short circuits or equipment malfunctions. If a fault occurs, the fuse will blow or the circuit breaker will trip, cutting off the electrical supply and preventing potential hazards like electrical fires or equipment damage.

The other options, while they may have specific uses in electrical systems, do not provide necessary overcurrent protection. An AC voltmeter across the incoming power source is useful for measuring voltage but does not contribute to safety in the event of an overload. An inductor in series with the AC power source could impact the current flow in some applications, but it does not offer protection against overcurrent. Similarly, a capacitor across the AC power source may filter signals or provide power factor correction but does nothing to safeguard the circuit from excessive current.