By Cameron J. Novak, MS, CFEI, MN-CFI
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.