"The discharge point of the main drain often is an overlooked detail in sprinkler system installation. Owners, designers, and fitters may try to find convenient, out-of-the-way places, without considering the consequences of the decision. NFPA13, 'Standard for the Installation of Sprinkler Systems', requires that main drain test outlets be installed at locations that will permit flow tests of water supplies and connections. The outlet should be arranged so the valve can be opened wide for a sufficient time to assure a proper test without causing water damage. It's common to find main drain outlets indoors where it is expected to flow into a recessed floor drain. If you look at the discharge from the 2-inch (51 mm) outlet in the picture, you can see that the amount water and turbulence with which is it discharged may be more than a typical floor drain can handle."
Coffee Break Training - Fire Protection Series No. FP-2009-14
United States Fire Administration: http://www.usfa.fema.gov/