USCG Exam PracticeElectrical fundamentals — DC/AC, motors, distribution
For a vessel with an AC electrical plant having an aggregate generating capacity of 900 kW, 46 CFR §111.51-3 requires the maximum asymmetrical short-circuit current to be assumed as:
- A. 10 times aggregate rated generator currents plus 6 times aggregate rated motor currents in operation
- B. 8 times aggregate rated generator currents plus 3 times aggregate rated motor currents in operation
- ✓ 10 times aggregate rated generator currents plus 4 times aggregate rated motor currents in operationCorrect
- D. 8 times aggregate rated generator currents plus 4 times aggregate rated motor currents in operation
Why C is correct
46 CFR §111.51-3(b) specifies that the maximum asymmetrical short-circuit current for an AC system below 1500 kW is 10 times aggregate rated generator currents plus 4 times aggregate rated motor currents in operation. The 10×/6× formula applies to DC systems; the 8×/3× formula is for average asymmetrical AC fault current.
Cited:46 CFR §111.51-3