While equipment changes may not be necessary to comply with NFPA 70E, the new regulations governing the prevention of arc flash, companies looking to improve workplace safety can now employ technologies designed specifically to limit arc flash hazards by installing new equipment or upgrading existing equipment.
NFPA 70E requires companies to:
- Perform an arc flash hazard analysis on all electrical equipment.
- Label electrical equipment designating the level of personal protective equipment (PPE) a worker must wear when working on energized equipment.
- Train workers and update work practice procedures to comply with the standards.
- Deploy products, solutions and methods to limit arc flash hazards whenever possible.
An arc flash occurs when insulation or isolation between electrified conductors is breached or can no longer withstand the applied voltage. As employees work on or near energized conductors or circuits, movement near or contact with the equipment (or a failure of the equipment) may cause a phase-to-ground and/or a phase-to-phase fault. Temperatures well over 5,000 degrees Fahrenheit and a powerful explosion can be produced by this arc flash.
Two types of short circuits
There are two types of short-circuit events: arcing faults and bolted faults. While arcing happens in both cases, it occurs in different ways and requires different protective solutions.
An arc fault occurs when electrical clearances are reduced or compromised by deteriorating insulation or human error, causing a conductive path between opposite phases or phase-to-ground. The arc burns in open air, creating a large amount of energy until an upstream overcurrent protection device opens to clear the fault. The amount of energy released during an arcing fault is a function of the available fault current at the point in the system, reduced by the dynamic impedance of the arc and the time the arc persists.
Bolted faults, on the other hand, are caused by mistakes made during installation or maintenance, when phases or phase-to-ground are connected together (bolted). The overcurrent protective device upstream of the fault opens to protect the system. Arcing occurs inside the protective device as the contacts open. When the device is a circuit breaker, arcing and gassing are vented outside the breaker through the arc chute.
Until now, bolted fault current was the standard by which the effectiveness of electrical protection equipment was measured. But extensive testing by the Institute of Electrical Engineers (IEEE) has quantified arc flash hazards in different types of equipment and led to a better understanding of the difference between arc fault current and bolted fault current.
Bolted fault current is the measure of current flowing through bolted bus bars, while an arc flash is a current flowing through the air. Because air creates a restriction or impedance, the arc fault current is always lower than the bolted fault current, which flows unimpeded through the metal of a bus bar.
This is important, because circuit breakers and fuses have traditionally been designed to disconnect current very rapidly when it reaches a specific bolted fault current level, yet allow a time delay when current magnitude is at a lower value. Since an arc flash occurs at a lower current level than the bolted fault, these protective devices may allow the release of greater arc flash incident energy at lower current values than at higher values.
This realization has led electrical equipment makers to consider a totally different approach to designing their circuit protection systems. These designs limit current at high values and interrupt current more rapidly at lower short-circuit values. They also enable electrical workers to reduce the amount of protective gear they must wear when working on energized equipment.
Fuses: benefits and limits
Fused switches and fused circuit breakers utilizing current-limiting fuses have traditionally been employed to provide additional arc flash protection. In a fused circuit breaker, the fuse protects the circuit breaker at fault current levels above its interrupting rating. The fuse provides quick clearing times during high-level short circuits and limits the amount of current available for the arc flash event.
In a fused switch, the fuse is the only overcurrent protective device in the circuit. A current-limiting fuse opens very rapidly at high current values in its current limiting mode, usually 20 to 35 times its continuous current rating. For continuous current ratings of 400 amperes and lower, both fuses and circuit breakers are very effective in limiting incident energy from an arc flash incident, based on calculations with values published in IEEE 1584-2002, Guide for Performing Arc-Flash Hazard Calculations. Whether a fuse or circuit breaker is used, incident energy values are within Hazard Category 1 or lower for all values of available fault current.
(Note: The charts for incident energy versus available bolted fault current were developed using IEEE 1584 calculations. They include the steps that convert the available bolted fault current to anticipated arc fault values, so that arcing fault values are considered in the calculation method but not seen in the charts. Only bolted fault current is shown for simplicity.)
While fuses with current ratings of 400 amps and below perform as well as circuit breakers, electrical system elements impacting the incident energy and bolted current become significant for ratings above 400 amps and up to 5,000 or more amps. The goal of engineering is to drive incident energy values down, so that the level of heat and blast is minimized if an arc fault does occur. For these higher current ratings, tests indicate that current limiting circuit breakers perform significantly better than fuses at the lower fault current levels associated with arcing faults and on a par with fuses at the very high fault current levels.
