Grounding an electrical system means one system conductor is connected to ground (earth by definition) and a reference to ground from the system is established. Installing and operating an ungrounded system means no reference to ground from the system conductors is established other than through capacitance. National Electrical Code (NEC) Section 250.30 provides specific rules for grounded and ungrounded separately derived systems.
To ground or not
From the earliest years of electricity use, there have been many discussions and even heated debates about the benefits of operating systems grounded versus ungrounded. The Code guides users whether to ground a system or not. As indicated in sections 250.20, 250.21 and 250.22 sequentially, certain electrical systems must be grounded, some systems are permitted to be grounded, and other systems are not permitted to be grounded. Part II of Article 250 provides the requirements for electrical system grounding. 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 a lightning strike, the system potential and the potential on conductive enclosures of the system will attempt to rise for the duration of the abnormal event. A ground-fault event attempts to force a rise in potential on grounded equipment and systems for the duration of the fault condition or until an overcurrent device opens the circuit. Grounding helps limit these above-ground potentials during abnormal events. System grounding is the process of establishing a connection from one system conductor (often a neutral) to ground (the earth). Therefore, when a system is grounded, one system conductor is connected to ground solidly; through an impedance device, resistor or inductor; or by some other means. The connection to earth is accomplished through a grounding electrode conductor installed in accordance with Part III of Article 250.
Ungrounded’s advantages and disadvantages
Ungrounded systems are often specified and installed in industrial facilities where continuity of power is desired for assembly lines and other continuous processes that would be damaged or could cause personal injury if a first phase-to-ground-fault event were to result in interruption of system power. The choice to install and operate this type of system is determined by the nature of the process, the operational characteristics of the process and the operators/owners’ desired method of operation. Where ground detectors are installed ungrounded systems, the sensors for such systems must be located as close to the supply source as possible. Listed ground detection equipment is available for use on ungrounded systems.
Functionally, a first phase-to-ground fault event on an ungrounded system will not cause overcurrent device operation, so continued service is achieved. However, it is important that personnel monitoring the system react to the annunciation, investigate the first phase-to-ground condition, and remove it. If the first phase-to-ground condition is not cleared and a second phase-to-ground fault develops on a different phase, the result is a simultaneous phase-to-phase short circuit and phase-to-ground-fault event. This type of event can lead to significant downtime and destruction of equipment in some instances.
The disadvantages of an ungrounded system is that a first phase-to-ground fault condition can be difficult to find and can take a considerable amount of investigation and time. The voltage to ground in an ungrounded system is, in theory, 0 volts (V) because there is no ground connection from any system conductor. But varying levels of distributed leakage capacitance can be present throughout such systems. Phase-to-ground voltage levels can appear as a result from capacitance coupling effects from the circuits supplied by such systems.
Breakers for ungrounded systems
The voltage to ground for ungrounded systems is clarified by the definition of “voltage to ground,” which indicates that the voltage to ground of a grounded system is the voltage between the given conductor and that point or conductor of the circuit that is grounded.
For example, in a 120/240V, single-phase system, the voltage is 120V from any ungrounded phase conductor to ground. However, for ungrounded systems, the greatest voltage between the given conductor and any other conductor of the circuit is also the phase-to-ground voltage.
For example, on a 480V, three-phase, 3-wire, ungrounded delta system, the phase-to-phase voltage is 480V. This voltage (480) is also the phase-to-ground voltage for this system, based on the definition. Installing circuit breakers on such systems requires an understanding of marked voltage ratings such as breakers marked with a straight voltage rating (240, 480, 600 and so forth), as compared to a slash rating (600/347, 480/277, 240/120 and so forth). See Section 240.85 for more information.