Nollajohdon

Nollajohdon, or neutral conductor, is a conductor in an electrical installation that carries current back to the source and is typically at or near earth potential during normal operation. It forms the return path for current in circuits that include a live (phase) conductor and a load. The neutral is distinct from the protective earth conductor, which provides a safety path for fault currents.

In many power systems the neutral is bonded to earth at the main service entry or at the transformer, establishing a reference potential for the installation. Different distribution schemes treat the relationship between neutral and earth in various ways: in TN-C-S and TN-C networks neutral and earth may be combined or bonded at certain points, while in TT systems the earth and neutral are kept separate within the installation.

The neutral conductor should be insulated and protected like other conductors. Circuit protection is typically provided on the live conductor, but the neutral also carries the same current as the live under normal operation. Although the neutral is near earth potential, it can present shock or equipment hazards if a fault or open-neutral condition occurs, or if grounding references are compromised.

Common issues include an open neutral leading to unstable voltages at outlets and malfunction of equipment, or improper operation of protective devices. Standards for installation and practices related to the neutral conductor are aligned with international and national codes, such as IEC 60364 and corresponding national adaptations.