The reason is that fuses have relatively high thresholds of current limitation compared with the lower instantaneous tripping levels of circuit breakers. This characteristic is valuable when fuses are used with current limiting circuit breakers. In that case, the fuse is designed not to limit the functioning of the circuit breaker in low-level over-current conditions. A 1600 A fuse-limiter, typically used to protect 800 A circuit breakers, will not operate within the threshold of current limitation until about 32,000 A. Below that level the fuse's opening time is considerably longer than the ½-cycle time at which it was thought to open, according to the IEEE study results. This additional time increases the potential incident energy during an arc flash event.
Limiting arcing current
Low voltage power circuit breakers and other main or feeder circuit breaker designs have traditionally used current path geometry to withstand the circuit forces for a short time. Magnetic forces within the circuit breaker keep the contacts closed, allowing downstream overcurrent devices time to open to clear a fault. As the current flow increases, this “blow closed” design increases the force keeping the contact assembly closed.
New current limiting designs such as Square D® MasterPact® Low Arc Flash circuit breakers act in the reverse, blowing open the terminal to interrupt the circuit. This unique design approach allows the circuit breaker to go into an accelerated opening mode at about 30,000A, comparable to limiting fuses. At a slightly higher current the circuit breaker goes into a fully current limiting mode. Even below that level, the circuit breaker will open instantaneously with a rapid opening time at current values down to its instantaneous setting. The same technology also provides very good arc flash limitation through a broad range of potential fault current levels.
The blow-open terminal in these circuit breakers is shaped so there is a reverse current loop in the moving arm. This reverse current flow creates a magnetic force proportional to the amount of current and, when the current is high enough, the force quickly pushes open the contacts, without waiting for the mechanism to unlatch and the springs to pull the moving arm open. The result is a faster opening time –
9 milliseconds at 200k -- than with traditional circuit breakers, limiting incident energy levels and the duration of an arc flash.
The circuit breaker also houses a filtered arc chute that contains an assembly of metallic grids and meshes that greatly reduce the gases released during an interruption. These grids de-ionize and cool the emissions, reducing the volume of vented gas and absorbing up to 95 percent of the energy. This compares to approximately an 80 percent reduction in energy in traditional circuit breaker designs.
Retrofitting older equipment
Aging equipment and inconsistent maintenance increase the likelihood of an arc flash incident. While most power circuit breakers will last 20 years or more, older installations require more maintenance to make sure they remain functional. As maintenance becomes more frequent on aging equipment, so do the opportunities for accidents or equipment failures that could cause an arc flash.
Some people mistakenly think that adding an extra transformer to an existing distribution system will increase impedance sufficiently to minimize the potential for an arc flash incident. Analysis shows that while the transformer may limit bolted current, it has minimal impact on arc fault and incident energy.
A better answer is to upgrade the equipment by replacing the existing circuit breaker with a new current limiting circuit breaker. While there are a number of options for circuit breaker retrofit, a power systems engineering analysis should be the first step in determining the best upgrade solution for each facility.
Replacing low voltage power circuit breakers provides an upgrade in technology with minimal downtime. The smaller size of modern circuit breakers makes this relatively easy because the new circuit breaker is generally much smaller than the one it replaces. In a direct replacement, the new circuit breaker's cradle is custom engineered to fit directly into the existing cell without any modifications to the cell. This ability to upgrade equipment with minimal disruption to operations is particularly important in process industries and other critical power applications where any type of equipment shutdown is to be avoided. It is also a solution that can be used in any manufacturer's equipment.
Another option is retrofill, where the existing switchgear cell is modified to accept the new low voltage power circuit breaker. Each circuit breaker cell is reworked to allow installation of a current limiting circuit breaker, cradle, racking mechanism and new cell door. While providing an upgrade in technology, a retrofill usually requires a main bus outage of some limited duration. Circuits supporting critical loads can be sectionalized and temporarily rerouted during the retrofill process to minimize downtime.
The process of complying with the new NFPA 70E standards for minimizing arc flash incidents presents an opportunity to re-examine your electrical system and procedures, gain a better understanding of potential weaknesses, instill new work practices to better protect employees and minimize financial risks to your company. Along with performing an arc flash hazard analysis on equipment, labeling equipment for the required PPE, training workers and updating work practice procedures, modern electrical equipment solutions can contribute significantly to a safer workplace.
Joseph Weigel is a marketing operations manager for Square D Services in Nashville, TN. A veteran of over 30 years in the electrical industry in the U.S. and China, he is a specialist in strategies to extend the life of electrical distribution systems in industrial and commercial facilities.
Jonathan Clough, PE is Western Region engineering team leader for Square D Services in Spokane, WA and Power Consultant for National Accounts. He has 27 years experience in heavy industry, including pulp and paper and aluminum rolling mills. He is an active member of the IEEE Power Engineering Society and the Inland Northwest Electric League
