Ground and neutral
In electrical engineering, ground and neutral (earth and neutral) are circuit conductors used in alternating current (AC) electrical systems. The neutral conductor returns current to the supply. To limit the effects of leakage current from higher-voltage systems, the neutral conductor is often connected to earth ground at the point of supply. A ground conductor is not intended to carry current for normal operation of the circuit, but instead connects exposed metallic components (such as equipment enclosures or conduits enclosing wiring) to earth ground. A ground conductor only carries significant current if there is a circuit fault that would otherwise energize exposed conductive parts and present a shock hazard. Circuit protection devices may detect a fault to a grounded metal enclosure and automatically de-energize the circuit, or may provide a warning of a ground fault.
For uses of the term "grounding" or "earth" in electricity but not in the context of mains wiring, see ground (electricity).Under certain conditions, a conductor used to connect to a system neutral is also used for grounding (earthing) of equipment and structures. Current carried on a grounding conductor can result in objectionable or dangerous voltages appearing on equipment enclosures, so the installation of grounding conductors and neutral conductors is carefully defined in electrical regulations. Where a neutral conductor is used also to connect equipment enclosures to earth, care must be taken that the neutral conductor never rises to a high voltage with respect to local ground.
Definitions[edit]
Ground or earth in a mains (AC power) electrical wiring system is a conductor that provides a low-impedance path to the earth to prevent hazardous voltages from appearing on equipment (high voltage spikes). The terms ground and earth are used synonymously in this section; ground is more common in North American English, and earth is more common in British English. Under normal conditions, a grounding conductor does not carry current. Grounding is also an integral path for home wiring because it causes circuit breakers to trip more quickly (ie, GFCI), which is safer. Adding new grounds requires a qualified electrician with knowledge particular to a power distribution region.
Neutral is a circuit conductor that normally completes the circuit back to the source. NEC states that the neutral and ground wires should be connected at the neutral point of the transformer or generator, or otherwise some "system neutral point" but not anywhere else.[1] That is for simple single panel installations; for multiple panels the situation is more complex.
In a polyphase (usually three-phase) AC system, the neutral conductor is intended to have similar voltages to each of the other circuit conductors, but may carry very little current if the phases are balanced.
All neutral wires of the same earthed (grounded) electrical system should have the same electrical potential, because they are all connected through the system ground. Neutral conductors are usually insulated for the same voltage as the line conductors, with interesting exceptions.[2]
Circuitry[edit]
Neutral wires are usually connected at a neutral bus within panelboards or switchboards, and are "bonded" to earth ground at either the electrical service entrance, or at transformers within the system. For electrical installations with split-phase (three-wire single-phase) service, the neutral point of the system is at the center-tap on the secondary side of the service transformer. For larger electrical installations, such as those with polyphase service, the neutral point is usually at the common connection on the secondary side of delta/wye connected transformers. Other arrangements of polyphase transformers may result in no neutral point, and no neutral conductors.
The IEC standard (IEC 60364) codifies methods of installing neutral and ground conductors in a building, where these earthing systems are designated with letter symbols. The letter symbols are common in countries using IEC standards, but North American practices rarely refer to the IEC symbols. The differences are that the conductors may be separate over their entire run from equipment to earth ground, or may be combined all or part of their length. Different systems are used to minimize the voltage difference between neutral and local earth ground. Current flowing in a grounding conductor will produce a voltage drop along the conductor, and grounding systems seek to ensure this voltage does not reach unsafe levels.
In the TN-S system, separate neutral and protective earth conductors are installed between the equipment and the source of supply (generator or electric utility transformer). Normal circuit currents flow only in the neutral, and the protective earth conductor bonds all equipment cases to earth to intercept any leakage current due to insulation failure. The neutral conductor is connected to earth at the building point of supply, but no common path to ground exists for circuit current and the protective conductor.
In the TN-C system, a common conductor provides both the neutral and protective grounding. The neutral conductor is connected to earth ground at the point of supply, and equipment cases are connected to the neutral. The danger exists that a broken neutral connection will allow all the equipment cases to rise to a dangerous voltage if any leakage or insulation fault exists in any equipment. This can be mitigated with special cables but the cost is then higher.
In the TN-C-S system, each piece of electrical equipment has both a protective ground connection to its case, and a neutral connection. These are all brought back to some common point in the building system, and a common connection is then made from that point back to the source of supply and to the earth.
In a TT system, no lengthy common protective ground conductor is used, instead each article of electrical equipment (or building distribution system) has its own connection to earth ground.
Indian CEAR, Rule 41, makes the following provisions:
Grounding problems[edit]
A ground connection that is missing or of inadequate capacity may not provide the protective functions as intended during a fault in the connected equipment. Extra connections between ground and circuit neutral may result in circulating current in the ground path, stray current introduced in the earth or in a structure, and stray voltage. Extra ground connections on a neutral conductor may bypass the protection provided by a ground-fault circuit interrupter. Signal circuits that rely on a ground connection will not function or will have erratic function if the ground connection is missing.