Everyone tenses up in anticipation as they hear the countdown, “three, two, one.” Then there’s an extremely loud BOOM and blinding light. Sparks fly everywhere, and smoke fills the test area. Laughter and perhaps even a high five frequently follow.
Blowing stuff up in the lab never gets old. As a boy growing up in rural Ohio, I was one of those kids that just loved fireworks. In hindsight, it was quite dangerous, but my destiny was apparently set at an early age.
Today, people contact me about performing forensic investigations involving arc flashes or to ask questions about how equipment may respond to one. Sometimes, the conversation ends up with the statement: “There is only one way to know for sure. Let’s blow it up!” Who knew that, all these years later, my affinity for explosions would become a part of my career in arc flash testing?
Arc flash testing was at the heart of developing the IEEE 1584 Standard—IEEE Guide for Performing Arc Flash Hazard Calculations, which was first published in 2002. The results of more than 300 arc flash tests were used to develop equations for this document, which is used globally to predict the prospective incident energy from an arc flash. Today, research and testing continues to develop the next generation of the IEEE 1584 Standard and further advance the understanding of this deadly electrical hazard.
Have you ever wondered how arc flash tests are conducted? Let’s go behind the scenes and take a look at what it is all about.
Arc flash testing requires the controlled environment of a high power laboratory with special capabilities. There are several of these labs around the world, and I have had the privilege of being involved with a few of them. One of these labs, Mersen, is located a short drive north of Boston in the town of Newburyport. I recently spent the better part of a week there conducting a series of tests with a colleague.
Wanted: large short-circuit current
An arc flash is a short circuit where the current arcs across an air gap. To conduct an arc flash test, an electrical source capable of producing thousands of amperes (A) of short-circuit current is needed. The magnitude of current can reach upward of 100,000A, depending on the test requirements.
Creating a short circuit for the test is not as simple as connecting a couple of conductors together on the lab’s electric power system. Like any other facility, the lab also receives its power from the local electric utility. If a short circuit were created using the utility as the source, serious problems could result. A short circuit causes the voltage to collapse near the point of the fault, resulting in dimmed lights and other potentially serious disturbances. To make matters worse, there is a real possibility of tripping a utility protective device, which could create a large power outage affecting others in the area. I would not want to explain to the neighbors why their power just went out.
To avoid these types of problems, most high power labs have their own generators that are used to produce short-circuit current. This gives them the capability to perform arc flash and short-circuit testing and have the test circuit completely isolated from the electric utility. The Mersen lab has two generators. Each generator is rated 10 megavolt-amperes (MVA) continuous with a short-circuit rating of 68 MVA and is powered by a 4,160-volt (V), 536-horsepower electric motor that is directly connected to the utility. When a test is conducted, the short-circuit current comes from the generator and not the electric utility.
This lab has the capability to produce up to 100,000A of short-circuit current at 480V. However, most testing requires the magnitude of current to be much less. To reduce the available current to a lower level, resistance and inductance is inserted between the generators and the test cell. The lab uses a large bank of switches to configure the test circuit with a specific amount of resistance and reactance. The exact amount that needs to be added to the circuit will depend on the specific magnitude of current required for the test.
Before the actual testing begins, there will be a test shot, aka, a confirmation test. During this shot, large shorting bars are bolted across the busbars where the test specimen will be connected. The generator is brought up to the desired speed and voltage, and the breaker is then closed into the bolted condition. Other than hearing the circuit breaker tripping in the next room, this is pretty uneventful. Since the connection is bolted with no air gap, no arcing occurs. During the test shot, the short-circuit current is measured to confirm that the circuit has been properly configured and the correct amount of short-circuit current was produced. Once the circuit is verified, the shorting bars are removed, and the test specimen is connected.
Arc flash duration
Since the generators cannot sustain thousands of amperes of short-circuit current for very long, and considering that most actual arc flash events are of a very short duration, a backup circuit breaker is used to limit the duration of the arc flash. Normally, the backup circuit breaker will be set to trip the short circuit offline in a predefined amount of time. This can be anywhere from several electrical cycles (1 cycle = one-sixtieth of a second) to a couple of seconds, depending on the magnitude of current and the generator’s capabilities.
Depending on the specific test setup, an arc flash will sometimes self extinguish after a few cycles, meaning it blows itself out and the backup breaker does not trip. However, sometimes the arc flash will continue for an extended period of time, in which case the backup breaker will trip to limit the duration and protect the generator and other equipment.
A trigger wire, sometimes known as a fuse wire, is used to initiate the arc flash. This is a small-diameter wire, typically ranging in size from a No. 18 to a No. 10 AWG. A real-world arc flash usually begins with accidental contact between energized conductors or between an energized conductor and a grounded surface. After the initial contact creates the short circuit, the current may arc across the gap between conductors to create the arc flash. The trigger wire is used to simulate the initial contact and initiate the arc as soon as the circuit breaker is closed at the end of the countdown.
Once the test has been set up, the calibration shot has been made, and the cameras and instrumentation are ready to go, the countdown begins. The countdown is necessary so the cameras can be started just before the circuit breaker is closed and, more important, to provide a warning to those involved to look away. As exciting as an electrical explosion can be, looking at the flash’s ultraviolet light can cause serious eye injury and even blindness.
Going on the record
Most arc flash testing will use an array of recording instruments to capture the voltage, current and incident energy. Incident energy is the primary variable used to quantify the severity of the arc flash’s thermal effects. Calorimeters measure the incident energy at a specific distance from the arc flash, known as the working distance. At the center of the insulating material is a copper slug of known mass. A thermocouple is located behind the slug and is used to measure the difference in temperature before and after the arc flash. This, along with the known mass of the copper slug, helps determine the energy required to raise the temperature.
Since incident energy varies exponentially with distance from the source of the arc, calorimeters’ placement is very important. Typically, the calorimeter is placed 18 inches away; however, other distances may also be used, depending on the type of equipment and test.
Let’s blow it up!
Arc flash testing can be performed for an almost infinite range of scenarios and conditions. The tests can range in complexity from using an empty box with electrodes—similar to the device used to develop the IEEE 1584 equations—to more complex testing that may involve actual electrical equipment. Testing can also be used in an attempt to recreate a specific event as part of a forensic investigation.
Having been involved with arc flash testing for projects, from basic equipment testing to forensic analysis involving earthquakes and nuclear plants, I can honestly say this stuff never gets old!