In February 1980, a 12-year-old boy climbed past a vent through a duct leading into an indoor transformer vault, which was installed in an apartment house basement in Ontario, Canada. While within the vault, he was seriously burned when he contacted an uninsulated terminal of a wall-mounted switch that was energized at 8,000 volts.
The transformer supplied power to the building’s residents.
The following were the transformer vault design, construction, and maintenance factors that contributed to this occurrence. Reference is made in this analysis to provisions of Part I of the Canadian Electrical Code (CEC), CSA Standard C22.1, 1966 Edition, entitled “Essential Requirements and Minimum Standards Governing Electrical Installations for Buildings, Structures, and Premises (All Potentials)”; the related U.S. National Electrical Code (NEC), and good engineering practice in general.
His required superior knowledge of the injury-producing effects of high-voltage electricity and of safe construction standards and methods gave the operator of a facility containing high-voltage parts a primary responsibility for ensuring its hazard-free operation.
It is fundamental for public safety that live parts of electrical equipment shall be inaccessible to unauthorized persons. This was stated in CEC Rule 36-002, which was violated in the construction of the transformer vault.
CEC Rule 26:158(4) required that “Ventilation openings be covered by durable gratings…according to the treatment required to avoid unsafe conditions.”
This wording is identical in NEC Rule 450-45(b). Again, this rule was not observed. The vent cover, which was visible and accessible from the street was inadequately secured. A youngster could breach it without using special tools or exerting much effort. Thus, the high-voltage equipment in the vault was inadequately guarded from entry.
CEC Rule 2-058 required that all operating electrical equipment be kept in safe and proper working condition. Assuming that the ventilation opening was initially guarded by a properly installed, durable grating, it could be concluded the facility maintenance was inadequate.
An outdoor high-voltage supply station enclosure must be rugged enough to deny access to unqualified persons. Outdoor electrical equipment enclosures must have sturdy chain-link “fabric” that is affixed to poles and gate frames, and is reinforced. (See CEC Rule 26-178.) By contrast, the grille installed to prevent passage through the ventilation duct into the transformer vault was relatively flimsy.
Furthermore, high-voltage energized equipment installed in the vault, whose exposed terminals were not covered with solid dielectric material, was inadequately isolated from contact. CEC Rule 36-014, as well as NEC Rule 110-34 required that bare parts energized at 8,000 volts be mounted at least 9 feet above a generally accessible place. NEC Rules 110-17 and 450(7)(c) required guarding of high-voltage energized parts.
CEC Rule 26-014 and NEC Rule 110-31(a)(1) required isolation of nonenclosed high-voltage switches so that they were accessible only to qualified persons. The term “readily accessible” basically means that there is no need to remove obstacles or to employ special means, such as a ladder, to reach an object. The switches were mounted on a wall 4 to 5 feet above the floor, in a space that could be entered without opening a locked door, and thus were “readily accessible.”
The transformer bushings and high-voltage conductors attached to them were enclosed and covered with solid dielectric material in accordance with CEC Rule 26-034. The photograph depicts such transformer terminals. However, the switch and its terminals were not so enclosed.
This was an inconsistent application of the CEC rule, given that the switch was readily accessible without employing special means. Even if access into the transformer vault was completely barred except for qualified persons entering through locked doors, isolation or insulation of live parts was nevertheless required for their safety.
Enclosed switches were available for this application. An insulated covering over the switch or its terminals, which the boy touched, would have prevented the escape of fault current into his body.
Warnings of the hazards of a high-voltage transformer enclosure consist of both fixed placards around the enclosure, and other means of communication, such as announcements, correspondence, and person-to-person contacts. CEC Rule 36-004 required the posting of external high voltage warning notices around the enclosure, and at the switches mounted on the wall within the vault. NEC Rule 415-7(d) also required a warning. Such warnings were not posted.
Solid guidance was available to the installer and operator of the indoor transformer vault to avoid this accident.
MAZER is a consulting electrical engineer who specializes in electrical safety issues. He can be reached at (202) 338-0669 or firstname.lastname@example.org.