Bonding in Electrical Equipment
Bonding Ground faults cannot be avoided in distribution systems. For various reasons, ranging from equipment failure to the accidental intrusion of moisture into a piece of electrical equipment, there is a system in place to protect personnel from accidental electric shock.
Bonding is the permanent joining of metallic parts to form an electrically conductive path that has continuity and the ability to safely conduct electrical ground-fault current. Non-current-carrying parts of the electrical system include items such as motor frames, metal raceways, and metal boxes and enclosures. If any non-current-carrying device accidentally becomes energized due to a ground fault, proper bonding provides an electrically conductive path that can safely conduct fault current back to the source.
A bonding path must be sized properly to carry all fault current that can be present during a ground fault. When impedance along the bonding path is low (as is required), sufficient current can travel back to the power source and the overcurrent protective device (fuse or circuit breaker) operates to de- energize the circuit and clear the fault.
A bonding jumper is installed to provide electrical conductivity between metallic devices of an electrical system. A bonding jumper is often a bare copper conductor, or can be made of green insulated copper conductor. In some applications, a bonding jumper is made as a solid metal strap or as a specially designed screw. Bonding jumpers can be installed at any location throughout the distribution system.
A main bonding jumper (MBJ) is a connection for electrical service equipment that ties together the equipment grounding conductor (EGC), the grounded conductor, and the grounding electrode conductor (GEC). The MBJ is used only at the service and is the connection between the grounded conductor and the EGC. See Figure 1.
An equipment grounding conductor (EGC) is an electrical conductor used to connect non-current-carrying metal parts of equipment or enclosures to the system grounded conductor or grounding electrode. An EGC connects the non-current-carrying metal parts of electrical equipment and systems back to the source of the power distribution supply. The power source is considered to be the service equipment when supplied from a utility or the source of a separately derived system, such as a transformer or generator. Should any non-current-carrying devices accidentally become energized as during a ground fault, an EGC can safely carry the fault current to its desired destination at the power source.
An equipment bonding jumper (EBJ) is a conductor that connects two or more parts of the equipment grounding conductor. A common application for an EBJ is the routing through FMC or LFMC to provide electrical continuity of the bonding path and prevent electrical shock. Improper bonding can result in overheating of systems and equipment, causing a fire hazard. If ground fault current encounters too much resistance or impedance along its return path to the source, not enough current may be present to operate the overcurrent protective device. Not only does the electrical shock hazard remain, but heat can develop at that point of high resistance. As ground fault current continues to flow, heat can build up enough to result in fire.
An EGC can be a conductor that is routed with circuit conductors or can be a conduit enclosing the circuit conductors. See Figure 2. Devices that can be used as an EGC include the following:
- Copper, aluminum, or copper-clad aluminum conductors
- FMC and LFMC
- Type AC cable armor
- Copper sheath (from type MI, metal- sheathed cable)
- Type MC Cable
- Cable trays
- Cable bus framework
- Other electrically continuous metal raceways and auxiliary gutters listed for grounding
Although the NEC allows the use of flexible conduit, under certain restrictive conditions, to be used as an EGC, they are usually not used as an EGC in most installations. A separate EGC must be routed with flexible conduit.
Copper conductors are commonly installed as an EGC and can be bare, covered, or insulated. In most commercial and industrial applications, insulated conductors are used. The color of the insulation is restricted when copper conductors are used for EGCs. Insulation must be colored green, or green with one or more yellow stripes. If an EGC is larger than 6 AWG, black-colored insulation may be used, and it is identified as an EGC at both ends and wherever accessible when it is installed. Identification is typically accomplished by wrapping green tape around the ends of the conductor insulation.
Sizing Conductors for Grounding and Bonding
When a grounded conductor, bonding jumper, or an EGC is installed, care must be taken that it meets minimum sizing requirements. Under all conditions, conductors installed for protection must be able to carry the amount of current that can travel in the system during fault conditions.
Conductor size requirements are located in different locations in the NEC. The minimum-size EGC for grounding raceways and equipment is based on the rating or setting of the automatic overcurrent device in the circuit ahead of the equipment. For example, equipment with an overcurrent device ahead of the equipment rated at 20A requires an EGC of 12 AWG copper. The NEC does not permit the use of conductors smaller than 14 AWG for copper or 12 AWG for aluminum or copper-clad aluminum for wiring in applications up to 2000 V.