Definition - What does Rubber Cap mean?
A rubber cap is a plug designed for insertion into the end of a pipe to stop the flow of water, wastewater, stormwater or sewage to allow work crews the opportunity to effect repairs to the pipeline.
Rubber caps may be conical, to fit into pipes ranging in size from the smallest diameter of the cone-shaped plug to the plug's largest diameter. They may be round inflatable or use a mechanical system to hold them in place.
Trenchlesspedia explains Rubber Cap
When a work crew must make a pipeline repair that involves "breaking" (disconnecting) an active pipeline, the rubber plug allows them to staunch the flow of liquid from the disconnected, upstream end of the line.
The plug is inserted into the line as quickly as possible once the line is disconnected, Once the plug is in place, the rehab and repair can commence, with minimal disruption.
When the repair-and-rehab is complete, the plug is removed from the line and the line reconnected.