What Causes Underground Outages?

Northern Lights lineworkers repair an underground line. While underground lines are less prone to outages, locating and repairing underground faults is time-consuming, especially in winter. Photo by Kristin Mettke

It is easy to understand what causes an overhead power line outage. Heavy winds, trees, ice loading, and broken poles create visible damage. With underground outages, it takes a bit more digging to find the issue.

While the lines are underground, junction boxes, transformers, and other equipment are often above ground. Above-ground equipment is susceptible to damage by snowplows and vehicles. Sometimes these can be easily replaced, and the cable connected back in place in a new box. However, if hit hard enough, there may be damage to the cable on both sides of the box.

Another human-caused issue is a dig-in. This is when someone is digging in the ground and hits and damages an NLI cable. This can easily be prevented by making a call to the 811 Call Before You Dig hotline and requesting a free locate in advance.

On occasion, we have had rodents or bugs build nests in equipment or cabinets. This can cause faults in the equipment.

Underground cable does not last forever. After it has been in the ground for 20 to 30 years, it becomes susceptible to cracks in the insulation. With the right combination of moisture and cracking, a fault can occur.

Don’t forget that NLI underground lines are supplied power from overhead lines. If the overhead line has an outage, the downstream underground line also will be out.

While underground lines tend to be more reliable and less likely to experience an outage, they generally also have a higher installation cost than an overhead line. Moreover, when there is an outage, the problem is often 3 feet or more below ground. Our lineworkers cannot easily fix what they cannot see.

To find an issue on a buried cable, lineworkers use a thumping device to find failed underground line sections. This device connects to the failed line and sends signals over the line to find the fault’s location. Once the general area is located, it’s a matter of digging up and finding the damaged section of cable. Snowy conditions and frozen ground add to the amount of effort needed to get to the fault.

Once repairs are made, everything needs to be put back together, which can be time-consuming.

Underground lines are a trade-off between fewer outages and time-consuming repairs. Ice and wind aren’t factors as they are for overhead power lines, but rectifying any problem can be time-consuming and expensive.

Kristin MettkeNorthern Lights Engineering & Operations Manager Kristin Mettke is an electrical engineer and has worked in the electric utility industry most of her career.