Designing Electrical Systems To Maximize Building and Worker Safety

Safety always has been an integral part of electrical system and product design. Recently, the increased understanding of, and focus on arc flash hazards has added another safety dimension that should be considered when designing an electrical distribution system. Careful system design has contributed substantially to fire safety and reliability of the electrical systems currently in use today.

05/01/2007


Safety always has been an integral part of electrical system and product design. Recently, the increased understanding of, and focus on arc flash hazards has added another safety dimension that should be considered when designing an electrical distribution system. Careful system design has contributed substantially to fire safety and reliability of the electrical systems currently in use today.

Arc flash accidents that involve a significant injury to a worker happen five to 10 times every day in the United States. Recent studies indicate that as many as 80% of all electrical injuries or fatalities involve life threatening or fatal tissue burns from arcing faults. These statistics are not new, but are reflected in the long history of reported electrical accidents.

This article will outline how to design an electrical distribution system to maximize building and worker safety against events such as arc flash. Adequate preparation during the design process typically will result in less problems later, and minimize the chances of equipment damage or personnel injury.

Arcing faults

Electrical system designers always have considered bolted faults the most significant event that occurs within a system. Electrical equipment is designed and built to withstand the electro-magnetic forces that result from a bolted fault long enough for an upstream protective device to clear the fault. Assigning an amps interrupting capacity (AIC) rating to electrical equipment has been a common industry practice for many years. Electrical equipment manufacturers use information from a system's short circuit study to build the equipment to this AIC rating. When bolted faults occur, all of the energy flows through conductive paths in the equipment. Because bolted faults always involve the maximum available current flow through the equipment, they will usually cause the upstream protective device to operate very rapidly.

But high-level bolted faults are quite rare events, especially after the equipment has been properly connected and is in service. Arcing faults are a far more common occurrence in electrical equipment, and the accidents that result in an injury or fatality most often happen when someone is working on the equipment while it is energized. Arcing faults release high amounts of thermal energy and concussive force into the surrounding air, which pose a threat to the equipment and to anyone near it. Arcing faults are usually the result of some unsafe act by the worker.

Because arcing faults flow through air, the air's resistance causes the arcing current to be somewhat reduced. In 480-volt, three-phase systems, arcing fault current will be approximately 40% to 60% of the total available (bolted) fault current. In medium-voltage systems, the arcing current is much higher, and often nearly equal to the total available fault current. While this current reduction may seem inconsequential, consider that the upstream protective devices only will see and react to the amount of current that is flowing during the fault. This current reduction causes overcurrent protective devices to somewhat delay clearing the fault, thereby allowing more energy to be released by the arc. This condition presents an increased hazard to anyone standing near the equipment. What's more, the amount of incident energy released in the arc is directly proportional to the clearing time. To achieve the goal of reducing this hazardous energy, clearing time must be as fast as possible without compromising coordination.

Circuit breakers, fuses and protective relays are often good choices for this role, and some are better than others, depending on the conditions. Large 600-volt current-limiting fuses such as class L will often not operate in their current limiting region at the level of arcing current that may occur, depending on the available fault current. The larger the fuse, the more fault current it requires to operate in its current limiting region. For example, a 600-volt, 4,000-amp class L fuse requires approximately 120 kA of available fault current before it becomes current limiting. Smaller current limiting fuses (400 amps and smaller) generally perform well at mitigating arcing faults over a wide range of conditions, as do thermal-magnetic molded case circuit breakers in that size range. Using bolted pressure switches as main or branch devices with large class L fuses may not be a good choice, because of the potentially slow action of the fuses. Power circuit breakers can be specified, as they perform significantly better at mitigating an arcing fault. Those devices should be specified with instantaneous trip functions if possible.


