The main drain, or test valve, on an automatic sprinkler system primarily drains water from the system for maintenance. The main drain also tests the operation of the water supply for the sprinkler system. Testing determines that all valves controlling the system are open and that water is available.
Documented drain test results on the Automatic Sprinkler Grading Report (ASGR) can help evaluate the water supply. Current testing of the main drain is a mandatory part of the ASGR. Without the testing, the sprinkler system cannot receive credit.