An evaporative cooler, also known as a “swamp cooler”, is an air conditioner that works by evaporating water. A float valve keeps several inches of water in the bottom of the unit. A pump takes water from the pan to the top of a series of vertical pads made of absorbent materials like wood fibers. The water flows by gravity through the fiber pads. A fan pulls hot air from outside the house through the soaked pads. Water in the pads evaporates, cooling the air and increasing its humidity. The cooled and humidified air is blown back into the house. Evaporative coolers need to be cleaned periodically.
In a recent case, a homeowner and accomplished guitar player was injured while attempting to clean his window-mounted evaporative cooler. He did not turn the unit off or unplug it before going outside and removing the back panel. Removing the back panel exposed the fan pulley and vee belt inside the evaporative cooler. While he was cleaning the unit, the machine started. When the motor started unexpectedly, the in-running nip point between the motor pulley and a vee belt cut off the tips of three fingers on his right hand, a “serious” injury by risk assessment standards. In addition to his pain and suffering, his guitar playing ability was greatly affected.
The seriously injured homeowner hired an attorney and sued the manufacturer of the swamp cooler. In order to prevail in a lawsuit of this type, the plaintiff must prove four things:
- A defect existed;
- The defect existed when the machine was manufactured;
- The activity being performed was foreseeable; and
- The defect was the cause of the injury.
The plaintiff theorized that the evaporative cooler was unreasonably dangerous and therefore defective. The claimed defects included:
- A motor that could overheat, stop, and start again without warning when it cooled;
- An unguarded fan motor drive; and
- A set of exterior panels that were not interlocked with the electrical circuit that powered the fan motor.
The plaintiff’s theory claimed that had the motor drive not restarted without warning, had the fan belt drive been guarded, or had the exterior panels been interlocked so that the drive could not start with the panels removed, then the injury would not have occurred.
The injured homeowner was hurt by a hazard in the evaporative cooler. A hazard is a source of harm. Most machines contain hazards. Designers try to minimize risks associated with hazards in machines, but some residual risk typically remains. The important question is: Were the residual risks tolerable? When the homeowner opened the rear panel and reached into the plugged-in-and-turned-on machine to clean it, there was an uncontrolled hazard and a risk of serious injury. Typically, a machine that contains an uncontrolled hazard is unreasonably dangerous and therefore defective when the residual risk is too high to be tolerable.
I testified on behalf of the machine manufacturer in a trial. I said that the manufacturer designed the machine properly. I said there is no need to interlock the guard because the manufacturer instructed the homeowner to turn the unit off and unplug it. The motor could not have started unexpectedly if the machine were turned off and unplugged. The manufacturer made over 2.3 million evaporative coolers, but no one had previously been injured while cleaning one. The probability of occurrence was certainly low.
The manufacturer made the evaporative cooler easy to turn off and unplug. The manufacturer warned users to turn the unit off and unplug it before cleaning. The manufacturer attached the panels to the unit with screws so they cannot be removed without a tool, in compliance with ANSI B15.1, the American National Standard for Power Transmission Apparatus. In my opinion, the machine was properly guarded when the panels were in place and the residual risk was low and tolerable. In my opinion, there was no reason to clean the unit while it was plugged in and would be unreasonable to do so. Importantly, the unit only needed cleaning every few months, much less often than the once-per-shift requirement of ISO 14120, the international standard for interlocked guards. Interlocking the exterior panels was technologically feasible, but was not required by any standard. The infrequent need to remove the panels, the manufacturer’s warnings, the need to use tools to open the panels, and the ease of unplugging the machine combined to reduce the residual risk of the in-running nip point to a tolerably low level without resorting to interlocks. The Arkansas Federal Court jury agreed and produced a verdict for the defense.
Jeffery H. Warren, PhD, PE, CSP, is the chief engineer and CEO at Warren specializing in mechanical, machine design and safety. His deep expertise in machine design and safety analysis makes him a frequent presenter, trainer and expert witness. In addition to investigating more than 2000 claims involving property damage and injuries related to machinery and equipment since 1987, Jeff has an undergraduate degree in Mechanical Engineering from the University of North Carolina as well as a Master of Science and a Doctorate in Mechanical Engineering from Virginia Polytechnic Institute and State University — both with machine design emphasis.