Prospective fault level

Modem machines are connected to power systems that often have relatively high prospective fault levels and so the generators and motors need to have their windings and terminations robustly braced to avoid movement during a major fault. General-purpose industrial machines may not be robust enough for such high fault level service. 

Protection against overcurrent and short circuit current is required and is mainly provided by circuit breakers fitted with overload trips. At the initial planning stage the supply company should be asked for the prospective fault level at the supply intake so that adequately rated equipment may be selected. At this position, if a switchfuse is used to control the installation and the BS 88 FIBC fuses specified in BS EN 60439-4 are employed, there should be no problem as the breaking capacity is not less than 80 k A at 400 V. If a moulded case circuit breaker is used, however, it is necessary to select one of adequate fault breaking capacity as they are made for a range of ratings. 

If the supply company provides a supply from its LV network, the quoted fault level is unlikely to exceed 16 kA, but on large sites where a substation is needed, a higher fault level could occur about 27 kA for example, at the output terminals of a 1000 kVA transformer with 5% impedance. The prospective fault level becomes progressively less as the distance from the intake increases, mainly due to the impedance of the distribution cables, so a lower fault rating for the MDAs and MCCBs is permissible. 

At the initial planning stage the prospective fault level should be calculated for several locations in the 400 V distribution system so that apparatus of adequate fault rating may be used. Inadequately rated apparatus, for instance an automatic motor starter, can often be protected by back-up HBC fuses, providing they have a suitable time/current characteristic so that they... 

Examples are HRC fuses (both LT and HT) and MCCBs and MCBs (LT only), which are available with current limiting features and are in extensive use. The tripping time of these devices is extremely low and much less than one half of a cycle of the current wave. They therefore do not allow the fatilt ctirrent to rise to its prospective peak. The protected devices and components can thus be selected based on the let-out energy of such devices on fault, which is extremely low, than the fault level of the system. If... 

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