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Grounded Power Systems

System grounding is illustrated in Figures 15 and 16, which show 120V/240V, three-wire, single-phase power and 120V/208V, four-wire, three-phase power.

System grounding is illustrated in Figures 15 and 16, which show 120V/240V, three-wire, single-phase power and 120V/208V, four-wire, three-phase power. In both cases, one of the current-carrying conductors is connected to earth. System grounding is defined as connecting to earth one of the conductors that carries power under normal operating circumstances.

The neutral conductor is the conductor connected to ground in most existing power systems. However, this is not the definition of the neutral conductor. The definition of the neutral conductor is the conductor that under balanced load conditions will carry no current. In a three-phase system where each of the three phases has an equal load, the current in the neutral conductor will be zero. In practice, it is easiest to ground the neutral conductor.

Equipment grounding consists of separate conductors that bond various elements of the ac system together and to earth. Under normal operating conditions, these conductors are not connected to ac power. In other words, equipment9rounding conductors only carry current under fault conditions. As a result of this, it is possible also to use equipment grounding conductors to ground sensitive electronic equipment or other devices that need a stable connection to earth.

The Kaufmann Experiment
This experiment, which was conducted in the early 1950s (Kaufmann, 1954), illustrates the importance of routing the ground conductor with the power conductors. The experiment setup (Figure 17) illustrates the typical arrangement that can be found on any job site. A current source was connected to the phase conductor and to pairs of possible ground return paths, which included a #4/0 conductor run with the phase conductor, and a #4/0 wire run 1 foot (0.3m) away from the rigid conduit and structural building steel. In a comparison of the relative impedances of 100 feet (30 m) of the rigid steel conduit vs. the insulated #4/0 ground conductors routed external to the conduit, 95% of the fault current flowed on the conduit, and only 10% flowed on the equipment grounding conductor routed outside the conduit. The impedance of the conduit was nine times less than the impedance of the grounding conductor routed external to it.

However, when the #4/0 equipment grounding conductor was routed with the phase conductor inside of the conduit, 80% of the fault current flowed in the equipment grounding wire, and only 20% flowed in the conduit. This experiment proves conclusively that the fault current that will flow through a ground conductor will be much higher when it is routed with the phase conductor. In the event of a fault, the circuit-protection device will be tripped much more quickly because of the high fault current, which minimizes the duration of the hazard. The results of the Kaufmann experiment are a primary reason why electrical codes require grounding conductors to be run with phase conductors.

When the building steel was compared to the rigid conduit, 95% of the fault current flowed on the conduit; only 5% flowed on the building steel.

Safety for people and equipment
We use power-system grounding (systems grounding and equipment grounding) to improve the reliability and safety of ac power systems. You must analyze several situations to understand how these systems work together to achieve this goal.
Lightning and transients: Lightning strikes are a particular problem for ac lines. In the case of a grounded system, the energy can be drained to earth through the ground connection. (See Figure 18.) If this ground connection did not exist, arcing from the conductors to nearby earth members would be the only way for this energy to leave the power system. Similarly, when switching large inductive loads on ac power systems, arcing can also occur. Again, connections to earth help reduce this possibility.

Faults to ground: In a completely floating power system, a short to ground anywhere in the system does not cause an excessive current to flow. However, in a system that already has a grounded neutral, if the phase conductor is shorter to ground, a large current will flow and the circuit-protecting device will trip (Figure 19). Because this device trips, the short to ground, which could include metal parts with which human beings could come into contact, does not occur for any period of time, because the circuit-protective device normally trips in well under a second. Without the grounded neutral, this short to steel could exist for a long time and could be dangerous. The neutral becoming grounded a second time does not pose any danger.

Double phase: If a fault to ground or structural steel occurs in a system with no grounding, a protection device will not operate and the fault will go unnoticed. When a second fault occurs in another phase, the short from phase to phase means both phases will be lost at one time and a full line-to-line (phase-to-phase) voltage will appear across the fault, with large currents resulting.

Fault to ground in multiphase system: If one phase faults to ground in a system with no grounding, the ground reference causes the other phase or phases to rise above the ground potential by the full line-to-line potential, as shown in Figure 20. This is an increase of 100% in residential systems and about 73% in three-phase systems. Most insulation can withstand these high voltages. However, they can break down if they are old or in poor condition.

System-to-system short: If an ungrounded system short has one conductor short to another system, it will be referenced to that system. Hence, it would be possible for a 120V circuit to have its potential raised several thousand volts should it short to a high-voltage system, as shown in Figure 21. If the system is grounded, circuit-protection devices will be blown immediately upon the system-to-system short.

The National Electrical Code
In the United States, The National Electrical Code, published by the National Fire Prevention Association, is the document used by all electrical power-system designers and inspectors. It specifies, in a rigorous manner, how to do both system and equipment9 grounding. It also stipulates all other aspects of power systems important from a safety standpoint. (At the time of this article) The most recent version of the National Electrical Code was issued in 1993. It can be ordered by calling 800-344-3555. A very helpful book in explaining the National Electrical Code is the National Electrical Code Handbook, which provides additional explanations and illustrations.

In Canada, the presiding code is the Canadian Electrical Code. The 1994 edition is published The Canadian Standards Association in Rexdale, Ontario.

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