Employee #1, a firefighter, was performing a pressurized hose test on a 4-inch fire hose. The employee was standing at the pump panel of the fire truck next to the discharge point where the hose was connected by a 2.5-inch double female adapter to the 3.5-inch fitting on the truck. The hose was being brought up to a pressure of 300 pounds per square inch. As the hose filled to approximately 150 pounds per square inch, it blew apart at the female adapter.