Some years ago, a customer called me with a unique problem involving underground feeders. The customer was a superintendent in charge of a multibuilding high school. He explained it was a well­-established older school that had been built approximately 25 years before.

There hadn’t been any major electrical additions or changes to the school since it was built. The layout consisted of a building with the electrical service, electrical distribution equipment and a hot water boiler, among other mechanical and electrical equipment. The classrooms were in multiple buildings connected with covered walkways. Each building was supplied by underground electrical feeders that originated from the distribution equipment in the power and boiler building and terminated in a panelboard in each classroom building. 

After explaining this design to me over the phone, I asked him about the problem he was having. He said that over the past five or six years, the power bill showed an unexplainable increase in electrical power consumption and asked me to start an investigation on the reasons for the increase.

An investigation was afoot 

I had access to several different types of electrical tracking, testing and monitoring equipment, so my assistant and I gathered it up and headed to the school. Upon arriving and meeting with the school superintendent, he gave us a tour. Since this was during the summer, there were no students or teachers, so we had unlimited access. We looked at the panelboards in each classroom building, including removing the dead-front covers from a number of the panelboards, and then eventually migrated to the power building and distribution system to start our investigation. 

We brought an underground wire and cable locator, which could be used to locate electrical lines, communications cables, metal fish tape installed in empty nonmetallic raceway, metallic pipes, invisible fence wires, and outdoor or landscape lighting cables. This locator is designed with a transmitter assembly, a receiver assembly, an inductive coupler (clamp), a built-in inductive antenna and two 8-foot test leads with heavy-duty alligator clips, making it a complete package for locating underground cables. This device can accurately trace the path of electrical cables and determine the depth of the buried cables, conductors over distances of 4,000 feet and at depths of up to 7 feet. 

By connecting the transmitter to the de-energized source of electrical power in the distribution switchboard with the receiver located at one of the panelboards in a classroom building, the handheld tracker operates similarly to a metal one, but it is tracking the signal imposed on the de­­-energized cables for each classroom building. 

The grass held the answers 

After setting this system up, I was outside with the handheld tracker following the signal in the underground raceway. I noticed that the grass along the path I was tracking was yellow, whereas in other places it was green. I immediately contacted the school superintendent, who was in his office, to see if he could explain this phenomenon. I had a fairly good idea about what was happening, but wondered if he had been around during the school’s construction. 

He said nonmetallic underground raceways had been installed. The grassy area between the power building and each classroom building was watered by irrigation, not by a sprinkler system. The underground raceways were nonmetallic, and once a week the grassy area was flooded with irrigation water. 

We also had a megohm (hypot) tester with us, which is a battery-powered DC meter used to determine the dielectric withstand testing of insulation up to 50 kilovolts DC. It can measure leakage current as low as 0.001A and resistance as high as 30 million ohms when testing underground cable. We usually only subject 600V insulation to about three times the AC voltage rating, or around 1,800V, and then only using low current for testing. 

The grassy area had been irrigated two days before, so the earth was still saturated with water. When doing the dielectric testing, we determined that the insulation on most of the underground cables was failing, causing current leakage that was not high enough to trip the circuit breakers. This internal leakage from conductor to conductor acted like an additional load whenever the irrigation water also flooded the raceways. 

Replacing the conductors in each raceway solved the problem, and the power bill returned to the lower consumption rate. Unique problem solved!

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