Arc Fault Protection Recommendation; Mccbs; Acbs; Fuses - ABB PCS100 UPS-I User Manual

PCS100 UPS-I User Manual
16.4.2

Arc Fault Protection Recommendation

Electrical arcs propagate through air and equals between 30 and 50 % of the bolted short circuit current
depending on arc distance. Released energy is proportional to current and time so protection must be designed
to minimize both. ABB highly recommends MCCB or ACB for arc protection.
16.4.2.1

MCCBs

Fast molded case circuit breakers (MCCBs) are the highly preferable choice for protection of PCS100 UPS-I
systems. Such MCCBs, such as ABB Tmax series use magnetic repulsion to successfully clear fault currents of 10 -
15 times the nominal current within about 6 ms. But note that below about 10 times the nominal current, the
breakers revert to the electronic trip and are specified as taking < 40 ms to trip.
16.4.2.2

ACBs

Mechanical air circuit breakers (ACBs) do not provide any sub-cyclic current limitation in short circuits and are
usually slower than MCCBs. The ABB Emax series is specified to clear in 70 ms, but EMax2 is specified to clear in
only 40 ms. Other ACBs may be slower than this and protection relays may add to the time. ACBs are applied to
systems from about 1500 A and above, where MCCBs are less commonly available. As with MCCBs, to minimize
arc fault energy, it is critical to choose instantaneous tripping under lowest expected arc current (typically the
ACB instantaneous trip level can be set to about 300% of the PCS100 UPS-I full load current rating).
To minimize arc fault energy, it is critical to choose instantaneous protection (no delay) and a tripping level less
than the lowest expected arc current.
16.4.2.3

Fuses

Careful attention must be paid when choosing to use fuse 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 that particularly in high current applications, fuse melt times are so long that fuses are
unsuitable for providing arc fault protection.
Protection on the MV side with systems with dedicated transformer will result in increased arc fault energy for
various reasons, such as MV breakers generally being slower than LV breakers.

16.5 Personal protective equipment

Due to the high levels of released energy and excessive heat it is highly recommended that the personal
protective equipment is used when working around live equipment.
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 Category Incident
(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.
Energy
Required FR Work Wear PPE
2 Non-melting Clothing
4 FR Shirt and FR Trousers (or FR coveralls) and PPE
FR Shirt and FR Trousers (or FR Coveralls), Cotton Underwear and
8
PPE
FR Shirt and FR Trousers, FR Coveralls (in addition to FR Shirt and FR
25
Trousers), Cotton Underwear d PPE
FR Shirt and FR Trousers, FR Coveralls (in addition to FR Shirt and FR
40
Trousers), Cotton Underwear, Full Coverage Arc Flash Suit and PPE
133