University of Extrication: Seatbelt Restraint Systems

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 recommended 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 ‘70s, pretensioners for seatbelts did not become popular across all makes and models until the late ‘90s. 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 actually 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 downwards toward the vehicle floor during the collision. These units might 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 backwards 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 which 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 backwards; both of which causes 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 either full deployment in the event of a crash or an adjustable, partial pretensioning grip that is released if no collision event occurs.

Training Tips for First Responders

EMS, fire, and rescue personnel should be capable of identifying seatbelt pretensioners in a vehicle, should understand the safety considerations for working near these systems, and must know how pretensioners can assist us with patient assessment and care. The following tips for responders should be considered:

Identification

• There are no visible identifiers for seatbelt pretensioners. Assume they are there on all front-seat three-point seatbelt systems at least. Outboard, rear three-point harness systems may have pretensioners also.

• An accordian-type sleeve, directly below the buckle end of a seatbelt, is a very good indicator that a pretensioner is present at the lower end of the buckle. If deployed during a collision, the seatbelt buckle may be at or even below the top of the seat cushion. The accordian sleeve will most likely appear compressed as well if the system has fired.

Responder Safety

• Strip the trim when pushing off a B-pillar, cutting the pillar for removal, or when opening or removing the roof. This will reveal the location of a pretensioner mounted anywhere along the inside of the B-pillar so you can avoid the unit and work effectively around it.

• Use the same precautions with an undeployed pretensioner as you would with an airbag stored gas inflator although it in no way presents as great a risk to responders as an undeployed inflator cylinder.

• Avoid cutting or crushing an undeployed pretensioner if possible. A recoiler-mounted unit may be able to be removed if it is in the way during extrication.

• Shutting down a vehicle’s 12-volt electrical system also shuts down power to undeployed pretensioners as well. Because pretensioners are wired into the airbag circuitry, they also have a similar “drain time” like airbags do after 12-volt power has been taken away.

 Patient Considerations

• To avoid creating or increasing injury to a patient, unbuckle or cut the seatbelt from your patient once you make patient contact.
• If your patient was not wearing their seatbelt at the moment of the collision and the pretensioner fired, you will typically find the seatbelt in a vertical position along the inside of the B-pillar. It will be drawn extremely tight due to the pretensioner operation. This is a valuable patient assessment tool; confirming that the pretensioner operated and that this seatbelt was not being worn at the moment of the collision. One exception to this is the Pontiac Vibe and Toyota Matrix automobiles. The seatbelt pretensioners on these “twins” will not deploy if the seatbelt is not buckled.

• If you arrive, are assigned to a front-seat patient and find the seatbelt unbuckled already, look at it. If it is extended and not retractable, then you know the pretensioner fired and that the seatbelt was being worn at the time of the collision.
• If you find an empty front seat and an extended, non-retractable seatbelt, then that seatbelt was also being worn at the moment the crash occurred. Locate that patient. Someone was belted into that seat when the crash occurred!

TASK: The rescue team shall identify the presence of seatbelt pretensioners, verify that the system has or has not deployed and employ recommended protocols for dealing with pretensioner systems at incident scenes.