Everyone in the electrical industry knows the importance of grounding. Just ask. Now ask the same people exactly what grounding is, and they'll just tell you what's involved. The fun starts when you put your interviewees in the same room together and play back the responses.
Let's start with the two distinct types of grounding recognized by the NEC: system grounding and equipment grounding. The first step in understanding electrical grounding is knowing exactly what these two concepts mean and how they differ. Then, we'll discuss where bonding fits. Per Sec. 250-20(d), you can apply this discussion to most power distribution systems, whether they originate at service equipment or separately derived systems.
This means intentionally connecting a current-carrying conductor of the electrical system to earth (or to some conducting body that serves in place of the earth). System grounding doesn't have anything to do directly with eliminating potential shock hazards. We don't build electrical systems with the idea that bare circuit conductors (even grounded ones) are safe to the touch. However, there are exceptions to this, such as service equipment bonded to a grounded service conductor. Sec. 250-2(a) of the NEC describes the performance objectives of system grounding:
(a) Grounding of Electrical Systems. Electrical systems that are required to be grounded shall be connected to earth in a manner that will limit voltages imposed by lightning, line surges, or unintentional contact with higher voltage lines, and that will stabilize the voltage to earth during normal operation.
In practice, system grounding usually refers to intentionally grounding a secondary transformer winding. In an ungrounded delta-connected 3-phase system, single phase-to-ground faults produce low values of fault current. These currents won't operate an overcurrent device. Instead, the first ground fault grounds one corner of the delta. If you install ground detectors, you can arrange an orderly shutdown at this point without randomly losing power.
A grounded wye-connected 3-phase system has its center point connected to ground. Such a system permits automatic clearing of accidental grounds. Why? Because, phase-to-ground faults produce currents that will operate overcurrent devices automatically. These systems have another advantage. You can carry the conductor from the grounded center point (a neutral) with the ungrounded conductors and use it in the distribution system for connecting loads at the lower voltage from phase to neutral.
Unlike system grounding that limits overvoltages on conductors and equipment to assure equipment functionality, equipment grounding has a different purpose, as described in Sec. 250-2(b):
(b) Grounding of Electrical Equipment. Conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected to earth so as to limit the voltage to ground on these materials. Where the NEC requires the electrical system to be grounded, these materials shall be connected together and to the supply system grounded conductor as specified by this article. Where the electrical system is not solidly grounded, these materials shall be connected in a manner that establishes an effective path for fault current.
By limiting voltage to ground on enclosure surfaces, the Code tries to assure safety by making electrical equipment as safe to touch as playing in a sandbox. How? By requiring you to bond the conductive surfaces to the sandbox.
How so? Any electrical power system, grounded or ungrounded, requires a continuous equipment grounding system connected to earth through a grounding electrode conductor (typically connected at the system source or disconnect). In grounded systems, you always connect equipment-grounding conductors to the system grounded conductor to provide a low impedance path for fault current. This allows the operation of overcurrent devices under ground-fault conditions.
For example, a metallic enclosure grounded only through the entering conduit contains a faulted phase conductor touching one of the surfaces. Suppose the grounding path back to the source provided by the conduit is poor (due to corrosion, etc.) or doesn't exist. If a person simultaneously contacts the enclosure and a grounded surface, some of the fault current will flow through the person.
On the other hand, if the enclosure has a solid equipment-grounding connection, the resistance through the person's body should exceed, by roughly ten thousand times, that of the ground return path. In this event, only a small percentage of the fault current will flow through the person. In addition, low impedance in the ground return path means a high value of fault current will flow, causing the overcurrent protective device to clear the fault quickly. Touching electrical equipment should be as safe as touching the earth. This, perhaps, is the principal human safety objective of Art. 250.
Note: Sec. 250-2 doesn't use the familiar phrase "fault current return path." This section covers both grounded and ungrounded systems; only solidly grounded systems have a fault current return path.
By definition, ungrounded systems have no point to return fault current. Still, the fault current path must be reliable. If two simultaneous faults occur from different phases on an ungrounded system, the equipment grounding system must safely carry the resulting phase-to-phase fault current.
