Every now and then you expect to get shocking news while on the telephone. Receiving a high-voltage electric shock while on the phone, on the other hand, is something few of us expect and even fewer experience. But that's just what happened to an unlucky man in an Eastern city after he dropped his 35 cents in an electrically charged public telephone.
After witnesses found the victim unconscious near the phone, local utility representatives arrived on the scene and discovered the potential for the powerful shock between the telephone stand and a nearby sidewalk grille. Subsequent to sustaining brain injuries that prevented him from continuing his career as a teacher, the victim filed suit against the utility and hired my forensic engineering firm to investigate the accident.
Our initial investigation uncovered the electric utility's report that a sump pump in a network transformer vault beneath the sidewalk grille had failed prior to the accident. At the time of the accident, the 1,000kVA underground utility network transformer was fed by a 13.2kV primary cable, which supplied 3-phase 120/208V power to the utility's low-voltage street network.
When we investigated the vault, we found a sump pump in one corner of the 200-sq-ft room. The sump collected water that accumulated in the vault and pumped it out of the area.
Typically, a utility installs a circuit breaker panel in the vault to provide single-phase 120V power for the sump pump. Then it runs a branch-circuit from the circuit breaker to the sump pump. The installation in question was not set up this way, and it violated applicable electrical codes.
We found the electrician in charge of the initial wiring had scraped the insulation off one of the 750-kcmil, 120V secondary cables and taped the bare end of a No. 16 insulated wire on the bare spot of the cable's conductor. Two No. 16 wires ran between the sump and the neutral bus in the vault and served as the 120V return and grounding wires. The sump's cast-iron discharge pipe ran through the wall of the vault to the street. The sloppy wiring created the potential for contact between the taped wire/conductor connection and ground.
In addition, we reconstructed the accident and concluded it took place after the sump pump motor winding burned out and brought the motor windings in contact with the metal parts of the motor. As a result, the pump had to apply 120V across the motor terminals to activate the pump's float switch, and the fault current from the motor winding to the motor burned through the grounding wire, which left the sump pump ungrounded. Now energized by the winding-to-frame contact, the sump pump had a voltage potential of 120V. This energized the metal discharge pipe that passed under the telephone to the same potential as the sump pump. As a result, the pipe energized the telephone stand. Although a portion of the sump pump's charge dissipated before reaching ground level, the telephone stand was still tested at 80V at the time of the accident.
The re-creation proved our client received the shock when he touched the telephone stand while his feet were in direct contact with the grounded metal sidewalk grille.
Our subsequent review of the electric utility's standards drawings for below-grade network transformer vaults correctly showed a panel with a circuit breaker and an outlet for installation of a sump pump, leading us to believe the installing electrician had not followed the plans properly. Our investigation and affidavit describing the improper installation led to the court granting the victim his motion on liability. As a result, he proceeded to a jury trial on damages and won an equitable award.
Kusko is vice president of Exponent Failure Analysis Associates, Natick, Mass.