Thread: Air Bags/Battery Cables
01-04-1999, 02:57 PM #1Lt46Firehouse.com Guest
Air Bags/Battery Cables
Hello Ron, Somewhere along the line I have heard that simply cutting battery cables to help minimize an accidental air bag activation, was not enough. Not only should the cables be cut but that both ends of the cables +/- should be brought together for 10-15 seconds, producing a dead short and thereby depleting any capacitor of any potential stored charge.
Mind you, I am not talking about the section of cable still attached to the battery posts but the "other ends", those that go to the frame and starter (I think). Does any of this ring true to you ? And does the reasoning make sense ?
01-07-1999, 04:44 PM #2rmooreFirehouse.com Guest
I have been told of this 'touching' story many times. It is an Urban Legend and is not true.
For three years back in 1989-1992, one automaker had one model in their line where this technique worked. Since then, vehicles have diodes installed in their electrical systems and the whole idea in now bogus.
The importance of shutting off the electrical power of a vehicle involved in a crash has long been understood by fire, EMS and rescue responders. Headlights and turn signals, radios, heater or air conditioner fans, even four-way flashers may still be operating when the first due companies arrive at the crash.
Electrical shutdown minimizes the possibility of post-crash fire caused by electrical arcing or sparking and prevents unwanted energizing of any electrical equipment on the vehicle. Most importantly, with loaded (undeployed) airbags still intact inside a vehicle, once power is shutdown, the airbag system's capacitor(s) begins draining their stored energy. Leaving an electrical system intact while involved extrication work is underway is how Dayton (OH) rescuers caused the only documented airbag deployment during an extrication in August of 1995.
Modern vehicles have state-of-the-art electronic electrical systems nowadays with semiconductor diodes integrated into the entire system.
Semiconductors are made from material somewhere between the ranges of conductors and nonconductors. Semiconductors, basically, are designed to do one of three things: (1) stop the flow of electrons, (2) start the flow of electrons, or (3) control the amount of electron flow. A diode is, in effect, a one-way valve. It will conduct current in one direction and remain non conductive in the reverse direction. When current flows through the diode, it is said to be "forward biased." When current flow is blocked by the diode, it is "reverse biased." Touching battery cables together as the story goes, is the reverse manner. Current will not flow.
So what do I suggest we do with battery cables? Here's my checklist;
• cut negative battery cable(s) first using care not to touch any metal part of the chassis with the cable or your tools, thus minimizing risk of creating a spark
• cut each ground cable a second time to remove a minimum 2 inch section
( if unbolting ground cables at battery terminal, fold cable and clamp onto itself and securely wrap with insulating tape to protect bare cable clamps from establishing a ground)
• check that ALL negative grounding cables have been disconnected from the battery
• assess if there is any evidence that power from the battery is still running any part of the electrical system of the vehicle
• cut positive battery cable(s), also cutting each 'hot' cable a second time to remove a minimum 2 inch section
• check the electrical system again to make sure it is not receiving power.
• if electrical system remains energized after this action, determine location of ADDITIONAL batteries and repeat shutdown tactics for second battery.
Shutting down a vehicle's electrical does not insure that a loaded airbag will not deploy during our extrication. Disconnecting the battery early in our rescue activities is, however, the most effective means of minimizing the potential for loaded airbag deployment. Vehicle rescue is still a gamble against time for both the patient and the rescuer.
By aggressively shutting down the vehicle's electrical system, as professionals, we are simply minimizing the risks.
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