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