In a typical television transmitter installation 30 years ago — at a time when big power was coming into play (three-phase, 460VAC) — power was fed from a transformer located just outside of the building. To design the building's internal power system, the station generally hired an electrician — not an architect or engineer. If the station was fortunate, the electrician contacted an engineer to discuss the system, but this was not likely.
Back then, the transmitter manufacturer stated the value of the fuse or circuit breaker desired to protect the equipment in an instruction manual. Too often, the next step was a trip to the electrical supply house to see what could do the job. That equipment was installed and would work fine for the next 30 years, as the transmitter would not develop a fault calling for the main disconnect to function. As a rule, smaller breakers connected to the individual circuits pick up most transmitter faults.
After 30 years, there's a problem. A short occurs in the power lines going to the high-voltage power supply. In essence, two of the individual phase lines short together. The transmitter goes down, but a greater problem rears its ugly head.
When the staff enters the transmitter room, smoke is coming from the main disconnect and from the wiring into the high-voltage power supply. The fuse or circuit breaker did not shut down the voltage as it should have. In a flurry of excitement, the technicians head for the main disconnect switch to kill the power to the transmitter.
When the engineers open the front cover, there is a huge flash of flames. Molten steel and copper spew from the disconnect. The force from the explosion knocks the technicians to the ground and scatters everything in the transmitter room. Both technicians receive second- and third-degree burns. The technician closest to the explosion may be blinded and will be lucky to survive.
Breaking down an arc flash
This phenomenon is called an arc flash. An arc flash can occur when insulation or mechanical characteristics break down, letting one or more power line phases short to the ground or to each other. The resulting short circuit produces a large amount of energy.
The results are amazing. Engineers have calculated the energy from 10,000A at 480V. This is comparable to 8MW instantaneously dumped into surrounding metal, wiring or people. It's roughly equivalent to the energy from eight sticks of dynamite.
To understand what happens during an arc flash, let's break down each step. When the transmitter's power system was originally installed, calculations should have been performed to determine the value of the short circuit current available at the primary disconnect. This involves physical parameters, such as the size of the feed conductors from the power company source, the material used, the physical placement of the affected components and the reactance values for the transformer feeding the building.
Calculating the short circuit parameters is a detailed process and should only be done under the supervision of a licensed engineer. This is not something your typical electrician can or should try to do.
The purpose of determining the short circuit current is to assure that the primary disconnect device is sufficiently rated to handle the possible current value. You need a device that provides normal overcurrent protection but also has the ability to interrupt the circuit in case of a failure. The reason the fire occurred in the earlier example is because the main disconnect didn't have the ability to break the circuit.
The determination of the short circuit current, sometimes called the bolted fault current, is part of the required work to perform a complete hazard analysis for possible arc fault situations. IEEE Standard 1584 establishes measures that can be taken to prevent arc flash. (See “Key steps to prevent arc flash.”)
The incident energy is expressed in calories per cubic centimeter squared. The flash protection boundary level is 1.2, which equates to a second-degree burn. As one gets closer than the boundary, the injuries increase.
The bottom line of this month's discussion is that arc flash calculations need to be performed on the electrical service to your building. That analysis should become part of a broadcaster's training program for technicians and part of its safety meetings. The proper safe area needs to be marked, and protective gear should be made available on-site and ready for use. All this is spelled out in NFPA Standard 70E. Finally, a professional engineer must make those calculations. Don't try this yourself.
Thankfully, there is a lot of information available on the Internet. One helpful Web site is www.easypower.com. The site contains free literature that explains both the problems and the solutions. Broadcasters should also obtain a copy of “NFPA 70E” and the “IEEE Guide For Arc Flash.”
If you don't think arc flash is serious, Google “arc flash photos.” One photo that is shown on many sites shows three guys looking into a main disconnect cabinet. Then it blows up on them.
Everyone in this incident went to the hospital. One person was placed into a coma to help him survive the burns, but he died. OSHA issued a huge fine to the company in this incident because the company knowingly sent the engineers to work on the switch without protective gear. The company's two supervisors face possible prison sentences.
It's estimated that arc flash injuries send five to 10 people a day to hospitals with burns. Direct and indirect costs to companies have run as high as $15.75 million for a single incident. These injuries can be avoided by the use of proper equipment and training. Even providing something as simple as a long pole that can be used to push the door open can save someone's life.
Now you know better
Now, it's time for the kicker. As a chief engineer, you can never again send someone to work on the electrical supply equipment unaware of the arc flash dangers and standards.
You have been informed about the necessary calculations, safe areas to be established, and the need for safety meetings, and protective tools and clothing. If you send someone to work on such equipment now and you don't provide the above resources, you would be doing so while knowing the possible dangers. The fine could be huge, and you could end up in the slammer. Sorry; I'll miss you as a reader.
Don't let this happen to you or your station. Hire a good electrical engineering firm that specializes in electrical power work. Determine the possible fault current and the arc fault problems. Then, take the measures necessary to protect your staff. While we joke about the cost of such studies, the real issue is the protection of your workers. No one wants his career tarnished because he failed to properly protect the staff.
Don L. Markley is president of D.L. Markley and Associates.
Key steps to prevent arc flash
- Collect system and installation data.
- Determine system modes of operation.
- Determine bolted fault current.
- Find protective device characteristics and arc duration.
- Document system voltages and equipment class.
- Determine arc fault current.
- Select the working distances.
- Calculate the incident energy.
- Calculate the flash protection boundary.
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