In case of d.c. generating stations, there are heavy thick copper bars which acts as positive and negative terminals for the entire stations. These bars are called bus bars. While connecting the d.c. generators in parallel, the positive and negative terminals of the generators should be respectively connected to the positive and negative terminals of bus bars. If the generator is connected with reverse polarity to the bars, it results in short circuit which will cause damage to the commutators and brushes, ultimately shutting down the station. Before making the parallel operation, it should be checked for reversal polarity for the generators otherwise breakers are tripped off as a result of heavy fault current.