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Procedure for IEEE 1584 based arc flash calculations.

All Arc Flash Calculators. Product Overview & Comparison

Incident Energy   +   Flash Protection Boundary   +   Arc Current Variation

Incident energy can be found using the equations below:

lg En = K1 + K2 + 1.081 * lgIa + 0.0011 * G   (1)

En - incident energy J/cm2 normalized for time and distance. The equation above is based on data normalized for a distance from the possible arc point to the person of 610 mm. and an arcing time of 0.2 sec.
K1 = -0.792 for open configurations, and is -0.555 for box configurations / enclosed equipment.
K2 = 0 for ungrounded and high resistance grounded systems, and equals -0.113 for grounded systems.
G - gap between conductors in millimeters.
Ia - predicted three phase arcing current in kA. It is found by using formula 2 a) or b) so the operating time for protective devices can be determined.


IEEE 1584 and NFPA 70E based Arc-Flash-Analytic v 3.0 tool for arc flash hazard analysis ( PC version for Windows 9X, 2000, NT, ME, XP ) is now available on CD


For 1000V and lower systems:

lgIa = K + 0.662 * lg Ibf + 0.0966 * V + 0.000526 * G + 0.5588 * V * lgIbf - 0.00304 * G * lgIbf   (2a)
lg - is logarithm base 10 (log10).
Ia - arcing current in kA.
En - normalized incident energy in J/cm2 as calculated by (1).
K - equals -0.153 for open configurations. and -0.097 for box configurations.
Ibf - bolted fault current for three phase faults in kA symmetrical rms.
V - system voltage in kV.
G - gap between condactors in millimeters.
Solve lgIa = 0.00402 + 0.983 * lg Ibf (2b) for applications with a system voltage ranging from 1 up to 15kV.

E = 4.184 * Cf * En * (t / 0.2) * (610x/Dx)   (3)
E - incident energy exposure in J/cm2.
Cf - calculation factor equal to 1.0 for voltages above 1 kV, and 1.5 for voltages below 1 kV.
En - normalized incident energy in J/cm2 as calculated by (1) above.
t - arcing time in seconds.
D - distance from possible arcing point to the person in millimeters.
x - distance exponent.
Flash Protection Boundary is found using the equation below:

DB = [4.184 * Cf * En * (t / 0.2) * (610x/EB)]1/x (4)
DB - distance of the boundary from the arc point in millimeters.
Cf - calculation factor equal to 1.0 for voltages above 1 kV, and 1.5 for voltages below 1 kV.
En - normalized incident energy in J/cm2 as calculated by (1).
EB - incident energy in J/cm2 at the boundary distance.
Ibf - bolted fault current for three phase faults in kA symmetrical rms.
t - arcing time in seconds.
x -
distance exponent.
EB is usualy set at 5 J/cm2 (1.2 cal/cm2 ) for bare skin, or at the rating of proposed personal protection equipment.

For protective devices operating in the steep portion of their time-current curves, a small change in current causes a big change in operating time. Incident energy is linear with time, so arc current variation may have a big effect on incident energy. The solution is to make two arc current and energy calculations: one using the calculated expected arc current and one using a reduced arc current that is 15% lower.
The
calculator makes possible both calculations for each case considered. The IEEE 1584 procedure requires that an operating time be determined for both the expected arc current and the reduced arc current. Incident energy is calculated for both sets of arc currents and operating times and the larger incident energy is taken as the model result. This solution was developed by comparing the results of arc current calculations using the best available arc current equation with actual measured arc current in the test database. The calculator predicts arcing fault current for a given configuration and bolted fault short circuit current. It also predicts bolted fault current required to cause 15% reduction of the predicted arcing current for the given configuration. Arc duration should be adjusted for the predicted and 15% reduced arc fault values.

This is the amount of thermal incident energy to which the worker's face and chest could be exposed at working distance during an electrical arc event. Incident energy is measured in joules per centimeter squared (J/cm2) or in calories/cm2 (5 J/cm2 = 1.2 cal/cm2 ) . Incident energy is calculated using variables such as available fault current, system voltage, expected arcing fault duration and the worker's distance from the arc. The data obtained from the calculations is used to select the appropriate flame resistant (FR) PPE.

 

The flash protection boundary is an approach limit at a distance from exposed live parts or enclosed live parts if operation, manipulation, or testing of equipment creates a potential flash hazard, within which a person could receive a second degree burn if an electrical arc flash were to occur. A worker entering the flash protection boundary must be qualified and must be wearing appropriate PPE. The Flash Protection Boundary is required to be calculated by NFPA 70E.

 

This is the minimum level of Personal Protective Equipment in calories per centimeter squared with the intent to protect the worker from the thermal effects of the arc flash at 18 inches from the source of the arc.

 

Classes of equipment included in IEEE 1584 and typical bus gaps are shown in table below:

Classes of equipment Typical bus gaps, mm
Open Air 10 - 40
Low-voltage switchgear 32
15kV switchgear 152
5kV switchgear 104
Low-voltage MCCs and panelboards 25
Cable 13

Equipment bus gap in mm. Gaps of 3 to 40 mm were used for low voltage testing to simulate gaps between conductors in low voltage equipment and cables. Gaps 13, 104 and 152 mm. were used in 5 and 15kV equipment testings. For cases where gap is outside the range of the model, the theoretically derived Lee method can be applied and it is now included in the Arc Flash Analytic v3.0.

 

Two grounding classes are applied in the IEEE 1584 procedure, as follows:
a) Ungrounded, which included ungrounded, high-resistance grounding and low-resistance grounding.
b) Solidly grounded.

 

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.
Arc-fash protection is always based on the incident energy level on the person's face and body at the working distance, not the incident energy on the hands or arms. The degree of injury in a burn depends on the percentage of a person's skin that is burned. The head and body are a large percentage of total skin surface area and injury to these areas is much more life threatening than burns on the extremities. Typical working distances are shown in table below:

Classes of equipment Typical working distance, mm
Low-voltage switchgear 610
15kV / 5kV switchgear 910
Low-voltage MCCs and panelboards 455
Cable 455

 
System Voltage, kV Equipment Type Distance exponent
0.208 - 1 Open Air 2.0
Switchgear 1.473
MCC and panels 1.641
Cable 2.0
> 1 to 15 Open Air 2.0
Switchgear 0.973
Cable 2.0

Use protective device characteristics, which can be found in manufacturer's data. For fuses, the manufacturer's time-current curves may include both melting and clearing time. If so, use the clearing time. If they show only the average melt time, add to that time 15%, up to 0.03 seconds, and 10% above 0.03 seconds to determine total clearing time. If the arcing fault current is above the total clearing time at the bottom of the curve (0.01 seconds), use 0.01 seconds for the time.
For circuit breakers with integral trip units, the manufacturer's time-current curves include both tripping time and clearing time.For relay operated circuit breakers, the relay curves show only the relay operating time in the time-delay region. For relays operating in their instantaneous region, allow 16 milliseconds on 60 Hz systems for operation. The circuit breaker opening time must be added. Opening times for particular circuit breakers can be verifed by consulting the manufacturer's literature.

 

Available 3 phase bolted fault current for the range of 700A to 106kA at the point where work is to be performed in kA.

 

The arcing current depends on the available 3 phase bolted fault current for the range of 700A to 106kA at the point where work is to be performed, configuration, system voltage and gap between conductors.

 

System Line to Line Voltage for the range of 208V to 15kV three phase. For cases where voltage is over 15kV, the theoretically derived Lee method can be applied and it is now included in the Arc Flash Analytic v3.0.

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