Trane Starters Engineering Bulletin page 51

To comply with this section of NEC without additional current-limiting devices, most chiller-motor
configurations will require a short-circuit current rating well above UL 's standard ratings.
Let-Through Current (LTC)
This is the fault current that passes through the circuit breaker before the circuit breaker trip
element has time to respond, typically equal to or less than the available fault current and lasting
three-quarter of an electrical cycle.
Current limiting
All components and wiring in an electrical distribution system offer some degree of resistance.
Under normal conditions, the heat produced when current flows against this resistance readily
dissipates to the surroundings; however, the enormous current generated during a short circuit
produces damaging heat at a much faster rate than can be safely dissipated. Interrupt the current
and you stop adding heat to the system. As
amount of heat (energy) added. An electrical short that lasts three cycles, for example, adds six
times the energy of an electrical short lasting just one-half cycle. It is in this sense that all circuit
breakers and fuses "limit" current.
Figure 38
interrupting device must open the circuit within one-quarter cycle (1/240 second), i.e. before the
fault current peaks.
Figure 38. Illustration of short-circuit current
100,000
CTV-PRB004-EN
Overcurrent protective devices (such as fuses and circuit breakers) should be selected to ensure
that the short-circuit withstand rating of the system components will not be exceeded should
a short circuit or high-level ground fault occur.
System components include wire, bus structures, switching, protection and disconnect
devices, distribution equipment, etc., all of which have limited short-circuit current ratings and
would be damaged or destroyed if these short-circuit current ratings are exceeded. Merely
providing overcurrent protective devices with sufficient interrupting ratings will not ensure
adequate short-circuit protection for the system components. When the available short-circuit
current exceeds the withstand rating of an electrical component, the overcurrent protective
device must limit the let-through energy to within the rating of that electrical component.
also shows the effect of a current-limiting device.To be truly current limiting, the
Current
Peak Fuse
"Let-Thru"
10,000
Current
t
Total Fuse Clearing Time
Figure 38 suggests, time is a critical determinant of the
Prospective available short-circuit current
that would flow when a fuse is not used.
Electrical System—Ratings
Time
51