There are several layers of protection elements on the electrical system to minimize the area of affected service when a fault occurs. The most common cause of customer outages are fuses and feeder breakers. Fuses usually serve 6-8 residential customers or 1-2 commercial customers. Feeder breakers commonly serve hundreds of customers.
The first and most commonly exercised level of protection is a fuse that is placed on each distribution transformer. A distribution transformer takes energy from a high voltage power line and converts it to a voltage that can be used in a home or business. On average, a distribution transformer provides service for around 6-8 customers. The most common cause of a blown fuse is animals. When a line is shorted with another line or a ground the fuse will disconnect the transformer to prevent harm to the rest of the system. This is usually identified by a loud boom and a loss of power to the customers being supplied by that transformer.
The next level of protection is a feeder breaker in a substation. The city currently has eight substations with a ninth substation currently in its planning stage. A substation distributes the high voltage power that supplies distribution transformers. When a fault occurs between a substation and a distribution transformer, high speed electronics cause a feeder breaker to disconnect that circuit from the rest of the city. Because this disconnects a large amount of customers, a device called a recloser will immediately reenergize the connection to check if the problem still exists. If a problem is still present the line will disconnect again and this process will repeat one more time 40 seconds later before disconnecting a final time of there is still a problem present.
The following graph shows the number of outages over the past year and is separated by the cause of the outage. Notice that the outages due to animals increase when the weather is nicer and weather related outages are more frequent when thunderstorms are more common.
December Outage Sources
12 Month Summary
2012 Average Response Time