Grounding and bonding in commercial and industrial systems

Grounding and bonding are among the most misunderstood parts of electrical work, and errors are dangerous: a missing bonding connection can leave metal enclosures energized during a fault, and an extra neutral-to-ground connection can send current through piping and cable shields. ELEC-130 covered grounding and bonding for dwellings. Commercial and industrial systems add larger services, several voltage systems created by transformers, standby generators, impedance-grounded systems that keep running after a ground fault, and large metal structures and piping systems that must be bonded. This lesson explains the purposes of grounding and bonding, the types of system grounding, service and separately derived system connections, sizing and installing bonding conductors, effective fault-current paths, grounding electrodes and testing.

Two different jobs: grounding and bonding

Grounding connects a system or equipment to the earth, through grounding electrodes. Its purposes are to limit the voltage imposed by lightning, line surges and accidental contact with higher-voltage lines, and to stabilize the system voltage to earth during normal operation. Earth is a poor conductor, so grounding does not clear faults.

Bonding connects non-current-carrying metal parts, such as enclosures, raceways, equipment frames and piping, together and to the system's grounded conductor at the source, forming a low-impedance path for fault current. When a phase conductor touches a bonded enclosure, fault current flows back to the source through the bonding path, high enough to trip the overcurrent device quickly. Bonding also keeps all touchable metal at nearly the same potential, so a person touching two metal objects is not exposed to a dangerous voltage between them.

In the NEC, Article 250 covers grounding and bonding: system grounding (Part II), grounding electrode systems (Part III), enclosure and raceway grounding, bonding (Part V), and equipment grounding conductors (Parts VI and VII), with sizes in Table 250.66 (grounding electrode conductors) and Table 250.122 (equipment grounding conductors). The NEC requires an "effective ground-fault current path" (250.4(A)(5)) and states that the earth shall not be considered an effective path.

The fault current path

For a ground fault to trip a breaker quickly, the fault current must be large compared with the breaker rating. The current is limited by the impedance of the whole loop: the source, the phase conductor up to the fault, and the bonding path back to the source. A long, small or poorly connected bonding path can limit the current so much that the breaker trips slowly or not at all, leaving enclosures energized.

System grounding types

How the supply system's neutral (or one conductor) is connected to earth determines how it behaves during a ground fault:

Solidly grounded wye systems are standard in commercial buildings because they supply line-to-neutral loads (347 V or 277 V lighting, 120 V receptacles) and clear ground faults quickly. In industrial processes where an unexpected shutdown is costly or dangerous, high-resistance grounding allows the plant to keep running after a single ground fault, while ground detection alarms and the fault is found and repaired before a second fault on another phase causes a trip. HRG systems cannot supply line-to-neutral loads, and they require ground fault detection, alarms and, at some voltages, procedures to locate and clear the first fault promptly. Ungrounded systems behave similarly but can experience dangerous transient overvoltages during intermittent arcing faults, which is why HRG has largely replaced them.

250.20 requires grounding of systems that supply line-to-neutral loads and of certain other systems; 250.21 permits ungrounded systems in specified cases with ground detectors; and 250.36 permits high-impedance grounded neutral systems from 480 V to 1000 V where qualified persons service the installation, ground detectors are installed, line-to-neutral loads are not served, and continuity of power is required.

The service connection

At the service, the system is grounded and bonded at one point. The grounded (neutral) conductor is connected to the grounding electrode system through the grounding conductor, and the main bonding jumper connects the neutral bus to the equipment bonding system and the enclosure. Downstream of the service, the neutral and the bonding conductors must be kept separate: neutrals on insulated neutral bars, bonding conductors on bonding bars connected to the enclosure. If the neutral were connected to the enclosure again at a downstream panel, normal neutral current would divide between the neutral conductor and the bonding paths (raceways, cable armor, piping), causing objectionable current, shock hazards, electromagnetic interference with data equipment, and confusing ground fault protection.

Separately derived systems

A separately derived system is a power source with no direct electrical connection to the supply conductors of another system, except through grounding and bonding. Dry-type transformers with separate primary and secondary windings (Lesson 4), and generators connected through transfer switches that switch the neutral, are the common examples. Each separately derived system must be grounded and bonded at its own source, just like a service:

• A system bonding jumper connects the secondary neutral to the transformer enclosure (or to the first disconnecting means), at one point only.
• A grounding conductor connects the neutral to a grounding electrode, which may be building steel or the building's grounding electrode system.
• A supply-side bonding jumper connects the transformer enclosure to the secondary panel enclosure where they are separate.
• Downstream of that point, neutrals and bonding conductors stay separate.

