250.80 Raceways and Enclosures Service Metal enclosures and raceways for service conductors and equipment must be linked to the grounded system conductor if the electrical system is grounded, or to the grounding electrode conductor for ungrounding electrical systems. Service metal parts of these structures must be electrically continuous from within the building to the exterior world with no open circuits or discontinuities.
Service connections to buildings should always be made in accordance with local code requirements. In some cases, these services may be made accessible from the outside of the building through a meter pit. In other cases, they may be made accessible from within the building via a special access panel. The method used to connect service conductors to these buildings is called a "raceway" or "enclosure". The term "raceway" will be used hereinafter to refer to both types of installations. A service metal enclosure is required for all branch circuits which supply 120-volt loads. These branches must be confined to small areas such as rooms or floors. They cannot be left unattended or exposed to weather conditions.
A service metal enclosure provides protection for wiring within it by preventing contact with live parts of the circuit and providing an electrical path back to the building's main service panel. An opening is provided at the base of the enclosure for connecting wires from the interior wiring network to its terminals.
Metal components located in a nonmetallic raceway run and isolated from possible contact by a minimum cover of 450 mm (18 in.) on all metal components are not needed to be linked to the grounded system conductor, supply side bonding jumper, or grounding electrode conductor. The only requirement is that they be connected to the same circuit as other metal components within the raceway.
Any metal component inside the metallic raceway can be linked to the system conductors if this will help prevent noise from being transmitted into the protected environment. For example, if there is a magnetic switch installed in the metallic portion of the raceway, it can be linked to one of the system conductors. This would be done so that when power is applied, the magnetic switch closes thereby turning off the circuit through which it is attached to. When used with fiber optics, magnetic switches can also be found on junction boxes located in the premises wiring closet. These magnetic switches control the connection of circuits feeding into and out of distribution centers or equipment racks mounted in the closet. They allow maintenance on these circuits without opening up walls or ceilings.
Magnetic switches are available in both mechanical and solid-state versions. Mechanical switches operate based on the movement of magnets relative to each other. This movement can be triggered by any number of factors such as light beams hitting their corresponding switches, fibers crossing their respective switches etc.
To guarantee effective electrical continuity, metal raceways, cable armor, and other metal conductor enclosures must be metallically bonded together to produce a continuous electrical conductor and must be connected to all boxes, fittings, and cabinets. Bonding can be done by means of solder, hot-melt adhesive, or mechanical fasteners.
The bond between the metal parts should be tight enough to prevent conductive moisture from reaching internal contact points where current might cause corrosion or other damage.
Bonds should also be loose enough to allow for some movement or expansion/contraction of the metal parts with respect to one another. For example, if cables are routed through walls, they should be able to move slightly so that there is no stress on the connection due to changes in temperature or other factors.
Finally, bonds should be strong enough to hold up under normal use conditions. For example, if someone were to repeatedly push on the metal parts of an enclosure, it would be possible for this pressure to weaken or break the bond between them.
Corrosion is the main concern when using metal for electrical connections. Corrosion will cause connections to fail over time. It is important, therefore, that any metal parts that are exposed to electricity are kept free of contamination by dirt or other substances.
The equipment grounding conductors must be connected to the service-equipment enclosures, and the grounded service conductor must be connected to a grounding electrode where the system is grounded. The "Grounding Electrode Conductor" is the conductor utilized to create this connection. It can be a metal rod set into the ground or an earth wire that is attached to a suitable point on the enclosure.
For systems without a grounded service conductor, such as single-phase systems, the equipment grounding conductors can be used as the only means of electrical connection to the system. In this case, each piece of equipment must have its own dedicated equipment grounding conductor. The term "dedicated" means that the conductor cannot be used for another purpose besides grounding. For example, a wiring diagram might show one side of a transformer being tied to the other side of the transformer with a cable, but if the other side of the transformer was supposed to be grounded then it would have to be done so with a dedicated equipment grounding conductor.
Dedicated equipment grounding conductors are required by law in some jurisdictions for specific types of equipment. These include appliances such as air conditioners, heat pumps, and freezers which use electric motors as well as tools such as drills and power mowers which have voltage levels applied to them while they are operating.
General-A connection to the grounding electrode conductor shall be made at any accessible point from the load end of the overhead service conductors, service drops, underground service conductors, or laterals, including the terminal or bus to which the grounded service conductor is connected at the service disconnecting...
National Electrical Code 2008 - Article 100 - Section 100-6. Grounding Electrode Conductors.
Grounding electrode conductors shall be used in conjunction with protective equipment for all abnormal conditions that could affect the ground system. They should be placed in accordance with their size so that the least number of conductors are necessary to provide effective ground protection. The person who installs the equipment shall not connect a ground conductor to an earthplate or other bonding point until the appropriate test procedures have been followed. Bonding points for ground conductors shall be indicated by markings on the wiring method used to install the conductors. A ground fault circuit interrupter (GFCI) installed in accordance with Article 210 shall serve as the required protective device for ground conductors.
The ground conductor shall be sized based on the expected ground current. If the ground conductor is larger than necessary, it will reduce the life of the conductor. Also, if the ground conductor is not long enough, it may not be able to carry the expected ground current and therefore place an additional burden on other conductors in the system.