This paper develops a new model for optimal placement of a combination of single-phase and three-phase Micro Phasor Measurement Units (μPMUs) in smart distribution networks with radial asymmetrical structure for complete observability. The presented model is formulated as a Binary Integer Linear Programing (BILP) problem. In order to take the full advantage of the zero injection properties to make distribution network observable, two types of Zero Injection Buses (ZIBs) are taken into account including Fully Zero Injection Buses (FZIBs) and Partially Zero Injection Buses (PZIBs). With regard to the location of a FZIB or a PZIB in the radial distribution network, an innovative technique is applied to reduce the number of μPMUs. In addition, the single μPMU outage contingency is incorporated in the model. The suggested model is examined on asymmetrical IEEE 13-bus and 37-bus and 123-bus test feeders for different cases and the obtained results are analyzed to confirm the presented model.@en