UFC 3 -520-01
June 10, 2002
Table 13-4. Typical ATS Fault Current Ratings
Available Symmetrical Amperes RMS Ratings at 480 Volts
Long-Time Withstand
Rating When
Rating When Applied
With Any Overcurrent
Applied With a
With Current Limiting Fuses
Protective Device
Rated MCCB
Symmetrical
Symmetrical
Symmetrical
ATS Rating
Time
Fuse Size Fuse
Amperes RMS
Amperes RMS Amperes RMS
(Amperes)
(Cycles)
Maximum
Type
at 480 Volts
at 480 Volts
at 480 Volts
30
10,000
1.5
22,000
100,000
60
J
70, 100
10,000
1.5
22,000
200,000
200
J
150
10,000
1.5
22,000
200,000
200
J
260
35,000
3
42,000
200,000
600
J
400
35,000
3
42,000
200,000
600
J
600, 800
50,000
3
65,000
200,000
1200
L
1000, 2000
65,000
3
85,000
200,000
2000
L
1600, 2000
100,000
3
100,000
200,000
3000
L
3000, 4000
100,000
3
100,000
200,000
6000
L
13-5.3.2.2 An ATS is rated in terms of the available RMS symmetrical current at a
specified power factor. The system short circuit study must consider the X/R ratio at the
ATS location. Higher than rated X/R ratios allow the fault current to peak higher and be
sustained longer than was certified during ATS certification testing. If the system X/R
ratio exceeds that of the ATS certification, discuss the available options with the
manufacturer. Higher withstand and X/R ratio ATS designs are available.
13-5.3.3 Arc Interrupt Capability.
13-5.3.3.1 When a set of contacts open that are carrying power, an arc will be drawn as
the contacts separate. The duration of the arc increases with higher voltages and lower
power factors. In an ATS, the arc between the openi ng contacts must be extinguished
before the other contacts close. Otherwise, a short circuit can occur between the two
sources.
13-5.3.3.2 When an ATS transfers from the normal source to the emergency source, an
arc might not occur because the normal source voltage might be zero. But, when the
switch retransfers back to the normal source, it will have to interrupt the full voltage of
the emergency source. During testing, the switch might have to interrupt full voltage
between both sources.
13-5.3.3.3 When evaluating an ATS application, review the manufacturer's test
documentation to ensure the ATS is capable of repeated arc interruption during transfer.
13-5.3.4 Inrush Current Capability .
13-5.3.4.1 When an ATS transfers to its alternate source, its contacts must be capable
of withstanding a substantial inrush current. The amount of current depends on the type
of load. Regardless of the load type, the ATS contacts must not weld when closing on
13-18
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