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Subject: Seatbelt Restraint Systems - Part 1
Topic: Seatbelt Pretensioners
Objective: Given a late-model vehicle, the rescuer will be able to identify the seatbelts with pretensioner systems, understand how the system operates and explain recommended protocols for dealing with these systems at an incident scene.
Task: The rescue team shall identify the presence of seatbelt pretensioners, verify that the system has or has not deployed and employ reommended protocols for dealing with pretensioner systems at incident scenes.
Part 1 of this University of Extrication series on seatbelt systems addresses seatbelt pretensioners and provides critically important information that fire and EMS responders need to know to operate most effectively at a vehicle incident scene where pretensioners are present.
Seatbelt pretensioners have been installed in vehicles for many years. They are one of the methods used to enhance occupant restraint system performance. Although the first systems were installed in vehicles in the late 1970s, pretensioners for seatbelts did not become popular across all makes and models until the late 1990s. Pretensioners are designed to remove slack from the seatbelt shoulder harness assembly during the moment of a collision; primarily a frontal impact. They will typically be provided on three-point seatbelts for both front-seat occupants and can be on the outboard rear-seat occupants or all three rear-seat positions. The seatbelt must be a three-point harness system, not a lap belt design.
The more aggressive pretensioner systems will pull the occupant back into their seat as it deploys. By maintaining the occupant securely on their seat and within their seatbelt system, they decelerate more smoothly. The frontal airbags can also deploy effectively with less chance of causing injury to the seated and belted occupant. Remember, seatbelts are the “primary” restraint systems; airbags are supplemental.
Most responders are aware that pretensioners located at the buckle or anchor end of a seatbelt shoulder harness assembly work by lowering the seatbelt buckle downward toward the vehicle floor during the collision. These units may be completely concealed within the seat structure or may be visible along the outboard edge of the seat frame.
Experienced responders also know that pretensioners can be commonly found attached directly to the retractor or take-up spool end of the seatbelt system. This design essentially spins the take-up spool backward during the crash, pulling excess slack out of the seatbelt and shoulder harness assembly. Two pretensioners can be provided on each seatbelt. The 2012 Fiat 500, for example, uses two pretensioners on each seatbelt to make sure the belt is tightened correctly.
There are mechanically activated pretensioner designs that do not use electricity to activate. These are not common. There are also pretensioners that are fired by an electrical signal during a crash but work completely mechanically by releasing a pre-stressed spring. The large majority of seatbelt pretensioners that responders encounter today are connected to the airbag wiring circuit. When activated, they fire off a small pyrotechnic charge of nitro-cellulose that burns rapidly in a small, enclosed chamber. This mini-explosion causes a plug to move up a tube as it pulls the seatbelt buckle downward or a sprocket to spin the retractor backward, both of which cause the seatbelt to retract, removing slack in the system. These pyrotechnic pretensioners do not use stored-gas cylinders like we are familiar with for airbag systems, although certain shapes resemble cylinders.
The newest pretensioner systems combine an electrical pre-tensioning mechanism as well as a pyrotechnic charge. These “smart” systems use information from components such as the vehicle’s forward-looking radar, inertia sensors, braking systems or speed monitors to provide full deployment in the event of a crash or an adjustable, partial pretensioning grip that is released if no collision event occurs.