There are many ways to calculate the short-circuit current for a marine electrical system, some very simple, others quite complex. The complexity of the calculation is not always a good guide as to the “worth” of the result that is produced. Some calculation methods involve extensive calculations, but the result obtained cannot be relied upon. Other methods that also involve extensive calculations, do not necessarily provide final results that are as equally reliable to those attained by less complex methods.
The “best” calculation method to use is difficult to determine and depends on the size of installation, the point of interest on the system and the purpose of the calculation.For any installation having an installed generating capacity of 500 kVA or less at 440 V, or 300 kVA or less at 230 volts the “10 times” calculation method is generally adequate. The advice would be to try it and provided the result justifies the use of the minimum rated circuit breakers (10 or 14 kA at the main switchboard), then additional calculations would seem unnecessary.The “ten times” rule calculation can be refined if the generator sub-transient reactance is known, however care should be taken to use the generator “internal” voltage and not the system voltage or the results will be inaccurately low.
For larger installations, it is strongly advised that a more complex method of calculation be used; if the “10 times” rule is used, the short-circuit current calculated values will result in vastly over rated switchgear being installed.
The reduction in short-circuit requirements due to feeder cables is significant, particularly for the lower rated feeders (e.g. 100 amps or less). Accordingly when calculating the short-circuit level at panel boards, distribution panels and other switchboards supplied from the main switchboard, the impedance of the feeder cables should be included. Account should be taken of both the feeder cable resistance and reactance otherwise the calculation results may be unacceptably low.
Calculation methods that include generator and motor short-circuit decrement will produce the lowest acceptable values of short-circuit current. Such methods are based on IEC Standard 61363-1.
For systems involving different sizes of generator, or generators at different voltages located on different switchboards, short-circuit current calculations at distribution and power panels that take into account current decrement are not straightforward. Essentially there are two approaches, one to ignore the time constant changes resulting from the feeder cable impedance, and one to take these into account. The later calculation can be completed using the Equivalent Generator approach outlined in IEC 61363-1 Section 7. For most “conventional” marine electrical systems, the Equivalent Generator method will involve extensive calculations and produce results marginally different from more simple methods.
For the majority of marine electrical systems used in the commercial marine industry, a calculation method based on IEC 61363-1 taking into account both generator and motor short-circuit current decrement will produce the lowest reliable calculated value for the short-circuit current. Even with this method, cognisance should be given to the accuracy of the data used in the calculation (sub-transient reactance tolerances etc.), in order to apply a calculation “error” margin to the result, particularly when choosing protection gear with rating close to the calculated values. The author generally applies a 5 – 10% margin to all calculated values.