We often hear grounding and bonding used in place of each other when describing grounding; but you can't use the two terms interchangeably. Bonding, as defined in Art. 100 means:
The permanent joining of metallic parts to form an electrically conductive path that will assure electrical continuity and the capacity to conduct safely any current likely to be imposed.
This means bonding, in reference to grounding, is how you achieve effective grounding. Bonding electrical equipment, metal raceways, and enclosures provides a continuous equipment grounding conductor and an effective low-impedance path. This allows for the flow of short circuit currents to ground (or between phases on ungrounded systems). This assures protective devices in the affected circuit will trip and "open" quickly. Also, a continuous equipment grounding conductor assures short circuit current flow will not cause arcing and sparking along such a path. As covered in a companion article in this issue, this concept drives special grounding rules in hazardous locations.
Sec. 250-2(d) covers how the fault current path needs to perform and the various bonding connections you make along the way comprise the "tools" you'll use to meet this requirement:
(d) Performance of Fault Current Path. The fault current path shall be permanent and electrically continuous, shall be capable of safely carrying the maximum fault likely to be imposed on it and shall have sufficiently low impedance to facilitate the operation of overcurrent devices under fault conditions.
In addition to bonding within an electrical equipment grounding path, other building systems may need to be bonded to the electrical system for safety reasons, as covered in Sec. 250-2(c):
(c) Bonding of Electrically Conductive Materials and Other Equipment. Electrically conductive materials, such as metal water piping, metal gas piping, and structural steel members, that are likely to become energized shall be bonded as specified by this article to the supply system grounded conductor or, in the case of an ungrounded electrical system, to the electrical system grounded equipment, in a manner that establishes an effective path for fault current.
This rule broadens the reach of Art. 250 to building steel and nonelectrical piping systems. This allows a low potential difference between conductive items located adjacent to each other. The Code requires this to prevent a person from receiving a shock if two items are touched simultaneously.
The earth is about the most unreliable equipment-grounding conductors imaginable. Why? The ground resistance limits the functional conductivity of the grounding electrode. The best electrodes have resistances in the 1-ohm range; on a 120V circuit, a fault into the earth would only pass 120A, by Ohm's Law.
This amount of current would never trip a 150A overcurrent device, and it would be below the instantaneous tripping points of lower-sized protective devices. A 30A device would see the 120A as an overload, such as a motor attempting to start. It would never trip promptly. Remember, typical grounding resistances to earth run much higher than 1 ohm, and you see why the final paragraph of Sec. 250-2(d) reads as it does:
The earth shall not be used as the sole equipment grounding conductor or fault current path.
Make as many supplementary connections to earth from the equipment grounding conductor(s) as you wish, but make absolutely certain that a Code-compliant equipment grounding conductor is in place to carry any fault current imposed. Sec. 250-54 restates this concept.
We started this article noting wide disparities in the concepts all of us apply to the term "grounding," and this final issue brings us back full circle. Here the "ground", the earth itself, doesn't really serve to meet the performance objectives of "grounding." Electrical power system grounding always involves the earth at some end point, but never relies on it to make systems function safely.
The parent language in Sec. 250-2 is a first in NEC history:
The NEC is a prescriptive-based (not a performance-based) Code. That is, it doesn't tell us what electrical protective protocols are supposed to accomplish. Instead, it tells us what we are to do to accomplish the safety objectives in Sec. 90-1. The prescriptive requirements in the NEC result from a consensus process whereby the collective wisdom of international experience boils down to actual requirements. If you want to know what a performance-based code would look like, imagine Art. 250 with just Sec. 250-2. Everything else is essentially prescriptive.
Although performance-based codes are all the rage, they require incredibly sophisticated engineering support and inspection to work. Many times inspectors have seen engineered plans that failed to comply with provisions of this or other articles. Often, the engineer will respond with the excuse of equivalent safety. However, if the real reason for the requirement was known (and it may not be, even to the inspector), it would have been obvious the proposal was not equivalently safe.
NEC requirements result from generations of participants applying their experience in millions of applications. That's a tall order for a performance-based code to match: particularly where many jurisdictions have no inspections and others have inspectors with no specialized electrical training.
250-2. General Requirements for Grounding and Bonding
The following general requirements identify what grounding and bonding of electrical systems are required to accomplish. The prescriptive methods contained in Art. 250 shall be followed to comply with the performance requirements of this section.