It seemed like an ordinary morning, as the dental clerk began her office startup routine. Yet when she tried turning on the motor-driven compressor from the generic wall switch, she heard a loud bang, felt a shock, and saw a flash that burned her right index finger. The shock left her with a severe and debilitating nervous dysfunction.

California workers' compensation laws shielded her employer from lawsuits. However, the owner of the building had liability exposure for maintaining an unsafe facility. It so happened that one of the dentists employed by the dental corporation was also the building owner. Her original worker's compensation attorney failed to pursue the building owner within the one-year statute of limitations, possibly denying her additional compensation. As a result, she hired a new attorney to sue her original attorney for errors and omissions (malpractice) covered by his liability insurance policy.

To plead this case, her attorney hired me to prove the building had an electrical defect and that the owner, builder, and/or designer was responsible. Ironically, the dentist who owned the building also partnered in the design and construction!

Although the switch was discarded shortly after the event, it was probably a 20A, 277V, heavy-duty, 2-pole, wall-mounted unit. The original general contractor returned to investigate the incident and later testified that the switch was black and red plastic. After having drilled the rivets to open the switch, he claimed there were no signs of damage, inside or outside. No one remembered who manufactured it. There was also conflicting testimony regarding possible blackening on the building's wall or the switch cover plate.

Her present attorney gathered a team of credible doctors to attest to her injuries. We had to substantiate that testimony with a plausible explanation: The physics of the motor-driven compressor, the switch, and electricity could combine to produce a burn and shock—even if the switch may not have shown marks or damage.

Although the motor-driven compressor had long since been replaced, the dentist remembered the manufacturer and the model number. A call to the factory revealed it carried a rating of 19A at 240V with no horsepower rating. The case rested on the adequacy of a 20A switch to control the motor load when evaluated against the NEC.

Sec. 430-81 of the NEC requires suitable controllers (the switch in this case) for all motors. Sec. 430-83 requires the controller to have a horsepower rating not lower than the horsepower rating of the motor. The defense tried to use Exception No. 1 to Sec. 430-83 (for motors 2 hp or less) that allows general use AC snap switches, as long as they have an ampere rating of at least 80% of the motor's full-load current rating. (Note: Non-snap and AC-DC switches do not satisfy the exception.)

However, Sec. 430-6(a)(1) states in part: "Where a motor is marked in amperes, but not horsepower, the horsepower rating shall be assumed to be that corresponding to the value given in Table 430-148, interpolated if necessary."

In this case, the Table equated 17A (at 230V) with 3 hp. The 19A-compressor motor was then, by interpolation, greater than 3 hp and not covered by the Sec. 430-83 exemption relating to 2 hp or less. Thus, the motor required a controller rated for more than 3 hp.

At trial, the defense was unable to produce a black and red switch from any manufacturer with a horsepower rating stamped on it equal to that which the motor required. We never considered the switch defective. The jury found the installed switch had an inappropriate rating, thereby causing the dental office building to have a dangerous electrical system design defect. These decisions meant a ruling against the original attorney, awarding the plaintiff a sum greater than $2 million.

Remember: Give serious consideration to the required horsepower rating of motor controllers, even if they are just simple snap switches.

Crawford is a Forensic Electrical Engineer and Contractor in Palo Alto, Calif.