Electrical systems are grounded to limit the voltage imposed by lightning, line surges, or unintentional contact with higher voltage lines and to stabilize the voltage to earth during normal operation. Electrical systems can be grounded in several ways. There are induction grounded systems, resistance grounded systems, and high impedance grounded systems among others. The most common grounded system is the solidly grounded system, where there is no intentional grounding impedance in the earthing or grounding circuit. Common solidly grounded systems are 3-phase, 4-wire, high-leg delta systems; 3-phase, 4-wire, wye-connected systems; and 1-phase 3-wire grounded systems.
Figure 1. Alternating current-circuits and systems required to be grounded
Generally, the serving utilities will not supply an electric service by a system that is not grounded. In the past, this was permitted under certain conditions. For example, in some industrial establishments, it was desired to have an ungrounded system to ensure continuity of service and eliminate costly downtime. Usually ground detectors were installed on these services. Current industry practices still make use of the ungrounded systems for many of the same reasons; however, these ungrounded systems are usually derived on the load side of the service equipment and are monitored for accidental ground faults by ground detection monitoring equipment. The decision to operate a grounded system as compared to an ungrounded system should be weighed carefully. Some advantages of the grounded systems are: the reference to ground from the system, stabilizing the voltages to ground, and localizing ground faults to the offending branch circuit or feeder.
Figure 2. Corner grounded systems
Section 200.2 reads as follows:
“”200.2 General.All premises wiring systems, other than circuits and systems exempted or prohibited by 210.10, 215.7, 250.21, 250.22, 250.162, 503.13, 517.63, 668.11, 668.21, and 690.41, Exception, shall have a grounded conductor that is identified in accordance with 200.6.””1
The NEC requires a premises wiring system to be connected to a grounded system. Section 200.3 reads as follows:
“”200.3 Connection to Grounded System.Premises wiring shall not be electrically connected to a supply system unless the latter contains, for any grounded conductor of the interior system, a corresponding conductor that is grounded. For the purpose of this section, electrically connected shall mean connected so as to be capable of carrying current, as distinguished from connection through electromagnetic induction.”2
Figure 3. Fusible switch for corner-grounded delta system
The uncommon grounded system is a 3-phase, 3-wire corner-grounded delta system. This is a system where one of the phase conductors of the 3-phase delta bank is intentionally grounded. Corner-grounded systems were used to supply services serving only a 3-phase load such as a well pump. Three-phase corner-grounded systems were also installed in the past as the main service for many commercial or industrial occupancies. These are becoming more and more scarce because of the increased use of grounded systems with usable voltages: 120-volts and 277-volts. In a corner-grounded system, the voltage-to-ground on the ungrounded phase conductors is the same as the voltage between phase conductors. An example is a 480-volt corner-grounded delta system. The phase-to-phase and phase-to-ground voltage is the same 480-volts. This affects which circuit breakers or equipment may be used on corner-grounded systems in conformance toNECrequirements (see figure 3).
Rules for Grounded Conductors
Whether the system is a common grounded system or one that is rarely installed these days, the rules for the grounded conductors of such systems are the same. Important requirements for the grounded conductor of grounded electrical systems or services are found in Article 200, which deals with identification of grounded conductors and their terminals. The grounded conductor is required to be an identified conductor of the system or service. Sizes 6 AWG and smaller generally are required to be identified by white or natural gray insulation for its entire length. Sizes larger than 6 AWG are permitted to be identified by white or natural gray insulation; by three white or natural gray stripes along the entire length of the conductor’s insulation, on other than green insulation; or at their points of termination by distinctive white markings that encircle the conductor, such as colored tape or painting. This identification is required for both grounded conductors and corner-grounded delta systems.
Other important requirements for grounded conductors are found in 240.22; these prohibit the installation of overcurrent devices in series with any conductor that is grounded. Two restrictive conditions listed in 240.22 allow this installation: The first is where the overcurrent protective device opens all conductors of the circuit, including the grounded conductor; and the second is where fuses are used for overload protection for motors and motor circuits in accordance with 430.36. Section 240.22 reads as follows:
“”240.22. Grounded Conductor.No overcurrent device shall be connected in series with any conductor that is intentionally grounded, unless one of the following two conditions are met:
(1) The overcurrent device opens all conductors of the circuit, including the grounded conductor, and is designed so that no pole can operate independently.
(2) Where required by 430.36 or 430.37 for motor overload protection.””3
Figure 4. Corner-grounded delta system
Section 230.75 requires a means be provided at the service equipment for disconnecting a grounded conductor. This disconnecting means may be a disconnect link, a terminal, or lug to which the grounded conductor connects at the service. The disconnect link is used in larger service equipment to accomplish required isolation for testing of dielectric values and testing of GFP equipment.
Another important requirement relative to grounded conductors of ground systems appears in 250.24(A)(5). Here it is clear that connections to ground by a grounded conductor downstream of the service disconnect is restricted. This is to prevent return current flowing on the grounded conductor from flowing over the equipment grounding conductors or other grounded equipment while returning to the source. Section 250.24(A)(5) reads as follows:
“5) Load-Side Grounding Connections.A grounding connection shall not be made to any grounded circuit conductor on the load side of the service disconnecting means except as otherwise permitted in this article.”4
The fine print note to this section indicates three locations and applications where this is still acceptable: (1) for separately derived systems, (2) for connections at separate buildings of structures, and (3) for use of the grounded circuit conductor for grounding equipment. Caution should be exercised here when determining whether any of these conditions stated in the FPN would be allowed on a grounded phase conductor.
