The black lines on a fuse box wiring diagram for a house represent hot or live wires. Hot wires can shock a person who touches them before shutting off power to the circuit. In a wiring diagram for a vehicle, the black solid lines represent the wiring that forms the circuit.
Electricity from the power company flows into a home through the fuse or breaker box. The electricity gets distributed through a pair of main power lines that emerge from the center of the main breaker. One wire is red and the other forms the first black line of the wiring diagram. Main breaker wires deliver live current from the fuse box to appliances and lights connected to the circuit.
Breaker wires carry live current that is isolated from the earth and grounded by the transformer terminals from the power company. If an alternative pathway, one that is similarly connected to the earth, touches a live wire, then the intervening pathway provides a secondary conduit for the electricity to flow from the power source to the earth. When the alternative pathway is a provided by a person, the electricity causes the person to experience a shock.
Black lines that form a zigzag pattern on the wiring diagram represent a light or appliance that is connected the electric circuit.