Generators illustrate the importance of this rule. If the transfer switch does not switch the neutral (a three-pole switch on a three-phase four-wire system), the generator neutral remains connected to the utility neutral, the generator is not separately derived, and it must not have its own neutral-to-ground bond. If the transfer switch does switch the neutral (four-pole), the generator is separately derived and must be bonded at the generator. Getting this wrong creates parallel neutral paths and can make ground fault protection at the service trip or fail to trip (Lesson 9).

Sizing bonding conductors

Equipment bonding conductors are sized from the rating of the overcurrent device protecting the circuit, because they must carry the fault current until that device opens.

Table 250.122 gives the minimum size of equipment grounding conductors based on the rating of the overcurrent device ahead of the circuit (for example, 12 AWG copper for 20 A, 10 AWG for 60 A, 8 AWG for 100 A, 6 AWG for 200 A, 4 AWG for 300 A, 3 AWG for 400 A, 1 AWG for 600 A). Where phase conductors are increased in size for voltage drop, the EGC must be increased proportionally by circular mil area (250.122(B)). Grounding electrode conductors are sized from Table 250.66 based on the largest service or derived conductor, and supply-side bonding jumpers from Table 250.102(C)(1).

Raceways, cable armor and other bonding paths

Metal raceways such as rigid conduit and EMT, and the armor of some cables, are permitted to serve as the bonding path when properly installed with tight, listed fittings, and metal enclosures connected by them are bonded through them. In practice, many engineers and owners specify a separate copper bonding conductor in every raceway anyway, because set-screw fittings loosen, raceways corrode and joints are disturbed by later work, and a wire conductor is more reliable. Flexible metal conduit and liquidtight conduit can serve as bonding paths only in short lengths and smaller sizes; for longer runs, flexible connections to vibrating equipment, and PVC conduit, a bonding conductor is required. Cable types such as TECK90 and MC cable include a bonding conductor inside the cable.

Bonding piping, structural steel and other metal

Metal systems that could become energized must be bonded so that they are at the same potential as the electrical system and fault current has a path back to the source. These include metal water piping, gas piping (bonded as required by the gas and electrical codes), other metal piping, exposed structural steel that is interconnected, metal ductwork in some cases, and metal raised floors in computer rooms. Bonding jumpers are sized according to the code tables and connected with listed clamps. Lightning protection systems must be bonded to the electrical grounding system so that a lightning strike does not create dangerous voltage differences between them.

Grounding electrodes

The grounding electrode system connects the electrical system to earth. In commercial buildings it typically combines several electrodes bonded together: concrete-encased electrodes (reinforcing steel or a bare copper conductor in the footings, often called an Ufer ground), metal underground water piping (supplemented by other electrodes, since plastic piping may later replace it), driven rods, ground rings around the building, and building structural steel where it is effectively grounded. Interconnecting electrodes lowers the resistance to earth and provides redundancy.

The resistance of a single driven rod depends strongly on soil resistivity; in average soil, a 3 m rod might measure 25 to 100 Ω. Codes accept a single rod in some cases only if its resistance is low enough, and otherwise require a second rod at least a minimum distance away. Facilities with sensitive equipment, telecommunications or lightning protection often specify a target of 5 Ω or less for the grounding system, achieved with ground rings, multiple rods, deep electrodes or chemical rods. Resistance is measured with the fall-of-potential method, which injects current between the electrode under test and a remote current probe and measures voltage at a series of potential probe positions, or with clamp-on testers on multiply grounded systems.

Isolated grounding and sensitive equipment

Isolated grounding (IG) receptacles have their grounding terminal insulated from the mounting yoke and connected by a separate insulated conductor back to the panel or source, intended to reduce electrical noise on equipment grounding. They must still be connected to the same grounding system at the source; isolated grounding never means connecting equipment to a separate ground rod, which would create a dangerous voltage difference during faults and lightning. Modern electronic equipment is far less sensitive to grounding noise than the equipment for which IG circuits were popularized, and most data centers now use a well-bonded, equipotential approach (signal reference grids and comprehensive bonding of racks and structure) instead of isolated grounds.

Inspection and testing

Common grounding and bonding deficiencies found by inspectors include missing or loose main and system bonding jumpers, neutral-to-ground connections in subpanels, missing bonding conductors in PVC and flexible conduit, undersized parallel bonding conductors, missing bonding of water and gas piping, painted surfaces under bonding connections, and grounding electrodes not interconnected. Testing during commissioning includes continuity of bonding conductors, insulation resistance between neutral and ground with the main bonding jumper temporarily removed (or checking for neutral current on bonding conductors with a clamp meter in service), ground fault protection testing by primary injection where required, and electrode resistance measurement.