Ideal branches are those with zero impedance. Examples of such branches are infinite voltage sources, ideal short circuits and an ideal switch in closed state. Standard electromagnetic transient solution algorithms using a nodal admittance matrix require every branch to possess a finite impedance. A zero impedance branch would yield an infinite admittance and would thereby lead to numerical problems.
In EMTDC, a provision has been made to allow for zero resistances and true infinite bus voltage sources. The algorithm used permits any combination of ideal branches - including loops. An exception to this is when two or more ideal branches, where one of these is a voltage source: This would create an infinite current in the other parallel branches.
The default threshold value for ideal branches is set to 0.0005 W. Therefore, to create an infinite bus you can either enter 0 or a value less than the threshold for the source resistance. Similarly, for a zero resistance branch, add 0 or a value less than the threshold for the ON resistance of a diode, close resistance of a breaker, etc.
NOTE: The ideal branch algorithm involves extra computations. Thus, a non-zero value of at least 0.0005 W (more than the ideal threshold) should be used wherever possible. See Switching and Non-Linear Elements for more.