Recommendations

Always include a single main device when designing switchboards, switchgear or substations. The practice of designing service entrance equipment without main devices using the “six main” or “six disconnect” rule always results in incident energy levels of more than 40 calories/cm2 (greater than NFPA 70E-2004 Hazard Risk Category 4 for personal protective equipment) for an arcing fault occurrence on the bus. Also, specifying barriers, hinged doors and insulated bus if available as options for switchboards and switchgear is also a good idea. Barriers help prevent arc fault propagation, hinged doors are safer to open than bolted covers and insulated bus help prevent shock injuries from accidental contact and may prevent an arcing fault if a worker drops a tool. Consider specifying switchgear designs that permit racking of the circuit breakers with the doors closed. The closed door will help protect the worker from an arcing fault should one occur during a racking operation.

An arc can be extinguished much faster than it can be interrupted through the action of an overcurrent protective device. Consider fault-making devices for metal-clad switchgear. The combined action of current and photo optical sensors in these products can react very quickly (less than

The choices of switchboards and switchgear also present issues for personnel protection. Switchboards do not normally have vertical barriers between sections. Often when working in a switchboard, even in one of the distribution sections, a worker will still be exposed to arc flash incident energy at the line side of the main device, which will be higher than in the other sections. Switchgear has more barriers around the circuit breaker and between vertical sections that help prevent propagation and escalation of an arcing fault condition.

It is always good practice to avoid oversizing electrical equipment beyond what is required for the load. Oversizing equipment leads to larger overcurrent protective devices than necessary, which may result in longer fault clearing times.

Consider also specifying an arc flash analysis during the project design phase. At this stage, all of the drawings are current, the equipment will be in optimal working order (i.e., new), and all of the device time-current curves are available. Because the equipment is not yet built, changes in the design can still be done to deal with areas where high incident energy conditions might exist. The short circuit and coordination studies are normally done at this time, and it is very easy and inexpensive to also conduct the arc flash hazard analysis before the equipment is shipped to the customer.

Several products are available that allow workers to remain outside the danger zone. For medium voltage metal-clad switchgear, remote racking options may be available that permit a worker to stay outside of the electrical equipment room when racking circuit breakers. For facilities that perform infrared inspection as part of a preventive maintenance program, inspection windows may be specified that are applied in the equipment covers. This allows equipment inspection without the need to remove the equipment covers, and eliminates the need for PPE.

Facility managers and consultants must keep one thing in mind: It is important to perform ongoing arc-flash hazard analyses to quantify the risks and take steps to prevent them.

Documentation is Key

Before you even consider an arc flash analysis, electrical documentation must be accurate. The facility one-line diagram is the culmination of all electrical documentation. If the one-line diagram is not accurate, further efforts toward analyzing arc flash are futile.

Once electrical documentation is in order, you can proceed to analyze the facility. Experts stress the necessity of a one-line diagram that matches the plant's electrical system as a starting point in assessing arc flash hazards. Every electrical distribution path in every plant is different. Each component in each of these paths is a variable that must be considered when evaluating potential arc flash hazards. An arc flash hazard analysis is more than recommended—it is urged.

Notice that arc flash analysis is listed within the steps to perform an arc flash analysis. This is not doublespeak; it is presented to emphasize the necessary involvement before and after making any calculations that quantify risks.

The physics behind arc flash is interactive. Available fault current is directly proportional to flash protection, limited approach, restricted approach and prohibited approach boundaries. Available energy depends on current and time. Electrical system impedance affects the magnitude of fault current directly. Fault current clearing time depends on the protective system. And an accurate one-line diagram of a facility's electrical system is crucial to determining available fault current.

Energized Equipment

Common sense suggests strongly that the power must be off if equipment is to be serviced. OSHA prevails where common sense fails. OSHA requires in its 29 CFR 1910.333(a)(1) that “live parts be de-energized unless the employer can demonstrate that the de-energizing introduces additional or increased hazards.”

The 2004 release of NFPA-70E includes language that details requirements of an energized electrical work permit, which calls for written authorization for work on or near a circuit that is energized at 50 volts or greater. There must be written documentation to prove that it is infeasible to work on equipment or circuits de-energized as required by the OSHA regulations. The permit intends to verify that electrical workers are fully aware of the hazards.

