Direct current circuits that have the negative pole of the power supply connected to the ground source are negatively grounded electrical systems. Typically the positive pole connects to one or more devices on the circuit, which have their negative terminals connected to the same ground source to complete the circuit.
A common application of this grounding technique is found in most automobiles where the vehicle's 12-volt battery is the DC power source. The usual configuration has the positive pole connected to all the devices that need electrical power to operate, including the ignition system, fuel pump, electronic fuel injection system, climate control system and lighting. The negative battery pole is connected to the car's frame, and each element that is connected to the positive pole has its negative terminal connected to the frame. DC power is a unidirectional current, so electricity flows out of the positive pole and returns through the vehicle frame to complete the connection.
Alternating current systems ground to the earth, so they need three connections instead of two in order to complete a grounded system. Typical 120-volt home electrical connections have a hot, a neutral and a ground wire. The ground wire connects to a separate ground source that runs into the earth, such as a water pipe or a ground rod. The hot and neutral wires connect at the home's circuit breaker or fuse box to complete the circuit.