I am often asked the question: “Can operating a device such as a fusible switch or circuit breaker cause an arc flash?” The exact answer is that it depends.
Learning the hard way
A client of a friend in Arizona found out the answer the hard way. The client had a contractor on-site to perform electrical work at their facility. The scope of work made it necessary to establish an electrically safe work condition at the 277Y/480 volt (V) main service switchboard based on NFPA 70E 120.5, Process of Establishing and Verifying an Electrically Safe Work Condition.
The main service switchboard contained four separate mains as permitted by the National Electrical Code 230.71. One of the mains was a 1,200-ampere (A) bolted pressure switch with 1,200A fuses that fed a distribution switchboard located in another room. As required by NEC 240.95, ground-fault protection was provided on the main since the disconnect exceed 1,000A and was a solidly grounded 277Y/480V system.
The first step was to interrupt the load by opening each of the smaller fusible disconnects at the distribution switchboard in the other room. Then, the 1,200A main was opened along with the other mains. However, that would not completely de-energize the main service switchboard since the bus would still be powered. To completely de-energize the main switchboard, the local electric utility company was called to take the transformer out of service. That was a very good decision!
Once the electrically safe work condition was established and verified, the contractor successfully completed the work. Restoring power began with the utility placing the client’s transformer back in service followed by the contractor re-energizing the four mains in the main service switchboard.
Switches and arc flash
The final step was to restore power to the individual loads by closing each of the disconnects in the distribution switchboard. Normally it is a routine operation, but this time one of switches exploded in the worker’s face. Yes, an arc flash!
Although this could have been catastrophic, the contractor was not injured—only shaken up a bit. Why? Because he was wearing arc-rated clothing and personal protective equipment during the switching. However, not everything survived. In addition to the damaged switch shown in the photo, his hard hat and face shield would never be the same again. This event is a vivid reminder that wearing your personal protective equipment (PPE) can make all the difference when the unexpected happens.
What caused the arc flash? It was attributed to the failure of a latching mechanism on one of the distribution switchboards fusible disconnects. It broke and fell into the line side of one fuse. When the switch was closed, an arc flash occurred and blew the 1,200A fuses in the upstream main service switchboard; the ground-fault protection did not operate.
Is today the day?
Electrical workers perform switching tasks like this every day without incident. But, how do you know if today is the day that an arc flash occurs? What is the risk?
NFPA 70E requires performing an arc-flash risk assessment. Risk has two components: the severity and the likelihood. The severity can be determined by performing incident-energy calculations. However, how do you determine the likelihood? Using a crystal ball is probably not the best idea.
To estimate the likelihood of occurrence of an arc flash, NFPA 70E Table 130.5(C) lists many different tasks including “Operation of a circuit breaker, switch, contactor or starter.” Each group of tasks in the table includes another line item known as “equipment condition,” which is classified as either “normal” or “abnormal.” If the equipment condition is normal, an arc-flash incident is not likely to occur. If the equipment condition is considered abnormal, additional protective measures are required based on the hierarchy of risk control and may include the use of arc-rated protective clothing and PPE.
What is the difference between normal and abnormal equipment conditions? Table 130.5(C) states that the condition is considered normal if all of the following circumstances apply:
- The equipment is properly installed.
- The equipment is properly maintained.
- The equipment is used in accordance with instructions included in the listing and labeling and in accordance with manufacturer’s instructions.
- The equipment doors are closed and secured.
- All equipment covers are in place and secured.
- There is no evidence of impending failure.
In hindsight, it is fair to say the switch that failed was likely not properly maintained. Although there may have been no observable evidence of impending failure, it did indeed occur. And the contractor is grateful that his PPE made all the difference in the outcome.