The basic premise in this protection is that the two sources should not be paralleled
since this raises the interrupting duty and the possibility of circulating currents. Therefore,
the transfer scheme must first drop the main supply and then quickly close the breaker for the
alternate supply. Before doing anything, however, the alternate supply must be checked to
ensure adequate voltage. In addition, the motor loads in the power plant must be checked to
make sure that there are no faulted circuits. The logic shown in Figure 18.15 is suggested for
this type of protection [5]