To understand the purpose of ground-detection equipment installed for ungrounded systems, it is best to have a fundamental understanding of ungrounded system performance and the rules that apply to them.
Section 250.4(B) provides performance requirements that apply to systems that are not grounded solidly or through any impedance. Ungrounded systems do not have an intentionally grounded conductor. Ungrounded conductors operate without a solid voltage-to-ground reference.
If an electrical system is not grounded, then there is no intentional connection between the system conductors and the earth. When conductors supplied by ungrounded AC systems are installed in grounded metal raceways and enclosures, the effects of capacitance coupling are present, which creates varying potential (voltage) differences between them and ground. This potential difference is the result of leakage capacitance and can appear as voltage-to-ground measurements. An example of an ungrounded AC system is a three-phase, 3-wire, 480V system. See the NEC definition of the term “voltage-to-ground.”
Although the system is ungrounded, electrical grounding and bonding are necessary and required for equipment enclosing circuit conductors supplied by ungrounded systems. The noncurrent-carrying parts of raceways, equipment enclosures and so forth must be grounded so as to limit overvoltages caused by lightning or unintentional contact with higher-voltage lines. Grounding equipment essentially places these conductive parts at or close to the same potential as that of ground, the earth.
Another important characteristic of a ground fault on ungrounded systems is that the path for fault current usually includes many conductive paths other than those specifically installed. Current—ground-fault or normal—will always try to return to its source. A ground fault on ungrounded and grounded systems introduces current in multiple paths, such as combinations of EGCs, metallic wiring methods, conductive electrical equipment enclosures and other electrically conductive materials, including conductive piping, structural building framing, metal ducts, cable shields and even the earth itself. This is why the word “effective” is not used for the fault current path constructed for ungrounded systems as provided in 250.4(B)(4).
A ground-fault event from any of the ungrounded system conductors will often accidentally and ineffectively ground the system. The word “ineffectively” is used because these events are typically ground faults, which indicate a pinched wire or similar failure of insulation creating this condition.
This differs greatly from the intentional system connection to the ground for solidly grounded electrical systems. Thus, a first ground fault on an ungrounded system is usually an intermittent and unstable connection that will often manifest through minor arcing and possibly become ineffectively attached to the conductive raceway or enclosure. This condition usually exists until a repair is made.
If a second phase-to-ground fault were to develop on a different phase than the first fault, a short circuit and ground-fault condition results simultaneously. Heavy levels of fault current are introduced into the system and feed into the faults until an overcurrent protective device opens the circuit. Effective bonding for metal equipment enclosures and metallic raceways is essential. Connections to these conductive enclosures must be made up tight and workmanlike to perform effectively in ground-fault conditions. The bonded conductive parts and equipment must maintain continuity and conductivity for any value of current imposed on them, including the highest value of fault current the system is capable of delivering. Bonding is an essential function of electrical safety and how the safety circuits operate.
Section 250.4(B)(4) addresses the path for fault current necessary for wiring methods and equipment installed for ungrounded systems. Again, a first phase-to-ground fault condition will unintentionally ground the system but not effectively because these are usually loose unintentional connections between any of the ungrounded phase conductors supplied by the system. The fault current path requirements for an ungrounded system are very similar to the performance requirements for the effective ground-fault current path in grounded systems.
As a ground-fault condition develops, the faulted phase conductor is forced to the same potential as that of ground because all wiring methods and equipment are grounded. In accordance with 250.21(B), ground-detection systems are required for ungrounded systems, so when a phase-to-ground fault event happens on an ungrounded system, an indication, typically audible and visual, is provided. The required detection system alerts qualified people to this condition so the problem can be addressed before a second phase-to-ground event on another phase develops. The first phase-to-ground fault condition on an ungrounded system is comparable to the calm before the storm. This condition usually exists until a repair is made.