Good business practice

Designing an electrical distribution system for maximum safety is good business practice for electrical system designers. Choices made during the design process will potentially affect how the equipment reacts to the possibility of an arcing fault condition, which can make all the difference between a safe working environment, and the safest it can be.



Assessing the Arc Flash Risk

Every year, arc flash kills between 200 and 300 people. Between five and 10 times a day, somewhere in the United States an arc flash explosion in electric equipment sends burn victims to a special burn center. This number does not include cases sent to ordinary hospitals and clinics, nor unreported case and “near misses.”

“An arc flash is an explosion with all the implications of that word,” says George Gregory, manager, Industry Standards, Square D/Schneider Electric, Cedar Rapids, Iowa. “It involves a fireball with temperatures at the center of the arc plasma that are over 5,000°F—hot enough to vaporize metal. The arc also produces a pressure blast that propels hot gases and molten metal outward. It presents a brown cloud of toxic materials. The ionized gases are conductors that will cause electrical shock or current paths through the body.”

Facility engineers and consultants alike must take arc flash seriously. Assuming that the danger does not exist in a given facility could prove fatal. It is important to perform ongoing arc-flash hazard analyses to quantify the risks and take steps to prevent them. The reasons for this risk analysis are to determine the incident energy for each electrical distribution path to each piece of electrical equipment and the level of personal protective equipment (PPE) required when working on energized circuitry.

Arc-flash hazard exists wherever electrical circuits have enough available energy to sustain an electrical plasma arc. The current required to clear a protection device is determined by bolted fault current. However, a bolted fault is always a worst-case scenario in terms of a dead short.

Arc flash presents a different problem. A worker drops a tool, insulation on aging conductors deteriorates, or animals create a momentary fault. Regardless of the incident, when a flash-over occurs, a plasma is created that conducts electricity. But it conducts with higher impedance than a dead short. Because the impedance through the plasma arc is higher, the current is lower. A problem exists when this current is insufficient to clear the fault at the protection device—regardless of whether it is a circuit breaker, fuse or tie breaker. Upstream protection devices have even higher ratings, allowing the fault to continue.

When the fault finally clears, the damage is done. The idea behind doing the analyses and taking corrective measures is to limit the amount of available fault current, and therefore the incident energy, at each piece of equipment and provide adequate PPE.

Above all, keep documentation up to date. Change the system and you must change documentation, and you must recalculate all affected energy paths. — By Jack Smith, Managing Editor, Plant Engineering

Assessing the Arc Flash Risk

Every year, arc flash kills between 200 and 300 people. Between five and 10 times a day, somewhere in the United States an arc flash explosion in electric equipment sends burn victims to a special burn center. This number does not include cases sent to ordinary hospitals and clinics, nor unreported case and “near misses.”

“An arc flash is an explosion with all the implications of that word,” says George Gregory, manager, Industry Standards, Square D/Schneider Electric, Cedar Rapids, Iowa. “It involves a fireball with temperatures at the center of the arc plasma that are over 5,000°F—hot enough to vaporize metal. The arc also produces a pressure blast that propels hot gases and molten metal outward. It presents a brown cloud of toxic materials. The ionized gases are conductors that will cause electrical shock or current paths through the body.”

Facility engineers and consultants alike must take arc flash seriously. Assuming that the danger does not exist in a given facility could prove fatal. It is important to perform ongoing arc-flash hazard analyses to quantify the risks and take steps to prevent them. The reasons for this risk analysis are to determine the incident energy for each electrical distribution path to each piece of electrical equipment and the level of personal protective equipment (PPE) required when working on energized circuitry.

Arc-flash hazard exists wherever electrical circuits have enough available energy to sustain an electrical plasma arc. The current required to clear a protection device is determined by bolted fault current. However, a bolted fault is always a worst-case scenario in terms of a dead short.

