Severed grounding connection uncovered as cause of plant worker's injury.
When a graphics production company found itself busting at the seams, it planned to move to a larger facility that offered more space to produce its signature signs, banners, and placards. In preparation for the relocation, a local electrical contractor was hired to disconnect some of the larger pieces of equipment, move them to the new location, and reconnect them to power. This task involved relocating a radio-frequency (RF) sealing machine — a piece of equipment that would ultimately shock and injure a worker who had used it hundreds of times before for up to 5 hours a day to produce large signs and banners.
The same electrical contractor involved in the moving process also worked on the construction of the new building. Because many of the machines were fairly unwieldy, the electrical contractor subcontracted a heavy moving company to load and transport several of the machines — at least one of which was damaged during transit. Much of the heavy equipment was moved over a weekend, about a month after the move first began. Most of the electrical work was completed the same weekend, getting the graphics company up and running by the following Wednesday.
No problems were reported with the RF sealer, which would later be in question, until two weeks after the move when the accident occurred. In order to understand the events leading up to the incident, it's important to take a closer look at the process of RF welding and how the electrical system was set up.
Setting the stage
In general, a sealing machine is used to heat and weld together sheets of plastic. One method uses a press containing a heating element to melt the edges of two sheets of plastic and compress them together to form one large sheet. Another method involves using high-frequency electromagnetic waves to heat the plastic. Just as a microwave oven excites water molecules inside a piece of food, an RF sealing machine excites the molecules in certain types of plastic.
In a typical RF sealer, the high-frequency waves (27-MHz range) radiate out from a heat-sealing die of a desired shape. The radiating waves travel down through the plastic material, which is set on a platen (i.e., a solid, perfectly flat steel plate). As the overlapping plastic sheets melt, the die compresses the plastic sheets against the platen, forming a weld. The RF waves are radiated for a few seconds and then shut off, leaving the weld to cool. The new larger plastic sheet can then be reloaded, and extended further.
The RF sealer, shown in Photo 1 and Photo 2, contains an integrated high-frequency generator and set of operational controls. The RF emitter is embedded in the sealing die. The lower platen rides on a movable arm that slides on wheels across a solid table. Two control panels with push buttons are mounted in front of the base table on wooden offsets (Photo 3).
In the RF sealer, most of the high-frequency waves are directed down through the medium. Although movable, the sealing die and lower platen are bonded to the bedplate, providing a ground path and preventing large voltages from building up on other metal parts of the machine. Some of the bonding straps are shown in Photo 4. The bonding straps are specially shaped for maximum surface area to absorb the RF radiation.
There were three distribution panels on the south wall of the production facility. EMT conduit traveled up from the east panel, across the ceiling, and down to a wall-mounted 60A disconnect, providing 208V power. Electrical spec sheets for the new building showed that a length of 1-inch EMT (containing three 6 AWG conductors) originally traveled from the disconnect to the RF sealer. The conductors traveled to a 50A circuit breaker internal to the generator.
According to his deposition, the worker was standing in front of the RF sealer at the time of the accident. As he simultaneously pushed the black buttons on each of the remote control boxes, he noticed a pop and flash emanating from underneath the table. At the same time, he received a severe shock to his left arm. Initially, he was unable to remove his hand from the button. Fortunately, his arm soon contracted, and he was thrown backward. Immediately taken to the hospital for examination, the victim was treated for a burn mark on his left hand. As time progressed, however, he suffered from tingling, numbness, and a loss of grip strength.
Some time after the accident, Anderson Engineering was hired by the plaintiff to investigate the accident and determine where and how the victim was injured.
Investigation and analysis
After extensive forensic investigation, we concluded that this case revolved around two possibilities for the origin of the shock hazard — both of which were tied to the damaged conduit and control conductors leading to the control box. At the time of the accident, the conduit was being used as a grounding path back to the RF generator. When the conduit was severed, so was the ground path. As the control boxes were mounted on a wood plate, they were electrically insulated from the RF generator and base table.
