  Radiality constraints are involved in both distribution system restoration
and reconfiguration problems. However, a set of widely used radiality
constraints, i.e., the spanning tree (ST) constraints, has its limitations
which have not been well recognized. In this letter, the limitation of the ST
constraints is analyzed and an effective set of constraints, referred to as the
single-commodity flow constraints, is presented. Furthermore, a combined set of
constraints is proposed and case studies indicate that the combined constraints
can gain computational efficiency in the reconfiguration problem.
Recommendations on the use of radiality constraints are also provided.
