8.10				Network Power Supply Issues: Power Line Problems
				8.10.1								Power problem classifications
There are three wires in a power cable, and problems that occur in the cable are labeled according to the particular wire(s) that are affected. If a situation exists between the hot and neutral wire, this is referred to as a normal mode problem. If a situation involves either the hot, or neutral wire, and the safety ground wire, it is referred to as a common mode problem. As shown in the figure, the explanation of the power problem code is as follows. In the first line the brown dot indicates that the ground wire is not connected. In line two, the brown dot indicates that the neutral wire is not connected. In line three, no dot is indicated, showing that the hot wire is not connected. In the next two lines the blue and white dot indicate which lines are reversed, and the final line indicates that the line has no power connections problems.
8.10			Network Power Supply Issues: Power Line Problems
			8.10.2								Normal mode and common mode
Normal mode problems do not, ordinarily, pose a hazard to you or to your computer. This is because they are usually intercepted by a computer's power supply, an uninterruptible power supply, or an AC power line filter. Common mode problems, on the other hand, can go directly to a computer's chassis without an intervening filter. Therefore they can do more damage to data signals than normal mode problems. In addition, they are harder to detect.
8.10		Network Power Supply Issues: Power Line Problems
		8.10.3			Typical power line problems
Unwanted voltage that is sent to electrical equipment is called a power disturbance. Typical power disturbances include voltage surges, sags, spikes, and oscillations. Another situation that can cause power problems is a total power loss. Surge A surge is a voltage increase above 110% of the normal voltage carried by a power line. Typically, such incidents last only a few seconds; however, this type of power disruption is responsible for nearly all hardware damage that computer users experience. This is because most computer power supplies that run at 120 V are not built to handle 260 V for any length of time. Hubs are particularly vulnerable to electrical surges because of their sensitive low voltage data lines. Sag/Brownout A sag is a brownout that lasts less than a second. These incidents occur when voltage on the power line falls below 80% of the normal voltage. Sometimes they are caused by overloaded circuits. Brownouts can also be caused intentionally by utility companies seeking to reduce the power drawn by users during peak demand periods. Like surges, sags and brownouts account for a large proportion of the power problems that effect networks and the computing devices that are attached to them. Spike A spike is an impulse that produces a voltage overload on the power line. Generally speaking, spikes last between .5 and 100 microseconds. In simple terms, when a spike occurs it means that your power line has momentarily been struck with a powerful hit of at least 240 V. Oscillations and Noise Oscillations are also sometimes referred to as harmonics, or noise. A common cause of oscillation is an excessively long electrical wiring run, which creates an antenna effect.
8.10		Network Power Supply Issues: Power Line Problems
		8.10.4								Sources of surges and spikes
There are numerous sources of electrical surges and spikes. Probably the most common one is a nearby lightning strike. Through induction, a nearby lightning strike can affect data lines. Utility switching operations performed by the local power company can also trigger electrical surges and spikes. Other sources of surges and spikes can be located inside your school, office, or building. For example, when equipment such as elevators, photocopiers, and air conditioners, cycle on and off, they create momentary dips and surges in power.
8.10		Network Power Supply Issues: Power Line Problems
		8.10.5							Surge and spike damage
A spike or a surge can wreak havoc on any type of sensitive electronic equipment, including networking devices. Consequences of electrical surges and spikes can be severe. Possibilities include the following: lockups loss of memory problems in retrieving data altered data garbling Protection products can save your data equipment from damage caused by direct contact with lightning, power lines, or electrostatic discharge. Primary protection devices are designed to protect people and buildings and are usually installed on the regulated side of a network by the local exchange carrier. Primary protection activates when lightning strikes, power lines cross, or when other situations that create high voltage occur, triggering the device to divert the surge energy to ground. However, primary protection devices do not respond fast enough and their clamping levels are not exact enough to protect today’s sensitive electronic equipment. Secondary protection installed behind primary protection will stop any damaging surges or currents that get past your primary protection. To protect the system equipment from surges introduced between the building entrance and the system equipment, install the inline surge protector between those two points and as close as possible to the equipment being protected. To protect the system equipment from surges introduced between the system equipment and the work area, install the inline surge protector between those two points and as close as possible to the equipment being protected. To protect the work area equipment that is connected to the Local Exchange Carrier (LEC), Campus Backbone Cabling or System Equipment. If the work area equipment operates over more than one-pair, install the inline surge protector as close as possible to the equipment being protected.
8.10		Network Power Supply Issues: Power Line Problems
		8.10.6						Surge and spike solutions
A common solution to the problem of surges and spikes is the use of surge suppressors. Theoretically, when surges or spikes come in, surge suppressors divert them to ground. In actual practice, however, it has been found that spot placement of surge suppressors can increase the incidence of electrical problems. For example, if equipment is not properly grounded when a surge suppressor channels a surge to ground, it actually elevates the ground potential. The resulting differences in ground voltages can create electrical current that flows in the ground circuit. Current flowing in a ground loop can damage non-protected devices; therefore, in any LAN installation, a good rule of thumb to follow is to protect all networking devices with surge suppressors. If your network is attached to a telephone line for modem or fax use, it is important that the telephone line be surge protected also. This is because lighting strikes to telephone lines are not uncommon. Even lightning spikes across the telephone lines to unplugged networking devices have been known to destroy components. As a general rule therefore, consider the telephone line to be part of the network. If you protect one networking device with a surge suppressor, then you should protect all devices, including the telephone line, in the same way.
8.10		Network Power Supply Issues: Power Line Problems
		8.10.7					Sag and brownout solutions
While surge suppressors can help resolve problems presented by surges and spikes, they cannot prevent the occurrence of sags and brownouts. A drop in AC power may cause only the faintest flicker of your electric lights; however, the same drop in power can be devastating to your data. This is especially true if you happen to be updating a file directory when a power failure occurs. Such a brownout could cause the directory, all subdirectories, and files along its path, to be lost. While the threat of power outages can be minimized by keeping current backups of all data, this measure will not prevent the loss of working files that are open on network computers. Every network should have some type of uninterruptable power supply.
8.10	Network Power Supply Issues: Power Line Problems
	8.10.8					Oscillation solution
The best way to address the problem of oscillation is to rewire. Although this may seem to be an extreme and expensive solution, it is probably the only reliable way that you can ensure completely clean, direct power and ground connections.