"Failure is not an option." This quote from Eugene Kranz, a flight director for NASA during the Apollo 13 space mission, defines the concept of mission critical in five words. Yes, the Apollo 13 mission may be an extreme example of mission critical, where every decision and every action made was essential to the survival of the astronauts aboard that unstable spacecraft. However, for those owning, managing, or running a mission critical facility, the operation of their electrical distribution system is essential to the survival of their business or organization. Whether that mission critical system is necessary for the protection of human life, such as an emergency-operation center, or essential for business continuity, such as a data center, they often share the same "failure is not an option" philosophy.
When describing the electrical distribution system for a mission critical facility, two of the key components are availability and reliability. The electrical system must be available when called upon to function (24/7) and it must not fail while in operation. Based on this "must not fail" philosophy, most of the protection systems for mission critical facilities traditionally have been designed to keep the system operating. As such, protection devices are set as high as possible to prevent them from tripping and de-energizing the critical load. This philosophy guards against dropping the critical load, but it does not protect the equipment or more importantly, the personnel working on the equipment from potential hazards such as arc flash. This article looks at the factors that determine the intensity of an arc flash hazard and how arc flash is affected by different protection schemes used in mission critical facilities. The article also investigates methods to mitigate arc flash hazards for mission critical facilities.
NFPA 70E-2015: Standard for Electrical Safety in the Workplace defines an arc flash hazard as "a dangerous condition associated with the possible release of energy caused by an electrical arc." An electrical arc occurs when the electrical current diverges from its intended path and travels through the air from one conductor to another or to ground. Arcing faults generate large amounts of heat that can severely burn human skin and set clothing on fire, making them extremely dangerous and potentially lethal. In addition to the intense heat, an arc fault can also create an explosive blast. During an arc fault, the high temperatures vaporize the electrical conductor, changing it from a solid state to gas vapor and causing it to expand outward with an explosive force. This explosive force can cause destruction to equipment, start fires, and injure employees working on the equipment as well as any surrounding bystanders.
NFPA 70E and IEEE 1584: IEEE Guide for Performing Arc Flash Hazard Calculations are the two standards used by the industry for guidelines and analysis regarding arc flash and arc flash safety. Arc flash hazards are usually expressed as a unit of incident energy (cal/cm2). Incident energy is a measure of thermal energy at a working distance from an arc fault. The three parameters used in an arc flash study to determine the incident energy and the severity of an arc flash injury are arcing current, working distance, and arcing time.
Arcing current: As previously indicated, an electrical arc occurs when the electrical current diverges from its intended path and travels through the air from one conductor to another or to ground. Arcing current is the current released during this fault condition. The magnitude of that arcing fault current is used in the calculation of incident energy. Data centers tend to be consumers of immense electric power; therefore, they have very sizable distribution systems. In the past, and even today, many data centers are being designed with large, multimodule uninterruptible power supply systems and large transformers to reduce the amount of equipment. These larger systems have a substantially higher let-through current that can allow increased amounts of fault current downstream in the distribution system.
Mission critical facilities also tend to have generators for backup power with closed-transition capabilities that allow the load to transfer from generator to utility without interruption (or vice versa). During this closed-transition period, the generator and utility operate in parallel and both contribute to the fault current, which results in increased amounts of fault current. In general, the more fault current available, the higher the arc flash thermal energy and the more dangerous the potential hazard becomes. Because of the larger systems and closed-transition transfers, data centers often have a higher magnitude of available fault current throughout the distribution system, leading to higher arc flash thermal energy. The higher the arc fault thermal energy becomes, the more dangerous the arc flash hazard becomes to those working on the equipment.
Working distance: Arc flash energy is established at the working distance from the arcing fault. The working distance is the distance between the arcing source and the worker's head and chest. The IEEE 1584a standard defines a working distance for each equipment type and voltage class. NFPA 70E Table D.4.3 provides typical working distances for different equipment and voltages.
The farther away a person can be from a fault, the lower the incident energy exposure. The 24/7 availability and reliability components of a mission critical facility require a comprehensive preventive maintenance program. Often, this preventive maintenance program demands that the equipment be operated or worked on live. For example, using infrared for scanning cable lugs to ensure all connections are tight. Working on equipment live puts the personnel working on the equipment close to the fault; therefore, they are at risk of severe injury.
When working on or near a potential source of an arc flash, personal protective equipment (PPE) must be worn. PPE typically refers to the protective clothing and equipment designed to protect the wearer's body from injury by reducing the workers' exposure to incident energy. The rating and type of PPE worn is based on the calculated exposure to incident energy. NFPA 70E Table H.3(b) provides guidance on the selection of arc-rated clothing and other PPE based on the incident-energy exposure.
Another issue with data centers is that electrical equipment, such as power distribution units and remote power panels, is often located in a data center's white space, which is occupied by unqualified electrical personnel. In addition to those working on the equipment, the explosive force caused by an arc flash can injure bystanders in close proximity to the arc flash. For this reason, an arc flash boundary is calculated in addition to the working distance. This boundary defines the distance from exposed live parts within which an unprotected person could receive second-degree burns. During equipment servicing, those without PPE should not be permitted within this boundary. Locating electrical equipment inside the data center, rather than in dedicated galleries adjacent to the data center, puts the information technology equipment at risk of damage and those employees working in the data center at risk of injury.