On a busy Friday evening at a local fast food restaurant, an employee had been making a large number of milkshakes when she received an electric shock. She reportedly had her left hand on the metal cup containing the ice cream and her right hand on the front, top-right corner of the mixer. She received the shock to her right hand, later noticing a red mark on her palm. This mixer was replaced with a new unit from the manufacturer a few days later, after which another employee reported being shocked in much the same manner.
An inspection of the scene revealed that the mixer was plugged into a ground-fault (GFCI) protected receptacle on a dedicated 20 ampere circuit. The mixer sat on a rubber mat on top of a stainless steel chest freezer, which housed the ice cream for the milkshakes. This freezer was hard-wired into the wall with armored cable. The outer case of the mixer was made with stainless steel.
This author was informed that prior to his arrival on scene, an electrician had found no issues that would cause the electrical shocks. An inspection of the electrical system found no open grounds, excessive voltages, or other issues that pointed towards the cause of the shocks. However, upon removal of the GFCI receptacle from the wall, an arc mark was observed on the metallic box extension within the box. The corresponding mark was located on the “load” side of the receptacle on the hot screw. These can be seen in Figure 1. This receptacle, box extension, and the circuit breaker supplying the circuit were replaced by the electrician.
Several weeks later, it was learned that employees were still receiving shocks. A subsequent inspection by an electrician revealed several problems with the installation and use of both the mixer and freezer. Removal of the back freezer panel revealed a loose ground connection and the presence of damaged insulation on the neutral wire. So, what was the
cause of these shocks? Why didn’t every employee receive a shock when they used the mixer? Let’s dig a little deeper. Several factors came into play to create the conditions necessary for the employees to receive a shock “from the mixer.”
It seems that when employees received a shock, they were leaning against the freezer while mixing the milkshake, causing the freezer to shake and vibrate. The ice cream had just been removed from the chest freezer, most likely causing the compressor to turn on to cool the interior.
It is most probable that the vibrations and movement of the freezer (caused by the employee and the running mixer) were enough to cause a poor ground and make the neutral come into contact with some grounded component of the freezer. As there was an inadequate grounding path back into the system, the stray current flowed from the case of the freezer to the grounded case of the mixer, using the employee as a path. But why wouldn’t the GFCI receptacle on the mixer trip? Most GFCI receptacles are set to trip at 6 mA or 0.006 amperes, which is the amount of current that a person can touch and still be able to let go of an energized component without being injured. This is 1/10th the current required to cause fibrillation in a human heart.
A GFCI receptacle provides protection by monitoring the current flowing into and out of a circuit on the hot and neutral lines. When these are equal, no problems exist. However, the GFCI will trip if an imbalance in current is detected, indicating that current is taking an alternate path to ground outside of the neutral conductor. Because the freezer was not protected by a GFCI receptacle and the amount of current was still within the limits of the circuit breaker installed to protect it, the problem was allowed to persist. The current entering the mixer failed to trip the mixer GFCI because it passed through using the equipment grounding conductor, which is not monitored by the GFCI.
Once the chest freezer was re-wired to replace the damaged wiring, no further problems were reported. However unlikely the scenario may be, it is often that “one in a million” chance that leads to a failure or fire. But as Jim Carey would say in the movie Dumb and Dumber… “you’re telling me there’s a chance.” We need to keep our eyes and ears, as well as our minds, open to any and all possibilities when it comes to the causes of incidents such as this.