Published In December 2001
While testing a household microwave oven near the end of a manufacturing and testing line, a worker received a fatal high-voltage (HV) shock. The microwave oven cover had been removed as a standard procedure for performing tests under energized conditions, which included a microwave energy radiation leakage test. The conveyor system was a standing type, table-high U-shaped conveyor. Manufactured items were placed at one end on a horizontal surface, which consisted of a series of rollers. Then, workers positioned on both sides of the conveyor would push a product along the rollers as they performed various tasks. Electrical outlets were positioned along the conveyor belt for energizing the products according to various manufacturing and testing functions being performed. HV was generated within the oven by means of a transformer for the operation of a magnetron, which creates the microwave energy for cooking food. Connections from the transformer to the HV-insulated conductors leading to the magnetron terminals were not completely insulated. The worker grasped the unit with both hands, preparatory to performing the test. Substantial burns on the worker’s palms signified a long-term “no let go” effect. The metal frame of the microwave oven, which was grounded into the 120-volt service branch circuit by a grounding conductor, was also the ground reference for the oven’s high-voltage transformer secondary. One of the deceased worker’s hands had been grasping the oven’s frame. Thus, the shocking current entered the oven through its 120-volt power cord, passed through the oven’s control circuitry into the HV transformer, passed from the HV circuit into the palm of one hand, through her body, exited from her other palm into the grounded chassis, and returned to the factory’s service entrance equipment. The transformer and microwave oven manufacturers should have known that the transformer would be installed at a specific site within the oven; and that its cover would be removed during tests and inspections, with the unit energized by the service voltage. Personnel would be working around the unit with the transformer’s terminals exposed. Thus, the transformer should have been covered or guarded in accordance with consensus standards and codes. Underwriters Laboratories Standard for Safety No. 923, “Microwave Cooking Appliances,” issued in 1977, states at Paragraph 7.1, “electrical parts of an appliance shall be so located or enclosed so persons are protected against unintentional contact with uninsulated live parts.” The NEC, 1978 Edition, states in Rule 110-17, Guarding Live Parts, (a) “live parts of the electrical equipment operating at 50 volts or more shall be guarded against accidental contact by approved cabinets or other forms of approved enclosures, or…(2) by…partitions or screens…” With the cover removed and the unit energized, the exposed HV parts would have required the application of NEC Rule 110-32, “Workspace About Equipment.” But this was unrealistic, since the specified minimum horizontal and vertical clearances would have made it impossible to perform the required testing functions on the conveyor belt. Consumer product safety standards and provisions generally differ according to whether it is foreseeable that a “qualified” or “unqualified” person will have access to the product with live conductors exposed. “Qualified Person” is defined as “one familiar with the construction and operation of the equipment and the hazards involved.” The production line personnel did not meet that definition with respect to the HV circuit. Their skills were confined to performing one or two repetitive tasks. NEC Table 110-34(a) would still require a knowledgeable repairman to work several feet away from this unit while it is energized because of the exposed HV conductors. Alternatively, HV-rated gloves, aprons, and floor mats are required. The HV transformer intended for use in the unit should have been designed with its terminals guarded, or recessed so that connections of insulated conductors made to the transformer would be inaccessible to worker’s hands. NEC Rule 450-7(d) Voltage Warning specified that the operating voltage of the exposed live parts of the transformer be indicated. A post-accident examination of the oven with the cover removed revealed the notation “Danger High Voltage”on a strut located about a foot from the transformer. However, on the transformer itself appeared the misleading notation, “120V 60 Hz.” The oven manufacturer and the conveyor system designer and manufacturer knew that these products would be energized while on the conveyor belt, since service voltage outlets were incorporated into the conveyor system. In the last few decades, HV circuits have been incorporated in many consumer products. However, the oven’s HV function is intended to deliver substantial amounts of raw power in the kilowatt range. Direct contact with such a circuit causes much more severe injury than with HV circuits engineered for low-power applications. Mazer is a consulting electrical engineer who currently specializes in electrical safety issues. His telephone number is (202) 338-0669, and his e-mail address is email@example.com.