Low Impedance Path
Section 250.24(B) requires that a grounded conductor of a service be installed with the service conductors, be brought to the service disconnecting means enclosure, and be bonded to the service disconnecting means enclosure. Service equipment is built and labeled either “Suitable for use as Service Equipment” or “Suitable for use only as Service Equipment.” One important feature of equipment that bears either of these markings is that provisions are employed in the equipment that accomplish the bonding of the grounded conductor to the disconnecting means enclosure. The grounded conductor connection in the service disconnect serves two vital purposes. In normal operation, the grounded conductor, which is usually the neutral conductor of the system or service, will carry the unbalanced return current to the source. In ground-fault conditions, the grounded conductor is required to serve as the low impedance path for fault current back to the serving transformer. The serving utility provides service that includes a grounded conductor, but very rarely an equipment grounding conductor. Sizing this grounded conductor must include provisions to carry the anticipated load current and any fault current for clearing overcurrent devices. The path for fault current must be effective, permanent and continuous, have ample capacity, and be of lowest possible impedance (see 250.24(D) and 220.22).
Figure 5. Application of circuit breakers on a corner-grounded system
Rules for Circuit Breakers and Equipment in Corner-grounded Systems
Circuit breakers used in corner-grounded systems must be chosen carefully and applied within their ratings. Slash-rated breakers must also be used within their ratings. Typical slash-rated breakers would be 120/208 volts or 277/480 volts. Typical markings on strait-rated breakers would be 240 volts, 480 volts, or 600 volts. Corner-grounded systems usually require strait-rated breakers to be installed in the system. The breakers are required to have a rating equal to the highest rated voltage-to-ground on any of the poles of the breaker. For example, breakers installed on a corner-grounded 480-volt system are not permitted to be rated at 480/277-volts. The lower voltage value in that rating is the phase-to-ground voltage rating. On a 480-volt corner-grounded system, the phase-to-ground voltage would be 480 volts, not 277 volts.
Equipment must also be properly rated to be used in corner-grounded systems. For example, service equipment for a corner-grounded 240-volt, 3-phase system must be marked “Suitable for use as Service Equipment,” and be marked with the appropriate ratings. Service equipment used on a 3-phase, 3-wire corner-grounded system is required to bear the markings indicating suitability. The equipment should be marked 3-phase, 3-wire 240-volts. If it is a 480-volt system, it should be marked 3-phase, 3-wire 480-volts. The slash-rated 480-volt equipment would be suitable for use on the 240-volt system because it would be used within its voltage limitations.
Some switchboards and panelboards carry dual or multiple voltage ratings. It is important to verify the types of circuit breakers allowed when used at the various voltage levels. Extra care should be taken here to properly apply the product within its ratings. The Code is silent on a required field marking indicating which voltage is applied, although it is probably a good practice to mark the multiple voltage equipment with the applied voltage in the field. The Code requires field marking of equipment when used in series rating combinations. The series-rated devices must be applied within their voltage test combinations and ratings. It is important if that equipment applied in a series rating combination at a voltage level above its rating be appropriately used. An example would be a 480-volt 3-phase, 3-wire series-rated combination system applied on a 240-volt 3-phase applied voltage. The key here is that the equipment is required to carry markings indicating suitability for such series ratings at the applied voltages. This indicates that evaluations have been made for conformance assessment by a qualified electrical testing laboratory.
Grounding (Earthing) the System
If the system is corner-grounded, then there will be a grounding electrode(s) to which the grounded phase conductor of the system is connected. The conductor used to connect the grounded conductor to the earthing or electrode system is known as the grounding electrode conductor. Grounding electrode conductors are required to be sized in accordance with 250.66, and must not be smaller than the values in Table 250.66, unless it is a sole connection to either a rod, pipe, or plate electrodes; ring electrodes; or the concrete-encased electrode. The minimum size conductor for a rod, pipe, or plate must be 6 AWG copper. The minimum size for concrete- encased electrodes is 4 AWG copper, and the minimum size grounding electrode conductor for a ground ring is 2 AWG copper. The installation of grounding electrode conductors must account for protection from physical damage; protection from magnetic fields (the choke effect) when installed in metallic raceways; and where aluminum is used for the grounding electrode conductor, it must not be installed or terminated within 18 inches of the earth in accordance with 250.64(A) through (E).
Installations and inspections of corner-grounded systems can be a bit intimidating both to seasoned veterans and to students of the Code. All the rules for grounded conductors must be applied identically to the grounded conductors of all systems, whether the grounded conductor is a grounded neutral conductor or a grounded phase conductor. The equipment utilized in corner-grounded systems must be appropriately marked and applied within their ratings. As the Code continues to migrate away from the use of the grounded conductor for grounding downstream of the service equipment or separately derived system, this holds true for the grounded conductor of corner-grounded systems as well. Grounded conductor requirements in the Code are applicable to corner-grounded delta systems as well as to the more common 3-phase, 4-wire wye- connected system, grounded high-leg delta-connected systems, and single-phase, 3-wire grounded systems. Training in voltage testing is critical for those who have the occasion to troubleshoot these systems. These systems are not as common but still are in use and can fool even the experienced. Caution must be used in troubleshooting at any voltages. Use of the appropriate personal protective equipment should be employed where equipment has to be worked on or inspected while energized (see NFPA 70E Standard for Electrical Safety Requirements for Employee Workplaces).
1 NFPA 70,National Electrical Code, Section 200.2, (Quincy, MA: National Fire Protection Association), p. 70-49.
2 NFPA 70, Section 200.3, p. 70-49.
3 NFPA 70, Section 240.22, p. 70-91
4 NFPA 70, Section 250.24, p 70-100