Dummy activities often have a zero completion time and are used to represent precedence relationships that cannot be easily represented using the actual activities involved in the project.
By convention dummies are always shown as dotted arcs in network diagrams.
A dummy activity is a simulated activity of sorts, one that is of a zero duration and is created for the sole purpose of demonstrating a specific relationship and path of action on the arrow diagramming method.

The essential condition for the decompression of an activity is that An activity could be decompressed to the maximum extent of its normal time.

Dummy activities are used to Maintain the required net work.

Guidelines for constructing Network Diagrams:
-Each activity is represented by only one arrow.
-An arrow can represent only one activity
-If two or more activities are identified with the same start & end events. Dummy activity / activities need to be introduced in the network
-Before an activity can be started all preceding activities must be completed
-The flow of diagram is from left to right.
-The arrows are not to the scale
-Arrows should be straight
-Unless absolutely unavoidable, arrows should not be crossed.