Mind Your Safety

Entering Outdoor Substations, Part 3 of 8

When you’re in a substation, shorten your stride because of a phenomenon known as ground potential rise: defined by Ohm’s law, current through resistance creates voltage drop. When a substation clears a fault to ground, and the current flows through the resistance of ground, you will have a voltage gradient on the ground from the centre of the station outward. We refer to this as ground potential rise.

It is there for the duration of the event, which could be seconds. This phenomenon makes it incredibly dangerous to do something as simple as plugging into an electrical outlet within a substation, then carrying the cord outside to conduct work. During a fault event, the ground potential difference will follow the extension cord outside the substation, and a worker at the end of that extension cord could be electrocuted.

Numerous problems have arisen over the years with telephone and other communications lines connected into substations that have had fault energy transmitted down their lines during lightning and other fault events.

The reason you don’t want to walk directly under any lines is due to unknown flash protection boundaries. After completing an arc flash study for a client, we discovered high IE in their 138-kV substation. Where the primary lines drop down to the transformer, there is a distance of 12 ft to the ground. Our study showed that the Cat 4 FPB (flash protection boundary) from these lines extended 8 ft, meaning that any person walking underneath those lines, past the transformer, was within the FPB. People had been walking there for years but, now, that common path had to be barricaded.

Do not walk quickly through a substation, and always know where you are; know what the equipment is, where it is, and where it is going. Continuously use your visual senses. Watch for leaks on the ground; transformers and oil circuit breakers will generally ooze leaks around the valves but drips to ground are significant. Constantly sniff for anything abnormal (burning, ozone... anything that is not normal air). Watch for distortion of panels and equipment, heat damage on metal and evidence of soot, and always listen.

When other workers have safety grounds installed, stay away from them—they are part of the protective system, and you don’t want to accidentally disturb them. During a fault, they whip like electrical snakes doing tremendous damage to anything close to them and, should one let go, the end swinging out will come out with hundreds of horsepower of force.

When you are responsible for substation inspections, you can get tremendous mileage from a set of binoculars. Most outdoor substation inspections are conducted during the day. It is wise to occasionally conduct one at night; a set of binoculars will help you identify small discharges that will lead you to a fault of which you had been previously unaware. For the most part, infrared and corona cameras, and partial discharge measurement will find problems your eyes will never be able to see. At the same time, never discount the quality of value of a set of trained and experienced eyes.

Recognize that a well-maintained substation is a controlled system of incredible force that becomes violent when control is lost. Entering a substation on a dry, warm, summer day far away from any buildings or habitation is different than walking into a substation after a storm, or in the winter time, or close to an area with a lot of contamination. (In one of our major cities, when a new highway overpass was built close to a substation, the maintenance frequency quadrupled for insulator cleaning due to road salt in an already humid area.)

Until next time, be ready, be careful and be safe.