A true story of the not-so-well grounded
The first part of power qualityis monitoring what is happening and determining why; the second part is finding a solution that eliminates the problem and doesn’t create a new one. Sometimes, the first part can be done even without a power quality monitor. The following photo essay is such a case.
In Virginia, just south of Washington, D.C., the evening of June 23 started out as a typical summer evening, with the threat of thunderstorms. The storm started around dusk and lingered in the area for at least six hours, producing a spectacular display of one of nature’s strongest forces.
The residents of a two-story colonial house were awakened abruptly around 2:30 a.m. by an extremely bright flash, a loud crack and the alarm system. Lightning had struck a tree at the rear of the property, blowing the bark off from 30 feet high down to ground level. In less than a millisecond, the lightning damaged the surrounding area and along the path from the tree to an outdoor outlet box.
The electricity traveled from the tree, under the fence, stepping stones and stone retaining wall to couple into the power system. Through the underground conduit powering the pump in the pond, lightning entered the house. The surge traveled through a series of basement outlets to the security system transformer and the breaker panel, tripping several breakers. The power surge coupled into the telephone lines through the security system’s circuit board, destroying the DSL filters and modem.
As it traveled through the ground, the moisture in the dirt boiled and caused an explosion, throwing the dirt and rocks from the retaining wall up against the fence. Several 1-inch-thick fieldstone slabs also shattered, propelling fragments into and over the fence.
What was roughly 10 to 100kV impulsive transient severed the line or “hot” conductor of the power wires for the pond pump while it destroyed the plastic sheathing. Although the outdoor outlets and the indoor outlet where the conduit cord plugged into were all ground-fault circuit interrupter (GFCI) outlets, they could not handle such a high-energy, sub-millisecond wide pulse.
Several halogen floodlights attached to the house on the lightning strike side were shattered, sending glass flying. Incredibly, the bulbs still worked despite the shock wave that knocked the protective lens cover out.
The outlet to which the cord from the waterfall pump in the backyard pond was connected was melted, deformed and fragmented.
The extension cord connecting the cable in the conduit to the GFCI outlet in the basement was only 4 feet in length and heavy enough to carry 15 amps. However, the plug and outlet suffered heavy arching, leaving both heavily carbonized and pitted. The heat generated was enough to melt the metal on all three prongs of the cord, indicating that a short circuit involved the hot, neutral and ground conductors. This is just one of the several flashovers that occurred at various outlets in the basement, which resulted in the breakers tripping.
Given that the flash-over voltage of a 120V outlet is approx 6kV, the energy level of the lightning-induced transients was still quite high after traveling from the tree into the ground, into the cable and into the house circuits. Since all of the outlet boxes in the basement area were plastic, the difference in the path that the
lightning-induced transient traveled through the house if they had been grounded
metal boxes is unknown.
A double switch box in the path between the conduit entry point and the circuit breakers suffered burns but continued to function after the corresponding breaker was reset. Both switches were in the off position at the time of the strike.
The outlet adjacent to the entry-point outlet suffered heavy burns. However, since nothing was plugged into the outlet at the time, only minimal carbonizing is visible on the surface. An air duct attached on the back side of the wall was screwed in between the Sheetrock and stud that the outlet was attached to. The arcing between the metal of the outlet in the plastic outlet box and the attachment of the air duct was enough to exploded a section of Sheetrock, sending fragments several meters. There were several outlets on the basement circuit where equipment was plugged in and was damaged, including a TV and VCR.
On the first floor of the house, additional equipment was damaged, including a toaster oven. Many types of equipment aren’t really turned off anymore when the power switch is “off,” such as televisions and microwaves. The fact that a clock is running and/or that it can be started by remote control means that there is a continual connection between the outlet power and the electronics inside the piece of equipment.
The 120:16V AC step-down transformer for the security system also suffered heavy burns. The energy passing through the transformer to the secondary side was enough to sever one of the internal wires and blow the plastic cover off.
From the AC power connection, the voltage potential was still high enough for it to arc to the metal box and then re-enter the circuit board at the phone connection side. At least one capacitor exploded on the circuit board and several integrated circuits were undoubtedly destroyed, causing the system to malfunction.
The arc back into the security system at the phone connection allowed the high-voltage transient to travel through the phone system to all the phones in the house and fused wires at the outdoor phone connection. Because of the DSL Internet connection, all phone connections were attached to DSL filters to block out the high-frequency noise. The filter attached to the phone in the basement exploded the most violently. The plastic cover was shattered and heavily carbonized. At least two capacitors on the filter board exploded because of the boiling liquid inside.
The moral of this story is that lightning, like any electrical circuit, will follow the path of least resistance, though it has the potential (1MV+) to “jump” gaps that normal 120V AC circuits cannot and the ability to couple into systems, such as the phone lines, that aren’t designed to handle the voltage and fault current levels that lightning can produce. Well-grounded, UL-listed surge protection should be installed on any metallic interface that extends outside of the structure, including the incoming electric service, phone lines and any exterior branch circuits. Clamping the voltage to non-destructive levels and giving the fault current a preferred path of least resistance are crucial to preventing significant equipment loss, as well as a potential fire. EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680. Scott Bingham is pursuing a master’s degree in engineering at Cornell University and can be reached at firstname.lastname@example.org.