1.1 This Outline of Investigation covers thermoplastic pipe and fittings for use in low pressure dry pipe sprinkler systems for fire protection service. In addition to the requirements in the Standard for Thermoplastic Sprinkler Pipe and Fittings for Fire Protection Service, UL 1821, the following are supplemental requirements that also apply to thermoplastic pipe and fittings for use in low pressure dry pipe sprinkler systems.