With this situation in place, if the control circuitry insulation was compromised and a bare conductor contacted the remaining conduit, the control box could become energized. If a worker touched the control panel, he or she could create a path to ground and be shocked. Unfortunately, because the original conduit and conductors were disposed of, no evidence of this type of arcing could be found. Although the RF generator contained 2.5A fuses on the control circuit, a current below this level could still injure a person and not blow the fuse.
Another possibility involved the high-frequency waves produced by the RF generator. As noted in SIDEBAR: Wave Basics below, the localized, high-powered, high-frequency signals can induce voltages in conducting materials. With the damaged control panel conduit compromised — and the bonding plates damaged — the ground path back to the generator (and eventually earth ground) was compromised. When the RF signal was generated, it could then induce a voltage on the metal control panel box. As a result, when a worker touched the panel, he could create a path to ground with his body and be shocked.
The manufacturer knew of this problem and attempted to mitigate the hazard by replacing the bonding plates and adding dedicated grounding conductors for the control panels sometime after the accident (see SIDEBAR: After the Fact below).
As is often the case with these types of legal disputes, our firm was brought into the equation a few years after the accident. One problem that comes with this situation is the challenge of sorting through conflicting statements and recollections. To complicate matters, the condition of the equipment at the time of the accident was somewhat unclear, as the machines and electrical connections had been altered several times before we ever examined them. Despite these obstacles, we established a few hypotheses.
Whether or not RF-induced voltages caused the accident, or if the event was prompted by a ground fault, both scenarios required a severed grounding connection to occur. The design of the machine used the remote control panel conduit as a grounding conductor. Because the machine had operated properly for many years prior to the accident, the conduit was most likely damaged during the move to the new building. If the machine had been examined after the move (and before it was reconnected to power), the damaged conduit could have been discovered and repaired.
Although the design of the machine and subsequent lack of maintenance to the bonding plates played a part, the accident would most likely not have occurred if the conduit was not damaged.
In the end, a lawsuit was filed against the heavy equipment movers and the electrical contractor in charge of reconnecting the equipment. Settled for an undisclosed amount, the case never made it to trial.
Paris is a forensic electrical engineer with Anderson Engineering in New Prague, Minn. He can be reached at firstname.lastname@example.org.
SIDEBAR: Wave Basics
According to Faraday's Law, a voltage will be induced on a conductor in the presence of a moving magnetic field. The magnetic field excites the atoms, creating an electromagnetic potential across the conductor. If a complete circuit path is present, electrical current will flow.
Man-made alternating electromagnetic fields are constantly flowing throughout the atmosphere. Fortunately, they are relatively weak, and their current-inducing effect is typically limited. Radio communications are one example. However, localized high frequencies can create problems. For instance, a typical microwave oven produces high-frequency waves that could interfere with other EM signals, and possibly injure the end-user. To prevent this, the microwave housing is shielded with a type of Faraday cage (in many cases, a fine metal mesh), which prevents the microwave signals from escaping the cooking chamber.
SIDEBAR: After the Fact
Following the accident, the shop foreman examined the radio-frequency (RF) sealer and could not find any problems. Therefore, he left the sealer idle until it could be further examined. Called in a few days later to repair the machine, a local electrician discovered a damaged flexible metallic conduit traveling to one of the control boxes. The conduit had pulled apart, exposing the internal conductors. The electrician pulled the wires from the conduit and installed them in a length of new flexible metal conduit as well as repaired the damaged insulation on one of the conductors with electrical tape. Also finding the bonding straps deformed, he proceeded to reshape them. Next, the electrician attached a ground lug to the table support and ran an 8 AWG ground wire up to the RF generator housing for an added ground connection.
A few months later, the RF sealer began acting up again when an operator noticed sparks while it was under operation. The original electrical contractor was called in to repair the machine. He apparently found a kink in the conduit supplying the left remote control panel (although no invoices were found) and replaced the conduit and conductors with a flexible nonmetallic cable.
A few years later, for unknown reasons, the RF sealer was inspected and repaired by a manufacturing representative (e.g., technician). He replaced the bonding plates and added a positive shuttle down switch, which prevents the generator from producing an RF signal until a ground connection to the bedplate is verified. The technician also replaced the flexible nonmetallic cable with flexible metallic conduit. He then ran a dedicated ground wire through each conduit back to the RF generator.