Arc flash presents a different problem. A worker drops a tool, insulation on aging conductors deteriorates, or animals create a momentary fault. Regardless of the incident, when a flash-over occurs, a plasma is created that conducts electricity. But it conducts with higher impedance than a dead short. Because the impedance through the plasma arc is higher, the current is lower. A problem exists when this current is insufficient to clear the fault at the protection device—regardless of whether it is a circuit breaker, fuse or tie breaker. Upstream protection devices have even higher ratings, allowing the fault to continue.

When the fault finally clears, the damage is done. The idea behind doing the analyses and taking corrective measures is to limit the amount of available fault current, and therefore the incident energy, at each piece of equipment and provide adequate PPE.

Above all, keep documentation up to date. Change the system and you must change documentation, and you must recalculate all affected energy paths. — By Jack Smith, Managing Editor, Plant Engineering

Assessing the Arc Flash Risk

Every year, arc flash kills between 200 and 300 people. Between five and 10 times a day, somewhere in the United States an arc flash explosion in electric equipment sends burn victims to a special burn center. This number does not include cases sent to ordinary hospitals and clinics, nor unreported case and “near misses.”

“An arc flash is an explosion with all the implications of that word,” says George Gregory, manager, Industry Standards, Square D/Schneider Electric, Cedar Rapids, Iowa. “It involves a fireball with temperatures at the center of the arc plasma that are over 5,000°F—hot enough to vaporize metal. The arc also produces a pressure blast that propels hot gases and molten metal outward. It presents a brown cloud of toxic materials. The ionized gases are conductors that will cause electrical shock or current paths through the body.”

Facility engineers and consultants alike must take arc flash seriously. Assuming that the danger does not exist in a given facility could prove fatal. It is important to perform ongoing arc-flash hazard analyses to quantify the risks and take steps to prevent them. The reasons for this risk analysis are to determine the incident energy for each electrical distribution path to each piece of electrical equipment and the level of personal protective equipment (PPE) required when working on energized circuitry.

Arc-flash hazard exists wherever electrical circuits have enough available energy to sustain an electrical plasma arc. The current required to clear a protection device is determined by bolted fault current. However, a bolted fault is always a worst-case scenario in terms of a dead short.

Arc flash presents a different problem. A worker drops a tool, insulation on aging conductors deteriorates, or animals create a momentary fault. Regardless of the incident, when a flash-over occurs, a plasma is created that conducts electricity. But it conducts with higher impedance than a dead short. Because the impedance through the plasma arc is higher, the current is lower. A problem exists when this current is insufficient to clear the fault at the protection device—regardless of whether it is a circuit breaker, fuse or tie breaker. Upstream protection devices have even higher ratings, allowing the fault to continue.

When the fault finally clears, the damage is done. The idea behind doing the analyses and taking corrective measures is to limit the amount of available fault current, and therefore the incident energy, at each piece of equipment and provide adequate PPE.

Above all, keep documentation up to date. Change the system and you must change documentation, and you must recalculate all affected energy paths. — By Jack Smith, Managing Editor, Plant Engineering

Assessing the Arc Flash Risk

Every year, arc flash kills between 200 and 300 people. Between five and 10 times a day, somewhere in the United States an arc flash explosion in electric equipment sends burn victims to a special burn center. This number does not include cases sent to ordinary hospitals and clinics, nor unreported case and “near misses.”

“An arc flash is an explosion with all the implications of that word,” says George Gregory, manager, Industry Standards, Square D/Schneider Electric, Cedar Rapids, Iowa. “It involves a fireball with temperatures at the center of the arc plasma that are over 5,000°F—hot enough to vaporize metal. The arc also produces a pressure blast that propels hot gases and molten metal outward. It presents a brown cloud of toxic materials. The ionized gases are conductors that will cause electrical shock or current paths through the body.”

Facility engineers and consultants alike must take arc flash seriously. Assuming that the danger does not exist in a given facility could prove fatal. It is important to perform ongoing arc-flash hazard analyses to quantify the risks and take steps to prevent them. The reasons for this risk analysis are to determine the incident energy for each electrical distribution path to each piece of electrical equipment and the level of personal protective equipment (PPE) required when working on energized circuitry.

