When an electric boiler control system fails for the fifth time, an electrician sustains sever injuries.
When the electrically powered boiler that provided heat for a 17-story bank in Topeka, Kan., shut down one winter day, the building manager immediately called the electrician who typically serviced that system to troubleshoot the problem. Shortly after opening the doors to one of the two control cabinets, the electrician was severely burned by electrical arcing that destroyed the control panel. Even after a four-month recovery, his injuries still limited certain activities. That was March 1973. Why cover a case that happened almost 30 years ago? Because this incident demonstrates a timeless lesson for each new generation of engineers and electricians: Proper electrical testing is just as important today as it was decades ago, and a lot of electrical equipment installed in the mid to late '60s is still around for engineers, contractors, and plant facility personnel to troubleshoot, service, and maintain.
After the accident, a personal injury lawsuit followed. The principal defendant was Manufacturer A (electric boiler system). The plaintiff's attorney called Manufacturer B (limit switch) as a codefendant on the assumption that it had provided a defective limit switch that caused the accident. Manufacturer B's attorney retained me to assist in its defense. To put the accident into perspective, let's take a look at the system specifications and operating conditions surrounding this particular piece of equipment to better understand why this accident occurred.
Designed and installed in the late '60s, when utilities offered special low rates to users who installed “electrically fueled” heating systems as a way to boost their base load, the electric boiler system provided supplemental heat for this office tower during the winter when the amount of recycled waste heat from the facility's cooling unit wasn't sufficient to heat the building.
Two 480V, 3-phase 1,500A feeders supplied power to separate control cabinets. A 1 kVA, 480/120V transformer provided the control power for the system via a transfer switch connected to the 480V feeder. The boiler contained 225 resistance heater units rated at 10kW each. These were connected in 75 delta loads for a total load of 2,250kW. A 3-pole contactor controlled each delta load. Photo 1 shows one of four panels that housed these contactors. A 20-step controller from Manufacturer B (Photo 2) operated the contactors in groups of three or four to modulate the heat rate for the boiler system to match the required heating load. If the temperature of the hot water deviated from the specified set point, the step controller would increase or decrease the heat rate accordingly. With sensors to shut down the unit in the event of a low-water or over-temperature condition, the boiler was designed to “reset to zero heat rate” in case of a shutdown.
After digging back a little further into the boiler control system's records, I uncovered a curious detail. Installed in 1969, the boiler had burned down four times prior to the incident in March 1973 — in fall 1969, fall 1970, fall 1972, and January 1973. Each time the system burned down, the original installing electrical contractor rebuilt the unit, and the boiler manufacturer sent a representative to the site.
In fall 1973, while building the system for the fifth time, the electrician discovered the “reset to zero heat rate” circuitry wasn't functional. A normally closed switch was intended to open when the step controller was moved off of the zero heat rate position. Under this circumstance, if the control voltage was shut down, the recycle relay would open and prevent control voltage from being applied to the contactors controlling the heating elements. With the recycle relay open, the step control unit would return to the zero heat rate position when power was turned back on. The arm of the limit switch failed to bear on the cam. Therefore, the normally closed contacts never opened, and the recycle relay was always energized when control voltage was present. The electrician used a limit switch with an arm that properly contacted the cam to replace the original. Armed with the facts from the past, it was time to investigate the circumstances surrounding the incident.
After thoroughly testing the original limit switch, I found it to meet all of the operational specifications of Manufacturer B for that particular component. I calculated the inrush current of the 3-pole power contactors to be 0.5A. Thus, if all contactors were simultaneously energized, the 1kVA control transformer was loaded to 375%.
Standard open-circuit/short-circuit testing methods determined the impedance of the control transformer referred to the low side as 0.879 ohms. If 76 contactors were energized simultaneously and the available control voltage was 110V, then the voltage at the coil of the contactors would be 76V, which is 12V less than the “pull-in voltage” for these contactors. Thus, for some heat rate setting less than full, the contactors would chatter rather than pull in solidly. This chattering would cause arcing, arcing would create ionization of the air, and ionization of the air would initiate ground-fault arcing from some of the components energized at 480V. The appearance of the burned cabinets is consistent with this theory.
Manufacturer A was unable to provide any credible evidence of written test procedures and/or detailed test data that were applicable to this product, nor could it produce any written test procedures or data that were applicable to the recycle relay subsystem. Although representatives of the electric boiler manufacturer had been on-site during each of the five builds, they never recommended testing the recycle relay subsystem for correct operation. Engineers from Manufacturer A, who designed the electric boiler control system, were the ones who specified the cam and the limit switch associated with the recycle relay subsystem, which were added to the step control unit. Based upon the findings, Manufacturer B's attorney filed a motion for summary judgment to remove his firm as a defendant in this case. The judge granted this motion in late 1979, more than six years after the accident.
Although it happened almost 30 years ago, this case demonstrates a valuable lesson for all electrical professionals. Overlooking regular electrical testing can prove to be inconvenient, costly, counterproductive, dangerous, and sometimes fatal. In this case, Manufacturer A could have prevented the accident if it had performed a few simple tests, including:
- Testing the prototype unit to verify correct operation of the recycle relay subsystem. If this had been done, someone would have clearly discovered the incorrect limit switch — regardless of a possible design or manufacturing error.
- Testing each production unit to ensure they met all functional specifications. This test would have prevented the first accident.
- Prescribing, in writing, complete functional testing of the rebuilt electric boiler control system after it was rebuilt on-site following the first accident. This test would have prevented the second accident.
Rummer is an engineering consultant and expert witness in courts throughout the central United States.