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| 3.1 |  | Characteristics of Physical Layer Failure Problems |
| 3.1.4 |  | Noncritical characteristics – power failures |

A major category of physical layer problems which may be encountered are power related. If power is not supplied to each network device within specific tolerances, then either a loss of service or damage to the unit may occur. Either the device will not have sufficient power to function, or it may be damaged, if the power fluctuates above the design specifications for that device. These fluctuations could include the following:

Power failure
A power failure is characterized by a loss of power. Incoming power is subject to blackouts, which are complete power outages, often caused by downed power lines or electrical failures.

Power spike
A power spike is a short burst of excessive power, usually lasting less than 1/60 of a second. Unprotected electronic equipment is vulnerable to this sudden, potentially massive, increase in voltage. Power spikes are similar in nature to power surges, which can last as long as several seconds.

Brownouts
Brownouts are in-line power reductions of ten percent or more. They are usually caused by utility company problems or a sudden drain of electricity from a particular part of the power grid.

Dirty power
Dirty power is caused by electrical circuits experiencing transients and noise. Transients are brief high-speed electrical fluctuations caused by lightning or improper grounding. Noise is electromagnetic or radio frequency interference in the power signal caused by disruption from the external power grid or by feedback from local mechanical devices such as printers and copiers.