Arc-flash hazard exists wherever electrical circuits have enough available energy to sustain an electrical plasma arc. The current required to clear a protection device is determined by bolted fault current. However, a bolted fault is always a worst-case scenario in terms of a dead short.

Arc flash presents a different problem. A worker drops a tool, insulation on aging conductors deteriorates, or animals create a momentary fault. Regardless of the incident, when a flash-over occurs, a plasma is created that conducts electricity. But it conducts with higher impedance than a dead short. Because the impedance through the plasma arc is higher, the current is lower. A problem exists when this current is insufficient to clear the fault at the protection device—regardless of whether it is a circuit breaker, fuse or tie breaker. Upstream protection devices have even higher ratings, allowing the fault to continue.

When the fault finally clears, the damage is done. The idea behind doing the analyses and taking corrective measures is to limit the amount of available fault current, and therefore the incident energy, at each piece of equipment and provide adequate PPE.

Above all, keep documentation up to date. Change the system and you must change documentation, and you must recalculate all affected energy paths. — By Jack Smith, Managing Editor, Plant Engineering

Assessing the Arc Flash Risk

Every year, arc flash kills between 200 and 300 people. Between five and 10 times a day, somewhere in the United States an arc flash explosion in electric equipment sends burn victims to a special burn center. This number does not include cases sent to ordinary hospitals and clinics, nor unreported case and “near misses.”

“An arc flash is an explosion with all the implications of that word,” says George Gregory, manager, Industry Standards, Square D/Schneider Electric, Cedar Rapids, Iowa. “It involves a fireball with temperatures at the center of the arc plasma that are over 5,000°F—hot enough to vaporize metal. The arc also produces a pressure blast that propels hot gases and molten metal outward. It presents a brown cloud of toxic materials. The ionized gases are conductors that will cause electrical shock or current paths through the body.”

Facility engineers and consultants alike must take arc flash seriously. Assuming that the danger does not exist in a given facility could prove fatal. It is important to perform ongoing arc-flash hazard analyses to quantify the risks and take steps to prevent them. The reasons for this risk analysis are to determine the incident energy for each electrical distribution path to each piece of electrical equipment and the level of personal protective equipment (PPE) required when working on energized circuitry.

Arc-flash hazard exists wherever electrical circuits have enough available energy to sustain an electrical plasma arc. The current required to clear a protection device is determined by bolted fault current. However, a bolted fault is always a worst-case scenario in terms of a dead short.

Arc flash presents a different problem. A worker drops a tool, insulation on aging conductors deteriorates, or animals create a momentary fault. Regardless of the incident, when a flash-over occurs, a plasma is created that conducts electricity. But it conducts with higher impedance than a dead short. Because the impedance through the plasma arc is higher, the current is lower. A problem exists when this current is insufficient to clear the fault at the protection device—regardless of whether it is a circuit breaker, fuse or tie breaker. Upstream protection devices have even higher ratings, allowing the fault to continue.

When the fault finally clears, the damage is done. The idea behind doing the analyses and taking corrective measures is to limit the amount of available fault current, and therefore the incident energy, at each piece of equipment and provide adequate PPE.

Above all, keep documentation up to date. Change the system and you must change documentation, and you must recalculate all affected energy paths. — By Jack Smith, Managing Editor, Plant Engineering

Assessing the Arc Flash Risk

Every year, arc flash kills between 200 and 300 people. Between five and 10 times a day, somewhere in the United States an arc flash explosion in electric equipment sends burn victims to a special burn center. This number does not include cases sent to ordinary hospitals and clinics, nor unreported case and “near misses.”

