From the accident files @ MSHA…
This incident involves a failure to LOTO equipment during servicing and maintenance, with a contributing factor involving the mis-programming of the Programmable Logic Controller (PLC) which permitted equipment to be inadvertently energized without warning.
A mechanic/repairman, age 37, was fatally injured about 12:30 p.m. on January 8, 1997 when the blade mill in which he was working started up. He had a total of one year and five months of mining experience, all with this company. He had not received training in accordance with 30 CFR Part 48 (Training and Retraining of Underground Miners). The vice president of concrete business, notified MSHA at 1:40 p.m. on the day of the accident. An investigation was started the following day.
Accident occurred at a Sand and Gravel wash plant. Principal operating officials were a vice president, superintendent, and plant foreman. The wash plant normally operated one 10-hour shift, five days a week. A total of three persons worked at the plant. Sand and gravel was transported by over-the-road trucks from an off-site pit to the plant for washing and screening. The material was then transported by conveyors to stockpiles. The finished product was sold, or used to supply the company’s ready mix operation located adjacent to the wash plant. The last regular inspection of this operation was completed on July 23, 1996.
PHYSICAL FACTORS
The accident occurred inside a Model 6500 blade mill. The mill, used to pre-condition aggregates prior to wet screening, was 22 feet 5 inches long, 7 feet wide, and 10 feet high. It was supported, about 18 feet above ground, by a substructure of I-beams. The mill was inclined about 15 degrees and had a railed work deck on three sides. The deck, about two feet above the base of the mill, was accessed by a vertical ladder. The top of the mill was about 8 feet above the work deck and could be reached with a portable ladder.
Blades and flights on the mill were 36 inches in diameter and attached to twin screws. The blades were replaceable NI-HARD paddle tips bolted onto NI-HARD outer wearing shoes. The shoes were on spiral flights that provided a scrubbing and abrading action to break up and dissolve waste material.
The mill’s twin screws were belt driven by two 40 hp., electric motors operated from a control center in a trailer about 100 feet from the mill.
The wash plant, including the mill, was controlled by a Programmable Logic Controller(PLC) that received power from a 230/120 volt panel board. The panel board’s main disconnect was a 230 volt, two pole, 125 amperes circuit breaker. For the past week an internal heat problem had caused the breaker to trip after about 10 to 15 minutes of operation. This would result in a loss of control power to wash plant components.
The wash plant control panel contained two emergency stop switches, a start-up warning switch, and start/stop stations for the individual motors within the plant.
A modification to the PLC in October of 1996 resulted in power being unintentionally returned to components following a power failure, if their switches had been left in the “on” position.
DESCRIPTION OF ACCIDENT
The victim started work at 7:30 a.m. on January 8, 1997, his regular starting time. He was assigned to work with the wash plant foreman. They were to thaw the frozen material inside the blade mill and then replace broken and worn paddle tips and wearing shoes.
The victim removed the plywood sheets that had been placed on top of the mill to retain heat generated by a propane heater located below. He signaled the foreman, who was at the motor control, to start the two blade mill motors, one at a time, to see if they were free of frozen material. The foreman started the mill motors, as well as those for the stacker conveyor and the feeder belt. Satisfied that the mill was free of ice and frozen material, the foreman loaded some empty propane tanks onto a truck to take them to a fill station located across the property.
Before leaving with the tanks the foreman returned to the motor control center and switched the four start/stop button switches to the “off” position, stopping all four motors that he had started earlier. He left at approximately 8:15 a.m., while the victim was preparing to make repairs in the mill, and returned about 9:00 a.m. The foreman and the victim worked together about two and one-half hours.
About 11:45 a.m. a contract electrician arrived and the foreman went with him to check out a faulty breaker. The victim remained in the mill. the foreman and electrician went to a 125 amp breaker that, for the past week, had been tripping out after being engaged 10 to 15 minutes. This breaker controlled numerous smaller breakers such as those for control center lighting, receptacles, and PLC power.
The foreman reset the tripped breaker and the electrician observed its operation for a few minutes. He then told the foreman he would remove the metal panels and tighten the terminal lugs and take ammeter readings to see if he could determine what was wrong. The electrician turned the breaker off, removed the panels and proceeded to troubleshoot the panel board. He determined that the circuit breaker had an internal heat problem and would replace it the following morning.
Ten to fifteen minutes after the electrician started troubleshooting, the foreman left to check on the victim. As he was leaving the motor control center he glanced over at the control panel and noticed that the two blade mill buttons were in the “run” position. He pushed them down to the “off” position then continued to the blade mill. He found the victim inside the mill, entangled in the blades. The foreman informed the electrician of the accident and then drove to the shop to call 911. Within 2 to 3 minutes paramedics arrived. The victim was pronounced dead at the scene.
Information gathered during the investigation suggests that the victim turned on the blade mill to clear some remaining frozen material after the foreman left to refill the propane tanks. The mill operated until the 125 Amp breaker heated up and kicked out. The victim then went back to work in the mill without shutting off any switches. Because the PLC was incorrectly programmed, the mill began operating when the 125 amp breaker was reset during troubleshooting of the electrical system.
CONCLUSION
The primary cause of the accident was the failure to lockout the two-blade mill electrical disconnects prior to working on the mill. A contributing factor was the mis-programming of the Programmable Logic Controller which permitted equipment to be inadvertently energized without warning.
CITATIONS/ORDERS
Order No. 4364005, Issued on January 9, 1997 under provisions of Section 103(k) of the Mine Act:
On January 8, 1997 a blade mill became energized and fatally injured a mechanic working inside the mill. This order was issued to insure the safety of persons until the affected areas of the mine could be returned to normal operation
This order was terminated on January 10, 1997 after the PLC was properly programmed.
Citation No. 7950070, Issued on January 9, 1997 under provisions of Section 104(d)(1) for violation of 30 CFR 56.12016:
On January 8, 1997 a mechanic was fatally injured while working inside a Kolberg blade mill when it inadvertently started. The two 480 volt, 3 phase, blade mill drive motors were energized through Allen Bradley starters that were located in a motor control center trailer. Power to the two 100 amp circuit breakers for the motors was not locked out nor were other measures taken to prevent the equipment from becoming energized without the knowledge of the individuals working on it. The plant foreman was aware that the motor’s circuits were not locked out and he was involved with mill repairs. This is an unwarrantable failure.
This citation was terminated on January 9, 1997 after the company re-emphasized with their employees their requirement for equipment lock-out.
Citation No. 7950071, Issued to H&N Electric Inc., contractor ID 2GS, under provisions of Section 104(a) of the Mine Act for violation of 30 CFR 56.12002:
On January 8, 1997 a mechanic was fatally injured while working inside a Kolberg blade mill that was not locked out and was inadvertently started. A General Electric Funac series 90-30/90-20 micro (PLC) programmable logic controller was used to control power to plant electrical equipment. Last October (1996) a normally open contact button was incorrectly installed in the program. This contact prevented run mode restarting of the timing control and would allow unintentional start up when power was restored after an outage.
This citation was terminated on January 10, 1997 after corrections were made to the segment of the electrical system that was permitting unintentional start-up.
/s/ Dennis D. Harsh
mine safety and health inspector
/s/ Arnold E. Pederson
mine safety and health inspector
Approved by: Fred. M. Hansen, District Manager
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