Curs PC si internet cap 8.7 Planning Structured Cabling: Cabling and Grounding

8.7 Planning Structured Cabling: Cabling and Grounding 8.7.1 Causes of ground potential problems Instructor Note Another way to visualize the problem of differing earth grounds is presented. Again, they key is that a person or a device can become part of an unintended circuit. In order to understand the conditions that must be present in order for there to be a problem, 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. In this example, the outer casings of the computer devices in building A have a different potential from the outer casings of the computer equipment located in building B. If a circuit were established that linked the computer devices in building A to those in building B, then electrical current would flow from the negative source to the positive source. Theoretically, in such a situation, if someone were to touch network devices with different ground connections, they would receive a nasty shock.In the example outlined above, can you explain why a person would have to simultaneously touch devices with different grounds for a shock to occur? As this theoretical example demonstrates, when devices with different ground potentials are linked in a circuit, they can produce hazardous shocks. In the real world, however, the chances of any such occurrence as that described above, are very slight, because in most instances a person would have to have extremely long arms to complete the circuit. There are situations however, where such circuits can be created. 8.7 Planning Structured Cabling: Cabling and Grounding 8.7.2 Networking devices and dangerous circuits Instructor Note The way that Cat 5 UTP (or any copper-based conductor) causes different earth grounds to be a problem is illustrated. As illustrated in the previous example, the closed circuit produced by your body and the UTP cable would allow electrons to flow from the negative source to the positive source through your body. If the ground wires for the devices in one location have a slightly different potential, to the common and hot wires, than the ground wires for the devices in the second location, the closed circuit produced by the use of UTP cable would allow electrical current to flow from the negative source to the positive source. Anyone touching the chassis of a device on the network would receive a nasty shock. A good way to avoid having current pass through the body, and through the heart, is to use the one hand rule. Simply put, this rule says that you should not use more than one hand at a time to touch any electrical device. The second hand should remain in your pocket. 8.7 Planning Structured Cabling: Cabling and Grounding 8.7.3 Faulty ground wiring problems Instructor Note The scenario where a difference in voltage exists between the network cabling and the chassis of an electronic device is described. Again, the problem is a human becoming part of an unintended circuit. When everything works correctly, according to IEEE standards, there should be no voltage difference between the networking media and the chassis of a networking device. This is because the standards separate LAN media connections from power connections. However, things don't always work as planned. For example, if there were a faulty ground wire connection to an outlet, there would be potentially fatal voltages between the LANs UTP cabling and the chassis of a networking device. To understand the potential consequences of such a situation, imagine what would happen if you were to place your hand on the computer's case, while simultaneously touching an Ethernet connector. It does not require a great imagination to see that by touching both the computer's case and the Ethernet connector, would allow electrons to flow from the negative source to the positive source through your body You would receive a painful shock. 8.7 Planning Structured Cabling: Cabling and Grounding 8.7.4 Avoiding potentially dangerous circuits between buildings Instructor Note The use of optical fiber -- which is electrically insulating (non-conducting) -- is proposed as a way to avoid creating potentially dangerous circuits between building. Since inter-building cabling is typically backbone cabling anyway, and since today most installations choose optical fiber as their backbone medium, this requirement does not present much of a problem. TIA/EIA-568-A specifications for backbone cabling permit the use of fiber-optic cable, as well as UTP cable. Because glass is an insulator rather than a conductor, electricity does not travel over fiber-optic cables. Therefore, when multiple buildings are to be networked, it is highly desirable to use fiber-optic cable as the backbone. 8.7 Planning Structured Cabling: Cabling and Grounding 8.7.5 How fiber optic cable can prevent electrical shocks Instructor Note The use of optical fiber -- which is electrically insulating (non-conducting) -- is proposed as a way to avoid creating potentially dangerous circuits between floors of a building. Since inter-floor cabling is typically backbone cabling anyway, and since today most installations choose optical fiber as their backbone medium, this requirement does not present much of a problem. Most network installers today recommend the use of fiber-optic cable for backbone cabling to link wiring closets that are on different floors, of the same building, as well as between separate buildings. The reason for this is simple. It is not uncommon for floors of the same building to be fed by different power transformers. Different power transformers can have different earth grounds, thus causing the problems previously described. Non-conducting optical fibers eliminate the problem of different grounds. 8.7 Planning Structured Cabling: Cabling and Grounding 8.7.6 Reasons for using UTP for backbone cabling between buildings Instructor Note As if the earth ground issue wasn't enough reason to discourage the use of copper-based media between buildings, another reason is presented. Lightning strikes can more efficiently couple into buildings, their networks, and their power systems if there is a copper conductor between buildings. The lesson is to just use fiber between buildings! While faulty wiring can present one type of electrical problem for a LAN that has UTP cable installed in a multi-building environment, there is another type of problem that can occur. Whenever copper is used for backbone cabling, it can provide a pathway for lighting strikes to enter a building. Such strikes are a common cause of damage to multi-building LANs. It is for this reason that new installations of this type are moving toward the use of fiber-optic cable for the backbone cabling.