“An arc flash is an explosion with all the implications of that word,” says George Gregory, manager, Industry Standards, Square D/Schneider Electric, Cedar Rapids, Iowa. “It involves a fireball with temperatures at the center of the arc plasma that are over 5,000°F—hot enough to vaporize metal. The arc also produces a pressure blast that propels hot gases and molten metal outward. It presents a brown cloud of toxic materials. The ionized gases are conductors that will cause electrical shock or current paths through the body.”

Facility engineers and consultants alike must take arc flash seriously. Assuming that the danger does not exist in a given facility could prove fatal. It is important to perform ongoing arc-flash hazard analyses to quantify the risks and take steps to prevent them. The reasons for this risk analysis are to determine the incident energy for each electrical distribution path to each piece of electrical equipment and the level of personal protective equipment (PPE) required when working on energized circuitry.

Arc-flash hazard exists wherever electrical circuits have enough available energy to sustain an electrical plasma arc. The current required to clear a protection device is determined by bolted fault current. However, a bolted fault is always a worst-case scenario in terms of a dead short.

Arc flash presents a different problem. A worker drops a tool, insulation on aging conductors deteriorates, or animals create a momentary fault. Regardless of the incident, when a flash-over occurs, a plasma is created that conducts electricity. But it conducts with higher impedance than a dead short. Because the impedance through the plasma arc is higher, the current is lower. A problem exists when this current is insufficient to clear the fault at the protection device—regardless of whether it is a circuit breaker, fuse or tie breaker. Upstream protection devices have even higher ratings, allowing the fault to continue.

When the fault finally clears, the damage is done. The idea behind doing the analyses and taking corrective measures is to limit the amount of available fault current, and therefore the incident energy, at each piece of equipment and provide adequate PPE.

Above all, keep documentation up to date. Change the system and you must change documentation, and you must recalculate all affected energy paths. — By Jack Smith, Managing Editor, Plant Engineering

Assessing the Arc Flash Risk

Every year, arc flash kills between 200 and 300 people. Between five and 10 times a day, somewhere in the United States an arc flash explosion in electric equipment sends burn victims to a special burn center. This number does not include cases sent to ordinary hospitals and clinics, nor unreported case and “near misses.”

“An arc flash is an explosion with all the implications of that word,” says George Gregory, manager, Industry Standards, Square D/Schneider Electric, Cedar Rapids, Iowa. “It involves a fireball with temperatures at the center of the arc plasma that are over 5,000°F—hot enough to vaporize metal. The arc also produces a pressure blast that propels hot gases and molten metal outward. It presents a brown cloud of toxic materials. The ionized gases are conductors that will cause electrical shock or current paths through the body.”

Facility engineers and consultants alike must take arc flash seriously. Assuming that the danger does not exist in a given facility could prove fatal. It is important to perform ongoing arc-flash hazard analyses to quantify the risks and take steps to prevent them. The reasons for this risk analysis are to determine the incident energy for each electrical distribution path to each piece of electrical equipment and the level of personal protective equipment (PPE) required when working on energized circuitry.

Arc-flash hazard exists wherever electrical circuits have enough available energy to sustain an electrical plasma arc. The current required to clear a protection device is determined by bolted fault current. However, a bolted fault is always a worst-case scenario in terms of a dead short.

Arc flash presents a different problem. A worker drops a tool, insulation on aging conductors deteriorates, or animals create a momentary fault. Regardless of the incident, when a flash-over occurs, a plasma is created that conducts electricity. But it conducts with higher impedance than a dead short. Because the impedance through the plasma arc is higher, the current is lower. A problem exists when this current is insufficient to clear the fault at the protection device—regardless of whether it is a circuit breaker, fuse or tie breaker. Upstream protection devices have even higher ratings, allowing the fault to continue.

When the fault finally clears, the damage is done. The idea behind doing the analyses and taking corrective measures is to limit the amount of available fault current, and therefore the incident energy, at each piece of equipment and provide adequate PPE.

Above all, keep documentation up to date. Change the system and you must change documentation, and you must recalculate all affected energy paths. — By Jack Smith, Managing Editor, Plant Engineering



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