Electrical and electronic equipment and factory machinery were allegedly damaged in a manufacturing plant in 1984.
Unfortunately, the post-accident investigation was seriously hampered because almost all of the damaged equipment was discarded before it could be examined and tested, leaving the plant with little recourse.
In this particular instance, one or more underground distribution cables previously installed between a utility riser pole and the plant's pad-mounted primary service disconnect switch were allegedly damaged while a trench was being dug with a backhoe in order to install a water pipe. While removing dirt from the trench, the backhoe operator "scratched" the ground surface with the teeth of the backhoe bucket. An electrical explosion occurred as the bucket snagged and broke a yellow and black plastic warning tape and a cable-one that had been installed on the factory's property in 1982 by direct burial in a 48-inch deep trench. Nearly six feet of limestone screenings had been laid above the cable, and the tape was installed 22 inches below grade.
Subsequently, the surface had been regraded for use as a parking lot, during which one to two feet of surface cover were removed. Thus the warning tape and cables were now one foot or less from the surface. Consequently, the cables were subjected to damage by crushing for the next few years.
Studies indicate that the preferred method of installing underground distribution cables is in pre-assembled conduit.
So equipment damage was likely to have been caused by years of wear and tear and improper cable installation and routing, not by one accident alone.
The electrical utility had recommended routing the cables from its riser pole in order to avoid potential moisture problems, pressure of heavy vehicles, ongoing industrial operations, and continual construction changes. The cable burial area was formerly a toxic waste dump, where varying deterioration depends upon the chemical formulation of waste materials. Installing cables in higher ground would have protected them from such damage.
Whether working with a backhoe or shovel, the operator should not have come closer to the energized cable than the distance of two feet specified in Table 422-1 of Rule 422B of the National Electrical Safety Code (NESC). Moreover, industry standards required close supervision of workers near high-voltage energized conductors, who were not specially trained in electricity. Rules 420C and 421C of the 1984 NESC required that electrically qualified workers direct and guide less experienced ones working near such equipment.
The electric utility company had originally installed a three-phase line recloser on its feeder near the plant, but had later removed it for an unknown reason. During the dig-in, this recloser would have locked out all of the factory's three phases in a short time, thereby substantially reducing damage to the underground cables, plant machinery, and electrical equipment. The utility's feeder conductors were also at risk during the fault.
Moreover, for electrical protection of underground distribution cables, it is frequently desirable to use nonreclosing (single-shot) tripping when single phasing of three-phase loads would be a problem.
Thermal damage to large motors requires several minutes of operation under single-phase conditions. The photograph depicts the stator of a motor that was damaged by single phasing.
Suitable upgrading of the electrical protection equipment on the plant's feeders and branch circuits should have been arranged. This upgrading would have fundamentally provided means for extremely rapid, selective de-energizing of electrical equipment in case of phase imbalance, over- or undervoltages, or ground faults.
Single-phasing, an extreme form of phase imbalance, is foreseeable under other circumstances than breaks in feeder cables, (e.g., tree branch contacts or lightning). The vulnerability of many of the plant's electrical equipments and processes to single-phasing made it prudent to employ single-phasing protection. The plant had earlier experienced single-phasing problems.
Rule 230-95 of the 1984 NEC required that ground fault protection be provided on service equipment rated at 1000 amperes or greater. Single-phasing protection could then be added at a nominal cost. A preference for three-phase circuit breakers over fuses for primary substation protection in an industrial plant is provided in IEEE Standard 277 because the breakers act more quickly to isolate faults. Circuit breakers were also preferred at the departmental and subfeeder levels within the plant.
Many plant losses were attributed to high temperatures reached by feeder and/or branch circuit cables and motor windings. The NEC required that all circuit conductors be provided with overcurrent protection to prevent thermal damage.
Motor feeder and branch circuits were required to have individual short circuit and ground fault protection. Motors and motor control apparatus were required by Rule 430-31 to have overload devices to protect them against excessive heating due to motor overloads and failures to start. All electrical equipment damage from excessive heating caused by the dig-in could be attributed to inadequate electrical protection of these components. Similar provisions even appeared in the 1962 NEC, when thermal protectors integral with motors were available for protecting them from damage due to overloads or failures to start.
Possible electrical and electronic equipment damage mechanisms included transient voltage surges succeeded by thermal damage due to power-follow currents, particularly during reclosings of the utility's substation circuit breaker. Rule 280 of the NEC contained provisions for surge arresters installed directly on a building's wiring systems. ANSI C62.2 provided information on proper selection of surge arresters.
Knowing the sensitivity to damage by voltage surges of the plant's feeders and branch circuits, plant machinery, and equipment that incorporated semiconductors, would have ensured the installation of suitable surge arresters. Harmonic suppression was also required in the plant because of SCR operations. Harmonic voltages were much higher than normal in the presence of voltage surges on the remaining undamaged incoming feeder cable.
If the inadequacy of the plant's electrical protection system against the effects of anomalous voltages had been recognized-particularly single-phasing of the incoming three-phase feeder-the maintenance response would have been based upon a specifically pre-planned, rapid reaction when only partial power was being supplied to the plant.
At the time of the dig-in, two phases were cleared by rupturing of the utility's riser pole fuses, but one distribution cable, feeding into the plant's main switch, remained energized for up to 30 minutes until it was manually tripped. Inexpensive monitoring equipment would have provided an audible or visual alarm, leading to an orderly shutdown. The prompt actions could have consisted of manual tripping of the main disconnect switches throughout the plant, in as little as 15 to 30 seconds.
This rapid response might have completely eliminated damage or destruction of electrical machinery and cables whose basic damage mechanisms were elevated temperatures. Instead, the initial reaction to the partial loss of power was to attempt to get the plant running again.
The requirement to accomplish an orderly shutdown of electrical equipment, in order to minimize hazards to personnel and equipment damage, was stated in Rule 240-12 of the 1984 NEC. Standard 277 recommended emergency or alternate power facilities for supplying critical electrical loads.
However, available emergency facilities were inadequate to prevent damage or destruction of equipment and material in process that required an orderly shutdown. For example, heat damage to continuously operating roller bearings could be foreseen if the cooling water supply failed, but no alternate supply was available.
A small mountain of equipment of various types was removed and replaced after allegedly being damaged by the dig-in. However, not a single item of this material was retained for subsequent examination, nor were photographs, schematic diagrams, handbooks, or even textual descriptions of the nature of the alleged damage to any item retained. This complete absence of evidence of damage left no firm basis for attributing the plant's equipment replacements or repairs to the dig-in, or alternatively to some other cause (e.g., earlier failure in service, or simply a desire to replace obsolescent equipment).
Damage occurred in three phases. The first occurred during the time of the dig-in, up to the time when two of the utility's riser pole fuses ruptured; the second was a 10- to 20-minute period during which attempts were being made to keep the plant working; and the third was during the re-energizing of electrical equipment re-energizing following rapid repairs and reactivation of plant machinery, which led to subsequent power losses or equipment damage.
Many lessons were learned from these events, but there were two principal ones. First, we learned the importance of foreseeing anomalous voltages in a plant environment with sensitive machinery. Second, we learned the importance of retaining damaged equipment and machinery for future critical evaluations and substantiation of claimed losses.
Dr. MAZER is a consulting electrical engineer who currently specializes in electrical safety issues. He can be reached at (202) 338-0669 or firstname.lastname@example.org.