8.6		Planning Structured Cabling: Electricity and Grounding
		8.6.1	Differences between AC and DC
Electricity is a fact of modern life. We use it to perform a variety of tasks. It is brought into our homes, schools, and offices by power lines that carry it in the form of alternating current (AC). Another type of current, called direct current (DC), is the kind found in a flashlight, car battery, and in the motherboard of a computer. It is important to understand the difference between these two types of current flow. DC flows at a constant value when circuits are turned on. To see how this works, refer to the Figure . As illustrated, the battery supplies current during a given period of time at a constant level of current flow. AC rises and falls in current values as it is manufactured by power companies. This rise and fall can be explained by the series of graphics shown here: The Figure depicts the rise to peak of current flow as the south pole moves across the core of the coil. The Figure depicts the fall to 0 current flow as the two poles straddle the core and balance current flow to 0 value. The Figure depicts the rise to opposite polarity peak (a negative value) as the north pole moves across the core of the coil. The Figure depicts the fall to 0 current flow as the magnet exits the coil area. AC power as manufactured for delivery to homes uses this concept.
8.6	Planning Structured Cabling: Electricity and Grounding
	8.6.2			AC line noise
After it reaches our homes, schools, and offices, electricity is carried to appliances and machines via wires concealed in walls, floors, and ceilings. Consequently, inside these buildings AC power line noise is all around us. If not properly addressed, power line noise can present problems for a network. In fact, you will discover as you work with networks, that AC line noise, coming from a nearby video monitor, or hard disk drive, can be enough to create errors in a computer system. It does this by adding unwanted voltages to the desired signals and preventing a computer's logic gates from detecting the leading and trailing edges of the square signal waves. This problem can be further compounded when a computer has a poor ground connection.

8.6	Planning Structured Cabling: Electricity and Grounding
	8.6.3	Electrostatic discharge
Electrostatic discharge (ESD), more commonly known as static electricity, is the most damaging and uncontrollable form of electricity. ESD must be dealt with in order to protect sensitive electronic equipment. At one time or another you have experienced what happens as you walk across a carpet. If the air is cool and dry, when you reach to touch another object, a spark jumps from your fingertips, and causes you to feel a small shock. You know from experience, that such ESDs can sting momentarily, but in the case of a computer such shocks can be disastrous. ESDs can destroy semiconductors and data, in a random fashion, as they shoot through a computer. A solution that can help solve problems that arise from ESD is good grounding.

8.6		Planning Structured Cabling: Electricity and Grounding
		8.6.4	Grounding electrical current in computer equipment
For AC and DC electrical systems, the flow of electrons is always from a negatively charged source to a positively charged source. However, for the controlled flow of electrons to occur, a complete circuit is required. Generally speaking, electrical current follows the path of least resistance. Because metals such as copper provide little resistance, they are frequently used as conductors for electrical current. Conversely, materials such as glass, rubber, and plastic provide more resistance. Therefore, they do not make good electrical conductors. Instead, these materials are frequently used as insulators. They are used on conductors to prevent shock, fires, and short circuits. Electrical power is usually delivered to a pole-mounted transformer. The transformer reduces the high voltages, used in the transmission, to the 120 or 240 volts used by typical consumer electrical appliances. Figure shows a familiar object, electricity as supplied through wall outlets in the US (other nations have different wall outlet configurations).. The top two connectors supply power. The round connector on the bottom protects people and equipment from shocks and short circuits. This connector is called the safety ground connection. In electrical equipment where this is used, the safety ground wire is connected to any exposed metal part of the equipment. The motherboards and computing circuits in computing equipment are electrically connected to the chassis. This also connects them to the safety grounding wire, which is used to dissipate static electricity. The purpose of connecting the safety ground to exposed metal parts of the computing equipment is to prevent such metal parts from becoming energized with a hazardous voltage resulting from a wiring fault inside the device. An accidental connection between the hot wire and the chassis is an example of a wiring fault that could occur in a network device. If such a fault were to occur, the safety ground wire connected to the device would serve as a low resistance path to the earth ground. The safety ground connection provides a lower resistance path than your body. When properly installed, the low resistance path, provided by the safety ground wire, offers sufficiently low resistance and current carrying capacity to prevent the build up of hazardously high voltages. The circuit links directly to the hot connection to the earth. Whenever an electrical current is passed through this path into the ground, it causes protective devices such as circuit breakers and Ground Fault Circuit Interrupters (GFCIs) to activate. By interrupting the circuit, circuit breakers and GFCIs stop the flow of electrons, and reduce the hazard of electrical shock. The circuit breakers protect you and your house wiring. Further protection - often in the form of surge suppressors and Uninterrupted Power Supplies (UPS) - are required to protect computing and networking equipment.
8.6	Planning Structured Cabling: Electricity and Grounding
	8.6.5			Purpose of grounding computer equipment
The purpose of connecting the safety ground to the exposed metal parts of the computing equipment is to prevent such metal parts from becoming energized with a hazardous voltage that may occur as a result of a wiring fault inside the device

8.6	Planning Structured Cabling: Electricity and Grounding
	8.6.6	Safety ground connections
An example of a wiring fault, that could occur in a network device, is an accidental connection between the hot wire and the chassis. If such a fault were to occur, the safety ground wire connected to the device would serve as a low resistance path to the earth ground. When properly installed, the low resistance path, provided by the safety ground wire, would offer sufficiently low resistance, and sufficient current-carrying capacity, as to prevent the build up of hazardously high voltages. Also, because the circuit would then directly link the hot connection to ground, any time electrical current passed via this path into the ground, it would cause protective devices such as circuit breakers to activate. By interrupting the circuit to the transformer, circuit breakers stop the flow of electrons, thus reducing the risk of electrocution.

8.6	Planning Structured Cabling: Electricity and Grounding
	8.6.7	Safety ground connection problems
Large buildings frequently require more than one earth ground. Separate earth grounds for each building are required in multi-building campuses. Unfortunately, the earth ground between buildings is almost never the same. Separate earth grounds for the same building can also vary. When ground wires in separate locations have slightly different potential (voltage), to the common and hot wires, they can present a serious problem. To understand this, assume that the ground wire for building A has a slightly different potential, to the common and hot wires, than the ground wire for building B. Because of this, the outside cases of computer devices located in building A would have a different voltage (potential) than the outside cases of computer equipment located in building B. If a circuit were established that linked computer devices in building A to those in building B, then electrical current would flow from the negative source to the positive source. Anyone coming into contact with any device on that circuit could receive a nasty shock. In addition, this errant potential voltage would have the ability to severely damage delicate computer memory chips