I would think that if you ever wanted the battery grounded it would be when the car potentially had an explosive situation. Note, that they used to advise that the battery be disconnected altogether. However, they now say that unless there is an obvious tank rupture that you should keep the battery connected (minus the ground) so that power seats, windows ect can be manipulated to better reach and position accident victims. What’s going on here?
Huh?
If you disconnect either lead on the battery then nothing is going to work. I can’t see how just leaving the positive lead on the battery is going to allow the seats and windows to work.
If you disconnect the positive lead, if you accidentally short something between the positive lead and any part of the car’s metal you can make a spark, which could potentially make a big boom in an accident. If you disconnect the negative lead, it’s harder to accidentally make a short circuit that would make a spark.
Note that “ground” in a car means referenced to the metal frame of the car. Since it isn’t connected to earth ground it doesn’t have quite the same safety implications as the protective ground in house wiring.
Roland, what EMT book are you referring to? None of the texts I’m familiar with recommend the procedure you mentioned…
No matter what the book says, I can tell you that the standard practice is not to disconnect anything unless there is an obvious tank rupture or extrication is required.
St. Urho
Paramedic
There is something fishy here. Either you are misunderstanding what the text says or the text is badly out of kilter. As the previous post said, if you disconnect either battery lead nothing electrical in the car will work.
There are always small loads, such as the clock, on the battery even when the ignition switch is off and when you disconnect either lead there is the possibility of a small spark when the current flow is interrupted.
With the positive lead disconnected and the ground connected, if something metal touches the positive terminal of the battery there will be a hell of a spark. I would suppose that there is a fairly high likelihood of something metal touching the positive terminal and the frame during rescue operations. With the ground disconnected you can touch either terminal and the frame with a metal object and nothing really bad will happen. Of course if you touch the negative terminal and the frame with something conductive there will be that same possibility of a small spark because the circuit to the clock will be re-connected.
With both terminals disconnected you can also touch either terminal and the frame and nothing bad will happen but it takes less time to disconnect just the ground which achieves the same purpose.
I suppose I can imagine some far-fetched scenarios where disconnecting the negative cable might have some slim safety edge over disconnecting the positive cable:
Some charged component that might somehow shift during the patient extraction and ground to the negative chassis could spark. If the neg cable isn’t attached, the ‘ground’ might be floating enough to not spark.
Or maybe there could be something having to do with chassis voltage differences between the wrecked car and the ambulance or other cars? So if the metal stretcher shorts the two chassises (or what ever plural for chassis is), any risk, no matter how tiny, is eliminated?
How disturbing. It IS standard practice not to cut cables, but…perhaps Roland is learning from what is the accepted standard in the United States, the Emergency Care 9th Edition, so-called The Brady Book.
I agree that it seems stupid to leave any cable connected, however the graph above does discuss the need for power windows and doorlocks, etc. This book is NOT a state-published manual. It’s privately published and any state protocols are issued under separate cover, as an augmentation to this book.
Cartooniverse, who never ever ever ever got into an M.V.A. wreck to touch a patient till the nice folks from the Fire Department said I could. ( Assuming major damage. Little fender benders, I’d jump right in. )
[quote=St. Urho No matter what the book says, I can tell you that the standard practice is not to disconnect anything unless there is an obvious tank rupture or extrication is required.
St. Urho
Paramedic[/quote]
These posts make good sense. After all no matter how the electrical system sparks or doesn’t spark on removal of a cable from the battery in the normal case, in a wreck there can always be a damaged wire shorting to something and that can result in sparking upon its disconnect.
The book in question is Emergency Care by Brady 10th edition on page 852 where this is discussed. It is confusing because I would think that the ground would make it LESS likely that a spark would occur. Unless they are basically saying "don’t mess with the positive lead cable EVER).
I’m wondering if the suggestion is not to disconnect the negative terminal of the battery, but to disconnect the wire that ties this terminal to the car body. That would leave all the electrical stuff working.
I’m not sure what benefit would be. Maybe to decrease the possibility of a loose/broken wire contacting the frame and completing a circuit?
