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The general requirements for grounding differ from those of bonding.
The grounding and bonding requirements contained in this column apply to solidly grounded systems that operate at not more than 600V (such as 120/240V, 120/208V, and 277/480V).
Article 250 begins providing grounding and bonding requirements in Section 250.4. It breaks these requirements down into two groups:
It may seem odd that the NEC has grounding and bonding requirements for ungrounded systems, but we'll explain that in a bit. However, 250.4 starts with grounded systems-so that's where we will start. The first requirement is for electrical system grounding-exactly what you won't find in an ungrounded system.
Grounding in grounded systems
The NEC requires you to ground (earth) system windings to limit the voltage imposed on the system from lightning, unintentional contact with higher-voltage lines, or line surges. Another function of this earthing is to "stabilize the voltage to earth during normal operation" by providing a common reference point.
The NEC also requires you to ground (earth) metal parts of electrical equipment in or on a building or structure. See 250.24(A) for services and 250.32(A) for separate buildings or structures. Figure 250-24. You accomplish this grounding (earthing) by electrically connecting the building or structure disconnecting means [225.31 or 230.70]-with a grounding (earthing) electrode conductor [250.64(A)]-to a grounding (earthing) electrode [250.52, 250.24(A) and 250.32(A)].
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Lightning and line surges.
When lightning occurs, high voltages drive high current (as much as 40,000A) into the earth for a fraction of a second (Figure 250-21). Typically, lightning strikes to wiring are to outside utility wiring. Therefore, grounding (earthing) the system windings will assist the flow of lightning into the earth.
When a ground fault over 600V occurs, the voltage on the other phases can rise significantly for the duration of the fault (typically 3 to 12 cycles). This voltage surge during the utility ground fault will be transformed into an elevated surge voltage on the secondary-possibly destroying electrical and electronic equipment. The lower the resistance of the utility grounding (earthing) system, the lower the secondary voltage surge.
But, grounding (earthing) has limitations:
To provide protection from voltage surges,
you must engineer a proper surge protection system. The design should address surge protection
devices (Articles 280 and 285) at service equipment, panelboards, and critical loads.
Also consider point of use surge protection in branch circuits (not covered by the NEC).
An "effective ground-fault current path" is a permanent, low-impedance path for fault-current, and it facilitates the operation of the circuit overcurrent protection device (OCPD) (Figure 250-04 A5 01). The earth is not an effective ground-fault current path. Because the earth is a poor conductor, it doesn't permit sufficient fault current to flow back to the system winding to open the OCPD. Thus, a rod or concrete-encased electrode will not assist in clearing ground fault [IEEE 142 Section 2.2.8] (Figure 250-04 A5 03).
A ground fault clears on a circuit through the automatic opening of the OCPD. The time it takes for an OCPD to open is inversely proportional to the magnitude of the fault current. Thus, the higher the ground-fault current value, sooner the OCPD will open and clear the fault.
To quickly remove lethal touch voltage from metal parts after a ground fault occurs, the fault-current path must have sufficiently low impedance for the fault current to quickly rise and facilitate opening the OCPD. For example, a 20A circuit with an overload of 40A (two times the rating) would trip a breaker in 25 to 150 seconds. At 100A (five times the rating) the breaker would trip in 5 to 20 seconds (Figure 250-9).
Thus, the effective ground-fault current path is critical. And you establish it through bonding. Start by connecting non-current carrying conductive materials of electrical equipment together and to the electrical supply source [250.4(A)(3)].
Do the same for electrical raceways, cables, enclosures, equipment, and other electrically conductive materials that are "likely to become energized" [250.4(A)(4)].
Whether something is "likely to become energized" is subject to interpretation by the authority having jurisdiction.
What about conductive materials other than electrical equipment? Can't you have dangerous voltages on metal water piping systems, metal sprinkler piping, metal gas piping, and exposed structural steel members? Yes. And if any such items are likely to become energized, you must bond them to the effective ground-fault current path (Figure 250-27).
When we say a system is ungrounded, we are referring to its supply configuration and wiring scheme. In a grounded system, the supply transformer secondary windings may be a wye-configured with the center tap grounded or it may be a delta-configured with a corner grounded.
Unlike a grounded system, an ungrounded system doesn't have a winding grounded at the supply transformer. This allows for continued operation if you have a ground fault on one phase. Presumably, qualified personnel will locate and repair the fault before a ground fault on a second phase takes the system down.
Ungrounded systems are common in factories in the southern United States. These facilities typically have ground fault monitors to alert maintenance personnel to a ground fault on any phase. For example, the Daramic plant in Owensboro, KY, has a ground fault monitor on the wall between the maintenance offices and the machine shop-where maintenance personnel pass by frequently.
But, maintenance crews at such facilities must repair the fault promptly. If a ground fault persists on an ungrounded system, the system could see overvoltage-to-ground as high as eight times the phase voltage. This excessive voltage can puncture conductor insulation and create additional ground faults.
Grounding in ungrounded systems
The general requirements for grounding electrical equipment in ungrounded systems differ in purpose from those for grounded systems. As there's no system grounding, there's no stabilization of the system voltage to earth. And for this reason, we limit the voltage (imposed by lightning, unintentional contact with higher-voltage lines, or line surges) at the equipment level rather than at the system level.
Bonding in ungrounded systems
Whether your system is grounded or ungrounded, you must bond enclosures and equipment together. In ungrounded systems, bonding of electrical equipment [250.4(B)(2)] serves a purpose similar to that set forth for bonding electrical equipment in grounded systems [250.4(A)(3)]. The difference here is you are bonding the equipment of an ungrounded system to each other, rather than to each other and the source.
One consequence of this arrangement is the equipment bonding path must be capable of carrying the maximum fault current likely to be imposed on it (250-04B2).
Remember, the bonding system must be able to remove dangerous voltage from a second ground fault.
The same difference and consequence applies to the bonding of electrically conductive materials and other equipment in ungrounded systems [250.4(B)(3)] vs. those in grounded systems [250.4(A)(4)]
Fault current paths
The requirements for establishing paths for fault current are similar for grounded and ungrounded systems. In either case, you cannot use the earth as the sole equipment grounding conductor or consider it a fault current path in either configuration.
And in either case, you must install electrical equipment, wiring, and other electrically conductive material likely to become energized in a manner that creates a permanent, low-impedance circuit. Yet, there is a key difference is this requirement:
A single ground fault cannot be cleared on an ungrounded system because there's no low-impedance fault-current path to the source. However, all metal parts of an ungrounded system must be bonded together so-in the event of a second ground fault (line-to-line fault)-the bonding path will provide a low-impedance fault-current path. This will allow the OCPD to clear the fault [250.4(B)(4)].
As with the definitions in Section 250.2, Section 250.4 differentiates between grounding and bonding. You will notice more of this differentiation as we continue our study of grounding and bonding in Article 250.
The above is an extract from my Grounding versus Bonding book.
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