By Norman E. Reifsnyder, WorleyParsons, Inc. and Robert A. Wilson, ABB, Inc.
Part 5 of 5
Documented Case Studies
There have been at least two arc flash incidents where dedicated optical arc flash relaying had been installed. One occurred in 2002 at the Detmarovice Power Plant in the Czech Republic and another occurred in 2003 at the Kemira GrowHow fertilizer plant in Finland. Fortunately, in both cases, there were no injuries.
Detmarovice Power Plant
During the morning shift at the Detmarovice Power Plant in the Czech Republic on June 26, 2002, two workers were exercising a 6.3 kV circuit breaker that had been withdrawn to its test position. Unfortunately, they forgot that the breaker was closed as they tried to return it to its full operating position. They also bypassed the mechanical interlocks in their attempt to insert the breaker. The closed breaker initiated an arc that could have been deadly. Fortunately, the switchgear was equipped with optical arc flash protection using the long sensor fiber technology. Within 82 milliseconds, the fault was isolated and a major catastrophe was averted.
Eyewitnesses reported that the breaker cubicle was full of white sticky smoke from the burned plastic but that was the extent of the damage. Repairs consisted of cleaning the breaker and cubicle as well as replacing the breaker rosettes and cubicle pins. The image below shows photos that were taken immediately following the incident. There was no permanent damage to the installation or surrounding equipment and the plant was quickly returned to service thanks to the fast reaction time of the arc flash relay. More importantly, both workers escaped injury and possible death.
Detmarovice Power Plant arc flash event
To those who may be unfamiliar with arc flash events and their consequences, the images above may seem severe. However, rarely is such an event confined to the cubicle where the arc flash originated. Quite often, the arcing fault will burn through the sidewalls, cascading from frame to frame before the fault can be totally isolated. Entire switchgear lineups may be destroyed, causing extended downtime and loss of production. In this particular case, even the circuit breaker where the arc originated was salvaged. Had the arc flash relay not been installed, the estimated direct cost of the physical damages would have been as high as $1.6 million. Indirect losses including extended loss of production would probably have been many times higher yet.
A similar accident at this same plant occurred in 1979, well before the installation of optical arc flash protection. That incident resulted in a 3-day outage. Lost production and equipment damages totaled several million dollars.
Kemira GrowHow Plant
Kemira GrowHow has a fertilizer plant located in Uusikaupunki, Finland. Energy consumption is 18 MVA and the plant has 7 MVA of on-site generation. Primary products are fertilizers for farms, greenhouses and forests.
In 2003, the Kemira GrowHow plant narrowly avoided a major catastrophe just one day after installing optical arc flash protection in their 1965 vintage medium voltage switchgear. The arc flash event was initiated when a disconnect switch serving a normally de-energized cable was manually operated. In this case, however, the cable was energized but otherwise unloaded. Due to the capacitive no-load current in the cable, the air disconnect switch was unable to extinguish the arc and the arc quickly progressed to the bus compartment where it evolved into a three-phase bus fault. Two workers were present in the vicinity of the arc flash when it occurred. Neither was injured although both were a bit shaken.
The just-installed arc flash relaying system detected the arc flash and tripped before the switchgear could sustain any significant damage. The plant was restored to service in a few hours. Had the optical arc flash relaying system not been installed, protection would have depended on conventional overcurrent relaying. The arcing time would have been at least two times higher. It is difficult to estimate the direct and indirect damages that would have occurred but it would most likely have been very expensive. One day of lost production costs millions of dollars.
Arc flash relaying was originally installed at this site as insurance to limit the direct and indirect damages associated with an electrical accident. Their investment paid back many times over in a single day.
The most significant danger to a worker performing routine maintenance on electrical equipment is the arc flash hazard. Recent improvements in electrical safety systems can reduce this hazard to a manageable level, thereby increasing worker safety. Such improvements can be applied when either designing a new facility or retrofitting an existing one. When designing a new facility, electrical equipment can be specified to provide added protection for workers. Protective relaying systems can be specified and engineered to reduce incident energy levels thereby further protecting workers. Existing facilities can be analyzed with the intent of establishing the level of arc flash hazard and then designing and implementing systems to reduce the hazard level.
Below are summaries of possible arc flash abatement procedures for both new and existing facilities. The steps to be taken to reduce the arc flash hazard when designing a new facility include the following:
- Specify arc resistant switchgear with closed-door racking capability during the design phase
- Perform an arc flash hazard analysis to determine incident energy levels without any high speed protective relaying
- Implement optical arc flash protection, fast relaying schemes, or other cost effective measures, to reduce problematic incident energy levels that are above acceptable levels
- Plan ahead for future system additions that could increase available fault current magnitude and hazard level
The steps to be taken to reduce the arc flash hazard in existing facilities include the following:
- Survey the existing electrical equipment with the intent of accurately documenting existing conditions
- Make sure that analytical models accurately represent what is installed
- Perform an arc flash hazard analysis to determine existing incident energy levels
- Investigate and implement cost effective measures to mitigate arc flash incident energy levels determined to be unacceptable.
- Communicate with and train workers on safety procedures that will protect them from danger
These steps provide a framework for applying the arc flash abatement technology discussed in this series of articles. However, when investigating possible future steps to take, it can be equally as informative to look at areas where these technologies have already been put into practice. To this end, documented case studies have been discussed, showing how retrofitting existing equipment with modern arc flash protection schemes can further protect workers from arc flash hazards and possible serious injury.
The safety standards discussed in this series of articles require that action be taken before an arc flash event occurs and a worker is seriously injured. Such actions include analyzing the hazard, labeling equipment thereby defining the hazard, establishing PPE requirements to protect workers and communicating with workers through implemented safety procedures. Providing the protection and communication required by the safety standards as well as taking steps to mitigate excessive arc flash hazard levels are the building blocks of a successful electrical safety program.
Electrical safety must be viewed as an ongoing program and not just a one-time effort. Arc flash hazard analysis studies need to be periodically reviewed and updated as the distribution system changes over time. Equipment labeling may be impacted and needs to be routinely evaluated. Personnel also need to be refreshed with proper safety procedures etc.