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|SUBJECT: Electrical Systems - Part 5|
|TOPIC: Confirming Total Electrical System Shutdown|
|OBJECTIVE: Understand that a crash-damaged battery can still produce current and energize a vehicle’s electrical system.|
|TASK: Establish protocols requiring confirmation of electrical system shutdown after cutting or disconnecting battery cables by conducting a “power size-up.”|
Originally Published: January 2002
Series Links: Part 1 | Part 2 | Part 3 | Part 4 | Part 5
RONALD E. MOORE
University of Extrication Editor
Photo By Ron Moore
This full-size van crashed head-on into a bridge pillar. The impact was at high speed, as can be seen from the extensive damage to the vehicle.
As a rescuer working on the battery cuts or disconnects all the ground and hot cables from the battery, it is easy to assume that the mission has been accomplished. The cables are off; the electrical system has to be shut down. This assumption is a mistake that should be avoided. This article proposes that personnel should not consider the electrical system shut down until a “power size-up” has been completed. The job is not finished until it is confirmed that power from the battery is “off.”
The National Highway Traffic Safety Administration (NHTSA) documented an interesting event that reinforces the need for surveying the vehicle after working with the negative and the positive battery cables. A crash test was conducted by GM engineers simulating an impact by one vehicle into the left front corner of another vehicle with a closing speed of 65 mph. As fire brigade members approached the crashed vehicle to inspect for post-collision fire, officials noted that the battery had broken open and smoke was rising from it. As was policy at that time, only the negative battery cable was disconnected by firefighters. The positive cable remained intact.
Photo By Ron Moore
The van’s battery, located in the very front corner of the engine compartment, had the cell covers blown off and the battery casing cracked wide open. During extrication, electrical wires remained energized, causing sparks during sidewall removal.
Several minutes later, an observer noticed that the vehicle’s taillights appeared to be “on.” Closer analysis revealed that in fact the taillight bulbs were illuminated. It was discovered that a sheet-metal screw had penetrated the side of the battery during the collision, creating a new ground circuit for the electrical system. Even though the battery was severely damaged during the crash, the ground cable completely disconnected by firefighters and three of its six internal cells were without any battery acid, the battery still generated approximately 7.5 volts of energy. The power and heat generated by the electrical current flowing through this unfused circuit were sufficient to light the taillights and to start a fire in the vehicle.
Photo By Ron Moore
Although the battery of this Hyundai automobile was crushed on impact, the parking lights and taillights remained illuminated. Note the glow of the amber-colored marker light.
To add to this battery challenge, consider that even a badly damaged battery can supply electrical power until it is shut down. It is more and more common to arrive at a night-time crash scene and find that the vehicle’s lights are still on. The engine may be stopped and the front end crushed, but current still flows from the battery.
What is even more unusual about these situations is the degree to which the battery can be damaged and still function. The battery casing may be completely broken open or crushed by collision damage, yet parking lights or interior lights may still be functioning.