The Earth Beneath Your Feet: Rules for corner-grounded systems

Michael Johnston
Michael Johnston
Published On
Mar 15, 2022

The NEC rules often determine which electrical systems must be grounded, which are permitted to be grounded and which are not permitted to be grounded. The provisions are broken down for each system application.

Part II of Article 250 provides the requirements for electrical system grounding. The specific rules for grounding electrical systems are found in 250.20. Section 250.21(A) provides a list of electrical systems that are permitted, but not required, to be grounded. Section 250.22 provides the electrical systems that are not permitted to be grounded.

Section 250.4(A)(1) describes the purpose of system grounding and what it is intended to accomplish. Grounded systems are connected to the earth in a fashion that limits voltage imposed by higher-voltage lines, line surges, lightning events and so forth. Grounding a system also establishes a reference to the earth from the system and stabilizes the voltage to ground during normal operation.

During abnormal events such as a line surge or lightning strike, the system voltage and the voltage on conductive enclosures of the system will attempt to rise for the duration of the event. A ground-fault event attempts to force a rise in voltage on grounded equipment and systems for the duration of the fault condition or until an overcurrent device opens the circuit. Grounding helps limit these aboveground voltages and keep the earth and the connected system and equipment equal during fluctuations from the causes stated in 250.4(A)(5), such as ground faults, line surges and lightning events. The effective ground-fault current path provides a means to quickly facilitate overcurrent device operation, thus reducing the amount of time the abnormal condition exists.

Systems can be solidly grounded in a few ways. How the system is grounded determines the resulting output voltage of the system. Wye-connected systems are typically grounded at the center or common point of the wye that connects all three of the phase windings.

The terminal in a dry-type transformer with a wye-connected secondary is generally identified as the XO terminal. This is the center of the wye and the neutral point of the system. Typical wye-connected systems are 480Y/277V and 208Y/120V systems.

Delta-connected systems can be solidly grounded using a few methods. One method of grounding a three-phase, three-wire, delta-connected system is to ground one of the phase conductors.

This creates a three-phase, corner-grounded system, sometimes referred to as an end-grounded system. The NEC term is corner-grounded system. The phase-to-phase voltage of a corner-grounded system is also the phase-to-ground voltage of that system. For example, the phase-to-phase voltage of a 480V delta system is 480V. If one phase is grounded, the phase-to-ground voltage from the other two phases is 480V.

It’s important to recall that it is generally prohibited to insert an overcurrent protective device in any conductor that is grounded, to comply with Section 240.22. See 240.22(1) and (2) for specific exceptions to this general rule.

Another important requirement to apply here is found in Section 240.85, which clarifies the difference between slash ratings and straight ratings for circuit breakers and their uses when installed on electrical systems. This section indicates that a circuit breaker with a straight voltage rating, such as 240V or 480V, shall be permitted to be applied in a circuit in which the nominal voltage between any two conductors does not exceed the circuit breaker voltage rating. A two-pole circuit breaker shall not be used for protecting a three-phase, corner-grounded delta circuit unless the circuit breaker is marked to indicate such suitability.

A circuit breaker with a slash rating, such as 120/240V or 480Y/277V, can be installed in a solidly grounded circuit where the voltage of any conductor to ground does not exceed the lower of the two values of the circuit breaker voltage rating and the nominal voltage between any two conductors does not exceed the higher value of the circuit breaker voltage rating.

The NEC included identification rules for grounded conductors, even grounded phase conductors, such as those in corner-grounded systems. Usually, a grounded conductor is used throughout the system. This conductor must be identified according to the requirements in 200.6. It is sometimes referred to in the field as an identified conductor, but in NEC terms, it is a grounded conductor that is identified. The more common term used in the field is neutral, because various other system conductors require identification. Section 200.6 has two basic identification rules. Section 200.6(A) deals with conductors in sizes 6 AWG and smaller and 200.6(B) covers sizes 4 AWG and larger.

About the Author

Michael Johnston

Executive Director of Standards and Safety, NECA

Michael Johnston is NECA’s executive director of standards and safety. He is a member of the NEC Correlating Committee, NFPA Standards Council, IBEW, UL Electrical Council and NFPA’s Electrical Section. Reach him at mj@necanet.org.

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