I thought that “The Case of the Shocking Sealing Machine” article, starting on page 16 of the May 2008 issue, was excellent, although a little reading was necessary to follow some of the points. I had to get all the way to the paragraph under “Investigation and Analysis” to understand the conduit was (unintentionally) being used as a “grounding path.” Could you please clarify the following questions?
Do you happen to recall if the flex conduit was intended to be the only grounding conductor? We are not sure if this is acceptable unless the length is very short, and maybe not even then. Maybe the original installation provided bonding of the control box as compared to the plywood mount shown in the photo?
Do you know if a grounding conductor was installed in this raceway and was it perhaps broken?
When the supply wiring was mentioned, it described the disconnect being connected to an internal breaker using conductors installed in EMT, but all this must have been changed from the original. It sounds like it was difficult to determine the wiring method because so much time had passed?
These are all good subjects and are quite typical of the situations encountered in the field — thanks again!
— Bob Dorazio, general building
contractor, Avila Beach, Calif.
Author's reply: I'm glad you enjoyed the article. By describing actual incidents where things went wrong, they remind electrical workers that safety and caution should always be at the forefront during any project.
As with most accident/injury cases, the scene and evidence had been changed multiple times since the original incident. Combine that with unclear and differing statements made years after the accident, and it can take some time to sift through and determine what actually happened. At times, critical facts are unknown or have to be inferred from all the evidence. With that in mind, I will try to address your questions:
1 & 2) I would like to note that as the intent of the article was general in nature, I did not specify all the NEC-related standards applicable in this case. I would point you and other readers to the 2008 NEC, Art. 665: Induction and Dielectric Heating Equipment. Dielectric sealing and RF sealing are analogous terms, so this article is directly applicable to this case. Specifically relating to grounding and bonding, Art. 665.26 states:
“Bonding to the equipment grounding conductor or inter-unit bonding, or both, shall be used wherever required for circuit operation, and for limiting to a safe value radio frequency voltages between all exposed non-current-carrying parts of the equipment and earth ground, between all equipment parts and surrounding objects, and between such objects and earth ground. Such connection to the equipment grounding conductor and bonding shall be installed in accordance with Article 250, Parts II and V.”
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.