I’m not an EMT but I am in the auto repair business, and I have done airbag safety training for several fire departments.
With the fire departments I have worked with, if there are un-deployed airbags after an accident, and extrication is required, it is standard practice to disconnect the battery. No battery = no big boom from airbag. If a fireman were to be leaning in the car and a bag deploys it could kill, or seriously injure. I saw a video of a fire captain in Pittsburg, IIRC, get thrown out of the passenger side of a car and knocked on his ass from a passenger side air bag that deployed as he was leaning in the car to check on a victim. :eek:
Disconnecting the ground cable first goes back to basic auto mechanics 101. If the ground cable is disconnected (or cut) and the tool doing the cutting or disconnecting touches the body of the car there will not be a spark given off. If the fireman goes to disconnect the positive first, and the tool hits the body there will be a spark. Actually spark is a bit of a misnomer, a huge arc will result as the fireman will be arc welding with hundred of amps flowing. Even not in the presence of gas the arc alone can cause injury to the fireman either directly from burns, or indirectly as he flinches and bangs his head or arm trying to get away. Remember firemen are NOT mechanics so they may not have the knowledge that a mechanic has, and they might not realize that touching the positive cable to ground is a bad thing.
so the lesson is:
- When in doubt disconnect the battery, especially if there might be an airbag that has not deployed (some cars may have as many as 6 airbags and 13 igniters, not all of which go off in any one single accident)
- When disconnecting the battery always disconnect the ground first, then the positive. When reconnecting the positive goes on first, and the ground connection last.
FriendRob if you leave anything connected to the battery, so that something works pretty much everything will work, and the potential for fire / airbag deployment is there. If you were to just cut the wire from the battery to chassis ground, you would not prevent any problems as the ground to the engine would still complete the circuit as the engine is grounded to the chassis. You must disconnect the battery to elimanate the risk.
I didn’t quite make sense of your scenarios, but I can assure you that it’s not at all far-fetched for the tool used for disconnection to accidentally touch something connected to vehicle ground (i.e. anything metal on the engine or body) and that in light of this disconnecting the negative cable has a huge safety edge over disconnecting the positive. (Rick explained this in more detail.)
10th Edition already?? Oye. Well, the cite is accurate for the edition I have here on the shelf, which was from my EMT class in 2001.
Pretty much this is a mistake that a technician only makes once. (unless he is an idiot) When you arc weld a $20 wrench to the body, and scare the fuck out of yourself the lesson tends to stick.
The most recent VRT (Vehicle Rescue Technician) class that I took instructed those of us on the firefighter side of the equation to communicate with EMS personnel regarding disconnection of battery should power seat ops be needed. Typically, once a CID (cervical immobilization device) is applied, so long as there is no patient lower extremity entanglement/entrapment, power seats can be operated to provide as much access as possible. Once that is done, then the battery is disconnected-both leads-negative first (assuming domestic negative ground passenger cars).
Disconnection of battery leads does NOT protect against airbag deployment. Capacitors in the firing circuit can retain charge from a few seconds to thirty minutes. Cages are on the market to place across steering wheels to retain airbag deployment, however I have no firsthand experience with them.
I have heard that 30 minute number myself, but have never been able to track it down to a reliable source, as in this model of this brand of car has an airbag system that remains live for 30 minutes after battery power is lost.
Everything I have read has been like you stated “some cars”
I have never found a system that held power for more than 10 seconds after power loss. most are less than 2 seconds. Also turning off the key renders the system safe in most if not all cars. After all the capacitors are only there to supply power for deployment if the battery is lost in a collision before the airbag threshold is reached. For a designer to build in a 30 minute window would be silly, unless of course he envisioned a crash taking 30 minutes to occur from loss of battery power until the airbag threshold is reached.
I hope you would err on the side of your safety is there is any doubt dances, but as a well trained doper, I have to wonder about a number that seems to make no logical sense.
Heh. The nickname “Tool” could well last for the rest of your firefighting career, you know.
