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PCS100 AVC-20 User Manual
14.2.2.2
ACBs
Above about 1600A, air circuit breakers (ACBs) are the only choice. These do not provide any sub-cyclic current
limitation in short circuits. Typical ACBs can clear in 60-80ms, but the ABB Emax2 series is preferred as it can clear
in only 40 ms. As with MCCBs, to minimize arc fault energy, it is critical to set instantaneous tripping under
lowest expected arc current – no more than 30% of the supply fault capacity.
14.2.2.3
Fuses
Careful attention must be paid if choosing to use fuses for arc protection. Current limiting fuses are useful when
the minimum arc current is in their current limiting range (e.g. 20 times or more). Fuses are not useful at reducing
arc energy when the arc current is a low multiple of their rating (e.g., 10 times or less). When choosing fuses it is
important to check that the melt time of the fuse is suitably short (e.g., 10ms) under minimum arc current
conditions. Note particularly in high current applications, fuse melt times are so long that fuses are unsuitable for
providing arc fault protection.
14.2.2.4
MV side protection
MV side protection generally involves discrimination issues, the need to avoid tripping on transformer inrush, and
much larger breakers with slower operating times. It is difficult to achieve clearing times of less than 100ms and
typically 300ms arise. If MV protection is employed much larger arc energy will arise.
14.2.3
Calculating ARC Energy
Various tools may be found on-line to calculate the arc energy according to NFPA 70E Annex D (referencing
references IEEE Standard 1584-2002 - IEEE Guide for Performing Arc Flash Hazard Calculations) and the system
design.
Once the energy is known, suitable keep out distances and/or personal protective equipment can be chosen.
14.2.4
Personal protective equipment
Personal protective equipment (PPE) should be worn in the vicinity of the AVC-40. The PPE should be selected
according to the calculated arc energy exposure. This should include clothing, footwear, eye and hearing
protection according to the calculated energy level.
NFPA 70E sections (14) and (15) provide guidance on the selection of suitable personal protective equipment and
ratings.
This can be summarized as:
Hazard Risk
Incident
Category (HRC)
Minimum cal/cm2
0
1
2
3
4
NFPA 70E does not have a Hazard Risk Category (HRC) above 40 cal/cm2. Working in environments above 40
cal/cm2 should be avoided because of the blast hazards caused by electric arc flash. Arc flash levels above 40
cal/cm2 can be fatal and usually result in a massive, pressurized blast with sound waves and projectiles. PPE is
available for 100 cal/cm2 however the force from the pressurized blast can be fatal regardless of the PPE.
14.2.5
Keep out areas
For larger systems the potential arc energy is so high that PPE is not a practical solution. In this case a keep out
area with barriers and labelling should be employed when the AVC-40 is energized.
14.2.6
Labeling
Appropriate labels should be applied by the plant owner to warn of energy levels, PPE requirements and keep out
zones
Energy
Required FR Work Wear PPE
2
Non-melting Clothing
4
FR Shirt and FR Trousers (or FR coveralls) and PPE
8
FR Shirt and FR Trousers (or FR Coveralls), Cotton Underwear
and PPE
25
FR Shirt and FR Trousers, FR Coveralls (in addition to FR Shirt
and FR Trousers), Cotton Underwear and PPE
40
FR Shirt and FR Trousers, FR Coveralls (in addition to FR Shirt
and FR Trousers), Cotton Underwear, Full Coverage Arc Flash
Suit and PPE
113
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