Commonly asked questions and answers regarding Arc Flash/Arc Fault?
Why, as of 2015, will the NFPA be referring to an Arc Flash Hazard Analysis as an Arc Flash Risk Assessment?
The reason why the NFPA has made this change is to help clarify that an electrical panel is not necessarily always a hazard. If the equipment is properly maintained and kept up to date the panel actually being a “hazard” or hazardous is very minimal but the “risk” for an Arc Flash Hazard is still there. Which is why proper precautions, such as the application of the appropriate voltage rated PPE, must still be taken.
The definition however is still basically the same but a bit more simplified…
What is an arc flash or Arc Fault?
When a fault occurs in an electrical circuit (or a second fault occurs in an underground circuit), an arc occurs. The arc is plasma that results from a current flowing through air. The Plasma converts some of the electrical energy into thermal energy at extremely high temperature. The temperature can approach 30,000°F. In most cases, the over-current device senses the fault and removes the source of energy, but the temperature frequently reaches 16,000°F to 18,000°F before the overcurrent device can clear the fault.
What hazards are associated with arc flash?
An arcing fault produces thermal energy and mechanical energy (a pressure wave). Some electrical energy is converted into visible light and other frequencies of the electromagnetic spectrum. Although the intensity of each of these forms of energy is not yet predictable, the thermal hazard is known to be severe, and the pressure wave is known to be significant.
In cooperation with NFPA, IEEE has initiated a multi-year research effort to determine all hazards and define a mechanism to predict their effects.
Can an arc flash hazard exist in a manhole?
Yes, Generally, cables and conductors in a manhole are high current of voltage and serve several areas or devices. Although terminations normally do not exist in a manhole, splices are common, and terminations sometimes exist. The insulation on old cables can become brittle and subject to damage by moving the cable or conductor. The environment on the manhole is likely to be wet and congested.
Any work in an manhole subjects existing cables and splices to physical damage. In some cases, physical damage is likely to generate an arcing fault. Failures inside a manhole are infrequent; however, when a failure occurs, the elevated temperature created in the arcing fault surrounds any worker in the manhole.
Are workers normally exposed to arc flash?
Workers who operate disconnect switches might be exposed to injury should and arcing fault occur. However, if the installation meets the consensus requirements for overcurrent protection, the risk of injury is reduced. Equipment that is not adequately maintained increases the risk of injury. Some electrical equipment is constructed with ventilation holes in the door or cover. In those cases, the risk of exposure to injury is increased. Employers and workers must assess both the hazard and the risk of injury.
What are the most important variables of an arcing fault?
Three variables that primarily affect incident energy in an arcing fault are the capacity of the circuit to maintain the arc, the duration of the arc, and the distance between the worker and the arc. Employers should ensure that the overcurrent protective device is functioning and operates in the minimum amount of time. A worker should position his or her body so that the distance between any body part and the potential arc source is as great as possible.
When is an arc fault most likely to occur?
Most arcing faults occur when something is moving. Opening a door, removing a cover, and operating a disconnecting means or a closing contactor are frequently actions that initiate an arcing fault. A worker’s movement also might result in an arcing fault.
What is the most frequent cause of an arcing fault?
Noted psychologist H.W. Heinrich suggested that human errors are the primary cause of most incidents and injuries. In the preamble to 29 CFR 1910 subpart S, OSHA suggests that up to 67 percent of electrical injuries result from inappropriate action of a worker.
The causes of all injuries and incidents can be divided into three categories:
- § Unsafe equipment
- § Unsafe conditions
- § Unsafe action
Unsafe actions cause about two thirds of the total number of injuries and incidents; unsafe equipment and unsafe conditions combined cause the remainder of injuries and incidents. Although equipment does fail and workers are sometimes injured due to an unsafe condition, the action of a worker is the principal cause of an arcing fault.
What work does practice is most important to avoid injury form an arc flash?
Workers are injured only when energy is released in an arcing fault or when they touch an exposed energized electrical conductor. Therefore, the only way to completely avoid the possibility of an injury from electrical energy is to remove the energy and take steps to ensure that the energy cannot re-accumulate. An arc flash event requires electrical energy. If no energy is available, no injury is possible. When that condition exists, the work task is considered to be in an electrically safe work condition. Therefore, the most important work practice is to create an electrically safe work condition.
How does a worker know if he or she is exposed to a potential arc flash?
In section 400.11, NFPA 70E suggests that a label be field installed on equipment that contains an arc flash hazard. In many instances, the label identifies the arc rating of PPE that is necessary for protection from the estimated hazard. If a label is present, workers know that an arc flash hazard exists.
If a label is not present, the analysis must take a different form. The electrical safety program should contain a procedure that defines the necessary steps to perform the hazard/risk analysis. If neither a label nor a procedure exists, the worker must determine the capacity of the source of energy. If the capacity of the energy source exceeds 125 kVA, then you are exposed to a thermal hazard.
What happens if the overcurrent protection fails to clear the fault?
Circuit breakers and fuses may be applied improperly. Devices are sometimes installed in circuits so that available fault current exceeds the device’s ability to clear a fault. Is the overcurrent device is applied improperly and does not clear the fault, the device could fail violently and expel parts and pieces. The risk of injury is elevated significantly.
Why do I get different answers when I use different methods to calculate incident energy?
Each current method of predicting the thermal hazard associated with an arcing fault uses different variables and different mathematical processes. Each method of calculating incident energy has both positive and negative aspects. All methods produce a number that is an estimate, at best.
Am I exposed to an arc flash hazard when the equipment doors are closed?
An electrical installation that meets the requirements of the National Electrical Code does not expose a worker to arc flash hazard when all the code requirements are met. Other national consensus codes may provide the same protection. However, as an installation ages, the integrity of the installation deteriorates. A worker must consider the age and state of maintenance to help determine if exposure to arc flash might be present.
It is important to note that some electrical equipment includes ventilation holes in the door. An arcing fault in the enclosed equipment will direct the heated gases through the ventilation holes.
Will rubber products protect me from arc flash?
Rubber products are designed to resist the flow of current. They are neither designed nor intended to serve as protection from a thermal event. However, one characteristic of insulating rubber compounds is that although they will burn, they may be difficult to ignite. Because an arc flash event is likely to be very short, insulating rubber will provide substantial thermal insulation.
How do I evaluate an arc flash hazard on a dc circuit?
No consensus method exists to estimate incident energy associated with a dc energy source. Direct current flow is inherently different from alternating current flow, because an alternating current reaches zero twice each cycle and the arc is extinguished. The alternating current must reignite the plasma when the direction of the electron movement changes. A dc arcing fault might be more intense that an ac arcing fault. All current methods of estimating incident energy suggest that duration of the arc and the capacity of the energy source to provide current to the arc are the most important variables. It is likely that the same variables are most important in a dc arcing fault.
A dc fault could be evaluated as if it were an ac fault. The estimate might use dc circuit characteristics and apply them to current estimating methods for ac circuits. The resulting answer is unlikely to be accurate; however, the answer would provide a basis for selecting PPE. Any arc-rated PPE is an improvement over ordinary work clothing. DC circuits associated with batteries include hazards that are not present in ac circuits. If the dc circuit contains a significant component of pulsating current, the arcing fault is more likely to resemble an ac circuit fault.
What is the difference between exposure to an ac circuit and a dc circuit?
The electrical hazards are the same: shock or electrocution and arc flash. However, the degree or intensity of the hazard is likely to be different. No public information is available that provides information about tests conducted on dc arc flashes. An IEEE/NFPA collaborative effort is underway. The resulting research effort will provide guidance about estimating an arc fault in a dc circuit.