1. In ac power distribution, the conductor that (a) is intentionally grounded on the supply side of the service disconnect and (b) provides a current return path for ac power currents. 2. In three-phase ac "Y," i.e., wye, power distribution, the low-potential fourth wire that conducts only that current required to achieve electrical balance, i.e., to provide a return path for any current imbalance among the three phases.