But Why? Understanding When Things Go Wrong

By Richard P. Bingham | Feb 15, 2017




When I arrived recently at a residential construction site to build a covered porch, the homeowner was busy cutting a tree away from the power lines to the house with a chain saw. It could have led to disaster, but his chain saw skills and carefully attached ropes dropped the tree away from the wires. He walked over with a section of the wires that had previously run to the house service from the utility transformer secondary. The evil in his hand was the source of destruction of numerous electronic and other electrical loads in the house earlier in the week.

The utility crew that replaced the wires had explained that there was a break in one of the conductors caused by the tree contact. It resulted in the damage to the equipment in the house. The question: “But why didn’t the main breakers open to protect my equipment?”

Trying to draw air schematics with my fingers didn’t really convey the explanation very well. The figure above shows more clearly how the 13-kilovolt (kV) distribution feeders are converted down to the 240-volt (V) split-phase circuit that comes into the house through the grounded conductor and two-phase conductors. Whereas the wye circuit has each phase 120 degrees apart from each other, the voltage on each phase conductor in a split-phase circuit is 180 degrees out of phase with the other.

Most loads in a residence are fed from one of the two 120V circuits that are derived from the 240V by the midpoint voltage reference resulting from the grounded (neutral) conductor. Current from each 120V leg sums in the neutral, which, in the ideal world, cancels each other out because of the 180-degree phase shift. Some larger loads are powered across the two-phase conductors, 240V. A grounding electrode is driven at the service entrance, which also comes into the breaker panel and is bonded to all the equipment grounding wires from each circuit in the house, along with each neutral or grounded conductor. Zng is ideally a low impedance, less than the 25-ohm value. Only very small amounts of leakage current should be flowing in the grounding conductor under normal circumstances.

Back to the damaged equipment and why the main breakers didn’t protect it. The residence was located in a sandy area, so the impedance (Zng) of the ground rod was probably relatively high. National Electrical Code Article 250.56, Resistance of Ground Rod Electrode, states, “When the resistance of a single ground rod is over 25 ohms, an additional electrode is required to augment the ground rod electrode, and it must be installed not less than 6 feet away.”

This house had one ground rod, but it is unlikely the test was done at ­installation—and certainly not since—to check the impedance. When the neutral conductor broke, the return path to the utility transformer was no longer low impedance. Without a good ground reference, the neutral-to-line voltage at the dwelling’s various receptacles was no longer 120V relative to ground on both sides. Based on current flows from the loads and impedances on each phase, the “mid-point” reference of the neutral is no longer in the middle of the 240V. One phase could be 150V and the other 90V, but still 240V phase-to-phase. Equipment powered by such voltage is likely to be damaged.

Also, most electronic equipment has a three-prong plug, with some components within the equipment referenced to the safety ground. Without adequate external or built-in surge suppression, semiconductor and capacitors can go “pop-pop-pop” in the presence of excessive voltage on the phase conductor. Unless the protection is properly designed to open the circuit when the protection devices get abused for too long, things will still go pop.

The homeowner still wanted to know why the main breakers didn’t operate to protect the equipment. Breakers (or fuses in the service panel) are designed to protect the wiring from being damaged from excessive current causing overheating, insulation melting, conductor failures and subsequent fire. The I2*t curve requires an overcurrent condition of a particular level to be exceeded for a specified time to trip. The higher the current, the faster the trip. Even so, the equipment could be damaged in milliseconds before the breaker operates.

Fuses within equipment are often an indicator that the equipment was damaged rather than protected, as failure of components inside the equipment often result in the overcurrent condition that blows the fuse. The main 100 ampere (A) breakers never saw enough current to trip. It was undetermined if individual circuit breakers tripped, but if a 15A or 20A breaker experienced enough current to trip, you can bet the TV or dishwasher or sensor spotlight electronics had already been damaged. The homeowner did switch off the main breakers after determining that something was wrong, but the damage had been done.

This wasn’t a one-off occurrence. Another homeowner had a similar neutral conductor failure that resulted in loss of the furnace and other equipment during a snowstorm. In October 2006, I covered the story of a phase conductor open-circuiting and the strange voltage phenomena that occurred for hours to houses fed from that transformer.

Loss of a phase in industrial/commercial facilities can wreak havoc on three-phase loads, such as electric motors. All conductors’ integrity is critical to safe operation of a facility, even the innocently named “neutral” conductor.

About The Author

BINGHAM, a contributing editor for power quality, can be reached at 908.499.5321.

featured Video


Vive Pico Wireless Remote

The Pico wireless remote is easy to install, it can be wall-mounted or mounted to any surface, and includes a ten-year battery life. See how this wireless wall control makes it simple to add lighting control wherever you need it.


Related Articles