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Case of the Shocking Sealing Machine Sparks Feedback

Jul 1, 2008 12:00 PM

Following that up, Art. 250.96(A) states:

“Metal raceways, cable trays, cable armor, cable sheaths, enclosures, frames, fittings, and other metal non-current-carrying parts that are to serve as grounding conductors, with or without the use of supplementary equipment grounding conductors, shall be bonded where necessary to ensure electrical continuity and the capacity to conduct safely any fault current likely to be imposed on them.”

The sealing machine in question can come from the factory in different configurations. Some show the push-button controls mounted to the sides of the RF generator housing, directly bonding them to the frame and ground. However, since different sized product tables can be mounted to the frame, the control boxes may need to be relocated so they are more accessible to the operator. That appears to be what happened in this case.

The information we received related to the move and condition of the machine directly after the accident. According to the electrician that repaired the conduit the first time, there were six red conductors in the conduit. He did not find any bare or insulated (green) grounding conductors [Art. 250.119], and assumed the conduit was being used as ground. The factory technician did not see a problem with the type of or location of the control boxes, but he did add a separate grounding conductor during his later repair. The control boxes do need to be grounded due to the dangerous voltages and RF frequencies. Based on the correct operation of the machine prior to the move, it is assumed that the control boxes were grounded, most likely through the conduit.

Although the language and restrictions have changed with time, listed flexible metallic conduit is allowed to be used as an equipment-grounding conductor [Art. 348.60] as long as it follows the provisions of Art. 250.118(5). One of those provisions, 250.118(5c), limits the length of a conduit used as a grounding conductor to 6 feet.

Therefore, a flex conduit could be used as an equipment-grounding conductor in this case, whether for protection against RF-induced voltages or a line-to-ground fault. Of course, its value as grounding path would depend on the type and condition of the original conduit and fittings, which unfortunately was unclear in this case.

3) Rereading the article, my description appears to be in error. The three conductors in EMT traveled from the distribution panels on the opposite wall, across the ceiling and down to the 60A disconnect. As shown in the photograph, flexible metal conduit traveled from the disconnect to the sealing machine and the internal circuit breaker.

Thanks again for the feedback.
— Andrew Paris, P.E., forensic electrical
engineer, Anderson Engineering of
New Prague, Inc., New Prague, Minn.

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