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Grounding Service Equipment
[ Please Note: No illustrations are included in this article ]
Because utilities provide grounded ac services and most facilities have at least one utility service connection, a grounded ac service probably provides power to your premises wiring system. When you have a grounded ac service, your premises wiring system must have a grounding electrode conductor connected to the grounded service conductor [250.24(A)].
This brings up the question of how to comply with grounding electrode conductor requirements. Because a grounding electrode conductor must connect the grounded conductor to the grounding (earthing) electrode, the question of how expands to include where. Can you make this connection just any place?
Location, location, location
Some inspectors require the grounding electrode conductor to terminate to the grounded conductor terminal at the meter enclosure. Other inspectors require the grounding electrode conductor to terminate to the grounded terminal at the service disconnect.
The Code says you can make this connection at any accessible location-from the load end of the service drop or service lateral, up to and including the service disconnecting means [250.24(A)(1)]. See Figure 250-53. The choice then becomes an engineering decision that balances such factors as installation costs, available space, and maintenance issues.
In this day of increased demand for uninterrupted power, many facilities are dual-fed. This means they have separate lines coming to the same service-we refer to such services as "double-ended." If the dual feeds are in a common enclosure (or grouped together in separate enclosures) and they employ a secondary tie, you can use a single grounding electrode connection to the tie point of the grounded conductors from each power source [250.24(A)(3)].
Whether your service is double-ended or not, you must install an unspliced main bonding jumper between the grounded terminal and the metal parts of the service disconnecting means enclosure. Ensure the bonding jumper complies with 250.28 and the installation complies with 250.24(C).
Your main bonding jumper is probably wire or busbar. Let's say it is. And, let's say you've installed this jumper from the grounded conductor terminal (or bus) to the equipment grounding terminal (or bus) in the service equipment. In this case, the NEC allows you to connect the grounding electrode to the same equipment grounding terminal (or bus or bar) to which you connected the main bonding jumper [250.24(A)(4)].
My name is Bond. Load (side) Bond.
A load side neutral-ground bond is a common cause of power quality problems. Such a bond creates ground loops, which allow undesired current to circulate in the system. Power quality problems often lead to the discovery and removal of such a bond. But, don't wait for power quality problems to reveal the bond. Another concern makes corrective action imperative. Load side neutral-ground bonds allow objectionable current to flow on conductive metal parts of electrical equipment-thereby violating 250.6(A). This objectionable current can cause lethal electric shock (Figure 250-55). And, it sets the stage for inadvertent flashovers, overheating of equipment, and other problems stemming from electricity in the wrong place.
So, don't make (or allow) a neutral-ground connection on the load side of the service disconnect [250.24(A)(5)]. There are two exceptions to this [250.142]. You can make this connection (Figure 250-5) for:
Separately derived systems if you follow the requirements of 250.30(A)(1).
Separate buildings, if you follow the requirements of 250.32(B)(2).
Electric utilities typically do not provide an equipment grounding (bonding) conductor to service equipment, and aren't required to do so. Thus, you must run a grounded conductor from the electric utility transformer to each service disconnecting means [250.24(B) and 250.130(A)]. See Figures 250-56 and 250-57.
Why is this important? Because the grounded service conductor provides the effective ground-fault current path to the power source winding. This path ensures that opening the circuit protection device will quickly remove dangerous ground fault voltage from the circuit [250.4(A)(3) and 250.4(A)(5)]. See Figure 250-58
Don't try to use the earth as a bonding jumper. The resistance of the earth is always too great for the earth to be an effective bonding jumper. Very little fault current returns to the power source winding if earth is the only fault-current return path.
But, let's suppose the earth is your only fault-current return path. What would be the consequences? For one thing, the circuit overcurrent protection device will not open and clear the ground fault. Consequently, metal parts such as metal piping and structural building steel will become-and remain-energized to circuit voltage (Figure 250-59 and/or Figure 250-24C2 02 250-28.cdr). The system then poses a high risk of shock, arc blast, and fire.
If you are so inclined, you can combine phasor or polar form network analysis techniques with Kirchoff's Law and Ohm's Law to quantify how much risk such a configuration imposes at a given point. You can mathematically determine, for example, the voltage on a metal enclosure due to an open service grounded conductor. Forensic engineers often crank out these kinds of numbers when investigating why someone died or why a facility blew up. It's easier just to comply with NEC requirements-to eliminate such a voltage in the first place.
So, you see the need for a grounded conductor-but how big should it be? Remember, this grounded service conductor serves as the effective ground-fault current path. Thus, you must size it so it can safely carry the maximum fault current likely to be imposed on it [110.10 and 250.4(A)(5)]. To accomplish that, size the grounded conductor per Table 250.66-based on the total area of the largest ungrounded conductor (Figure 250-60). The grounded conductor must also have the capacity to carry the maximum unbalanced current, per 220.61.
Test your understanding with a quick quiz. Here's the question:
What is the minimum size grounded service
conductor required for a 480V, three-phase service, where the ungrounded service conductors
are 500 kcmil and the maximum unbalanced load is 100A (Figure 250-61)?
Now, here's the explanation. The unbalanced load requires a 3 AWG grounded service conductor-rated for 100A at 75?C per Table 310.16 [220.61]. However, the grounded service conductor cannot be smaller than 1/0 AWG (Table 250.66). This minimum size requirement ensures the conductor will accommodate the maximum fault current likely to be imposed on it. Thus, the answer is (d) 1/0 AWG.
What if you parallel your service conductors? Does that mean you use just the one conductor, or do you parallel your grounded conductor the way you parallel the current-carrying conductors? Answer: No to both.
First, you must install a grounded conductor in each raceway whenever you parallel your service conductors.
Second, you cannot simply divide your grounded conductor into two smaller equal conductors. Yes, doing so would satisfy the requirement that the grounded conductor must have the capacity to carry the maximum unbalanced current per 220.61. But, it could also result in a grounded conductor that is too small for a given raceway.
To eliminate such a problem, size each grounded conductor per Table 250.66-based on the total area of the largest ungrounded conductor in the raceway. Note that regardless of the number you come up with, the grounded conductor in each parallel service raceway can never be less than 1/0 AWG [310.4].
Let's review this with a quick quiz. Here's the question:
What is the minimum size grounded service
conductor required for a 480V, three-phase service installed in two raceways, where the
ungrounded service conductors in each of the raceways is 350 kcmil and the maximum unbalanced
load is 100A (Figure 250-62)?
Now, here's the explanation:
The unbalanced load requires only a 3 AWG grounded service conductor, per Table 310.16 [220.61]. However, the grounded service conductor in each raceway cannot be smaller than 1 AWG (Table 250.66). As before, this is to ensure it will accommodate the maximum fault current likely to be imposed on it. But, ungrounded service conductors run in parallel cannot be smaller than 1/0 AWG. Thus, the answer is (d) 1/0 AWG per raceway.
Properly grounding and bonding service equipment improves safety while eliminating a common cause of power quality problems. You just have to make the right connections in the right places. If you think in terms of providing a low impedance ground-fault path back to the source, you will have no problem doing so.
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