What is in this article?:
- The Case of the Shocking Sealing Machine
- SIDEBAR: Wave Basics
Severed grounding connection uncovered as cause of plant worker's injury.
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.