A power distribution unit assembly, comprising: a power distribution unit housing pressingly mounted onto a stationary sliding rack, the housing including a first groove and a second groove longitudinally extending along either side of the housing respectively, characterized by a retaining device for stopping the relative movement between the housing and the sliding rack, comprising: a first member having a first hook portion for releasably engaging the first groove; a second member having a second hook portion for releasably engaging the second groove; and a handle member pivotally mounted to the first member via a pivot; wherein the second member comprises a stopping portion, and the handle member comprises a first contacting edge and a second contacting edge which are configured to selectively abut against the stopping portion by means of the pivotal movement of the handle member, wherein a distance between the second contacting edge and the pivot is greater than a distance between the first contacting edge and the pivot, such that the first hook portion is at a first distance from the second hook portion when the first contacting edge abuts against the stopping portion, such that the first hook portion and the second hook portion are loosely mounted into the first groove and the second groove in such way that they are undetachable from the housing, and that the first hook portion is at a second distance, which is smaller than the first distance, from the second hook portion when the second contacting edge abuts against the stopping portion, such that the first hook portion and the second hook portion engage to the bottoms of the first groove and the second groove. a power distribution unit housing pressingly mounted onto a stationary sliding rack, the housing including a first groove and a second groove longitudinally extending along either side of the housing respectively, characterized by a retaining device for stopping the relative movement between the housing and the sliding rack, comprising: a first member having a first hook portion for releasably engaging the first groove; a second member having a second hook portion for releasably engaging the second groove; and a handle member pivotally mounted to the first member via a pivot; a first member having a first hook portion for releasably engaging the first groove; a second member having a second hook portion for releasably engaging the second groove; and a handle member pivotally mounted to the first member via a pivot; wherein the second member comprises a stopping portion, and the handle member comprises a first contacting edge and a second contacting edge which are configured to selectively abut against the stopping portion by means of the pivotal movement of the handle member, wherein a distance between the second contacting edge and the pivot is greater than a distance between the first contacting edge and the pivot, such that the first hook portion is at a first distance from the second hook portion when the first contacting edge abuts against the stopping portion, such that the first hook portion and the second hook portion are loosely mounted into the first groove and the second groove in such way that they are undetachable from the housing, and that the first hook portion is at a second distance, which is smaller than the first distance, from the second hook portion when the second contacting edge abuts against the stopping portion, such that the first hook portion and the second hook portion engage to the bottoms of the first groove and the second groove.