A Matter of Safety Interlocks

Poor maintenance and unsafe working practices lead to injuries.

What was supposed to be a routine maintenance job for two workers turned into a near life threatening experience. A mechanical malfunction of electrical equipment coupled with improper working practices and lack of proper personal protective equipment sparked a hazardous chain of events these workers will never forget.

When Bob, a maintenance electrician, and Chuck, a contract field service technician, reported to a remote facility housing ventilation equipment for a highway roadway tunnel, their purpose was to check out an abnormal vibration on a medium-voltage 1500-hp motor powering a ventilation fan. The motor's 4.16kV full-voltage across-the-line contactors consisted of double-stacked construction (one contactor assembly on top of another). There were several fans at the site, each with its own motor controlled separately by the switchgear. The seven-year-old switchgear and other equipment in the fan equipment room appeared to be well maintained, and the facility's general housekeeping was excellent (Photo 1 in the original article).

As work progressed, Chuck was on the far side of the room, to the rear of the switchgear where Bob was working. Bob had racked out the contactor for the No. 2 fan that Chuck was to repair. As Bob performed a meter measurement in the open compartment of the 4.16kV motor's starting contactor, he called Chuck over to watch.

A blinding flash and burned electricians

As Bob placed the leads of his digital multimeter between the bottom of the motor contactor's 4.16kV fuse and the metal cabinet, a blinding flash came from the compartment. Bob recoiled, knocking Chuck over. Temporarily blinded by the flash, Chuck could not help Bob, who also had burns on his hands and face. A few minutes later, Chuck was able to call for medical assistance.

The company operating the highway tunnel asked us to independently review the accident. Our forensic investigation began that same afternoon. First, we performed an overall inspection of the electrical system, giving special attention to the switchgear. Since the injured workers were away from the facility recuperating from their injuries, the facility manager gave us a recap of their story. From this summary, we could not determine what measurement the workers were performing at the time of the flash (Photo 2, in the original article).

Next, we documented the condition of the switchgear components. We de-energized the switchgear. The main disconnect was locked out and tagged, and the switchgear bus work remained grounded. We performed tests to determine and record the position of all draw-out contactor assemblies in the switchgear. We also took dimensions from full-racked in to full-racked out positions, as well as the position of all safety interlock brackets and lever arms.

To provide documentation of how the interlock operated, we made a videotape of a simulated operation of the switchgear contactor draw-out procedure, performed according to the manufacturer's recommended maintenance instructions. The compartment door would open with the contactor partially racked out and still energized. When we performed the same test on all the contactor compartment doors of the switchgear, two out of three failed-opening while the contactor remained energized.

How did the accident happen?

In our preliminary findings, we determined the draw-out contactor had not been completely racked out and disengaged from the energized stabs at the rear of the cubicle before the workers began their maintenance activities. The draw-out operating handle was in the mid-position, and the compartment door remained open (Photo 3 in the original article). Since we did not have the opportunity to interview either of the workers, we never determined exactly why Bob used a low-voltage meter to perform a measurement on equipment operating at 4.16kV.

After piecing together the facts, we found several factors contributed to this accident. First, no one had maintained the switchgear in accordance with the manufacturer's recommended practices. The door's safety interlock mechanism had, over time (possibly by abuse), become misaligned (Photo 4 in the original article). This situation allowed the compartment door to open when the contactor rack-out mechanism, operated manually by the draw-out handle, had not completely disengaged the contactor from the 4.16kV connection stabs. With the door open, the workers apparently assumed the rack-out of the contactor was complete.

Secondly, the pair did not follow proper safety practices, nor were they wearing proper working attire and safety equipment to protect themselves from an arc-flash. Lastly, Bob's use of a low-voltage instrument on medium-voltage equipment was obviously inappropriate (Photo 5 in the original article).


The workers could have prevented this accident. First, and perhaps most importantly, had the workers performed the manufacturer's recommended maintenance and routine check of the safety interlocks, they would have identified the defect before the accident.

In one of our tests, not included in the manufacturer's procedures, we attempted to open the contactor compartment door by pulling on the door as the draw-out handle operated mechanism moved through its entire motion.

This test determined the integrity of the door interlock as the contactor moves from its fully racked-in position to its fully racked-out and de-energized position. We revealed when the handle was in the halfway position, the door opened, and the contactor remained energized.

Manufacturers take time and effort to develop maintenance instructions to include with their equipment upon shipping. Make sure you never discard this information when you install any equipment, and include it in your company's maintenance department procedures. Finally, don't let unqualified workers perform tasks beyond their expertise or capabilities.

Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.