A system interlock must automatically shut off the injection system if the irrigation pump stops operating or if variation in water flow adversely affects product injection rate or product distribution uniformity. The operator must be able to demonstrate that backflow cannot occur.

(1) Pressurized injection systems or injection into a pressurized portion of the irrigation system requires either an electrical, hydraulic, or mechanical system interlock device.

(2) When the injection point is at a nonpressurized section of an irrigation water distribution system, a slide metering scale or batch tank may function as the system interlock.

(3) With venturi systems.

(a) Booster or auxiliary water pumps must be connected with the system interlock such that when pressure in the mainline changes to the point where product distribution is adversely affected automatic shutoff of product supply will occur.

(b) The supply line must contain either a normally closed solenoid-operated valve connected to the system interlock or a normally closed hydraulically operated valve that opens only when the main water line is adequately pressurized. If a booster or auxiliary pump is used in conjunction with a venturi system, the normally closed solenoid must be electrically interlocked with the source pump for the irrigation system.