Why was my power off when there were no storm emergencies?

Power outages happen for a wide variety of reasons. Many of these are compounded by storm events such as high winds or heavy rains, while others are not related to the environment. Below is a list of some of the more common causes of electrical outages.

Tree Damage: Trees can conduct electricity at the high voltages at which TUA's distribution system operates. Any time a tree limb falls from a tree and lands across two or more of the wires on an overhead power pole an outage will occur. Obviously, this happens more during storms, when the winds and rain break limbs from the trees. We limit the amount of damage caused by trees with a vigorous trimming program, but in Tullahoma trees are likely to always be a problem for power reliability.

Lightning: Lightning causes electrical outages through several mechanisms. First, lightning can induce large currents on the electric system that blows fuses as it is taken to ground by the system's lightning protection. Second, occasionally, poles or other facilities are hit and destroyed by lightning. Trees that are hit by lightning also frequently fall into electric lines.

Animals: Tullahoma is a very beautiful place to live, and the environment lends itself to proliferation of various animal species. Unfortunately, these animals sometimes find their way into harms way with the electrical system. Most commonly, squirrels like to climb on the poles and use the wires as highways between trees. These animals frequently contact energized parts of the system. The resulting short circuit will blow fuses on the line and cause a power outage. We have also had occasional problems with snakes, opossums, skunks, cats, ground hogs and birds. As a note, we have several systems in place to limit the amount of animal involvement in the system, but even the best state of the art systems are not foolproof.

Accidents: Frequently automobile accidents involve utility poles. If the pole is broken, the wires can come down. Even if the pole is not broken, the wires on the pole may clap together. Either of these events causes electrical outages. Note: Even though the system is designed to cut off current to a line that falls to the ground, you should never assume this. Treat every downed line as though it is live-stay clear!

Mechanical Failures: The electric system is made up of a large number of different components. These include breakers, controls, and simple wire connections. These components can and do fail on occasion. Like many other systems, the wires and connections see their highest stress during peak loading conditions. This means that they are more likely to fail when it is extremely hot or extremely cold.

Planned Events: TUA routinely works with 13,000-volt systems energized to minimize inconvenience to our customers. This said, we will not undertake any job "hot" if the crew on site considers the work too dangerous. If a job affects only a small area, the crew will knock on each door to attempt and notify customers that their power is going to be interrupted. For larger areas, the newspaper is notified in advance, and a notice of the upcoming work is announced. We attempt to schedule work where it will cause the least inconvenience to our customers, including late night outages. For persons with a life support need, TUA will attempt to notify them of an upcoming outage by telephone or post card. We cannot however, notify persons of unplanned outages (else we would take steps to prevent them!).

Why do my lights blink?

In the answer for the first question, we addressed the major causes of power outages in Tullahoma. Many of those outages are caused by "temporary faults."

A temporary fault is a short circuit caused by a short-lived event. For example, a tree limb that falls onto the lines bounces off about 80% of the time. Likewise, a fault caused by an animal climbing on a distribution transformer is usually cleared (isolated from the rest of the system) by a fuse.

The system is designed to cut off power during such events, but then to check and see if the fault has cleared itself. For example, if a tree limb falls on a line, the circuit breaker will cut the power to the circuit for about two seconds. After that, the circuit breaker will turn back on. If the limb is clear, the power will stay on. If the limb is still there the power will turn off again for two seconds. Depending on the exact location in the system, if the fault stays on the line through three cycles or four cycles, the power is permanently turned off and a repair crew must come, clear the fault, and re-energize the circuit.

For discussion purposes, if your lights blink and then come back on, it is extremely likely that one of three things has happened:

• A tree limb has fallen onto a line and bounced off.
• A squirrel or other animal has been electrocuted and the system has isolated the short circuit.
• A lightning stroke has operated a lightning arrestor, resulting in a temporary fault that has been isolated by the arrestor.

Why do my trees have to be trimmed?

Trees are one of the major enemies of the electric system. Tree limbs can cause short circuits by simply brushing against lines. During snow or ice emergencies, the limbs can interrupt power to thousands of customers.

TUA tries very hard to strike a balance between system reliability and the aesthetic qualities of the community. This is a losing battle. Frankly, on a clear sunny day, any trimming is too much. When the snow or ice is on the tree and the power is off, any amount of trimming was not enough.

If I have underground service in my subdivision, why do my lights go out during bad storms?

There are no totally underground electric areas. Starting at the generating plant, overhead 161,000-volt lines feed the main substations. Overhead 48,000-volt lines feed the distribution substations. In nearly all cases, overhead distribution lines feed to the underground areas.

These lines are susceptible to all of the problems that an overhead line can have. Additionally, the underground lines are actually more vulnerable to lightning. The lightning can enter through the overhead portions and race down the wires until they cause a fault in the underground cable. (As Mr. Murphy would have it, the ruined cable is frequently under a major street intersection.)