What are the results of arc flash assessment?
The arc flash analysis should result in calculated incident energy, incident energy at arc flash boundary (AFB) and AFB at each piece of electrical equipment under study. The calculated incident energy values at working distance are used to determine the appropriate levels of personal protective equipment required by qualified personnel working on or near the equipment as well as in keeping with the requirements of NFPA 70E, facility safety policies and other applicable standards. In addition, the results are used to provide equipment labeling that contains information that can be used by the qualified person to perform the hazard risk assessment as required by NFPA 70E and CSA Z462.
The arc flash study should be performed by or under the direction of a qualified person with the necessary experience and knowledge about power system analysis and arc flash hazard assessment. Engineering licensing requirements of individual jurisdictions may require the analysis to be performed by or under the direction of a registered professional engineer.
The results of the arc flash hazard analysis would typically be shown in form of the arc flash hazard table. The table shows the parameters upon which the arc flash hazard is based:
- Bus Name
- Upstream Protective Device Name
- System Voltage - System line voltage
- Total Available Short Circuit Current (ASCC) – The current flowing to a bus fault that occurs between two or more conductors or bus bars, where the impedance between the conductors is zero
- Bus Arcing Fault Current - total predicted arcing current flowing to the bus fault
- Part of the ASCC through Upstream Protective Device – The portion of the total bolted fault current that flows through a given upstream protective device
- Protective Device Arcing Fault Current – The arcing current flowing through the protective device feeding the electric arc fault
- Arc Duration – The time required for the protective device to operate for the given arcing fault condition plus the time required for a breaker to open after receiving a signal from the trip unit to operate. For low voltage circuit breakers and fuses, the trip time is assumed to be the total clearing curve or high tolerance of the published trip curve. The combination of the Trip/Delay time and the Breaker Opening time determines the total time required to clear the fault. Arc duration would generally be set to a maximum of 2 seconds. Reaction time, mobility, and equipment working clearances are considered as factors in setting this value
- Electrode Configuration
- Enclosure Size
- Gap between Conductors - Equipment bus gap
- Arc Flash Boundary (AFB) – The distance from exposed live parts within a person could receive a 2nd degree burn
- Working Distance - Typical working distance is the sum of the distance between the worker standing in front of the equipment, and from the front of the equipment to the potential arc source inside the equipment
- Incident Energy – The amount of energy on a surface at the specified working distance away from the arc
- Required Protective Arc Rated (AR) Clothing Category – Indicates the Personal Protective Equipment (PPE) required to prevent an incurable burn at the working distance
during an arcing fault.
The arc flash hazard analysis can reveal panels with higher incident energy levels than would be expected. This is due to the fact that the fault current level is below the instantaneous trip point on the molded case breakers, which allows the arc current to flow for a longer period of time.