07-06-2000, 10:41 PM #1Fir, M GAUTHIERFirehouse.com Guest
i would like to know why do we have to disconnect the negative of the batterie first. When we disconnect the positive and we touch the frame of car it will spark any how. Is there any documentation on this subject?
07-09-2000, 09:53 PM #2pokeyfd12Firehouse.com Guest
There's always a fight in my house about which side to disconnect first.
I figure if you disconnect the negative first, you have isolated the ground and unless you happen to put a metal tool across the negative terminal to the frame or engine, there shouldn't be any sparks. It's all in the theory of electrical circuits. Just don't complete the circuit.
Actually, I think the best way to not have any problems is to disconnect both terminals and wrap them with duct tape. If you have the time to disconnect one, why not do the other one for safety.
Rescue Lt. Kevin C.
07-09-2000, 11:31 PM #3MetalMedicFirehouse.com Guest
Well.. maybe I'll be sorry someday, but I seldom spend alot of time even deciding which one to disconect first anymore. Many times I find that I may only have access to one terminal anyway due to the damage from the accident. If that is the only one I can get to, I will disconnect it, clear it from the battery and that breaks the circuit. If I can get the the second terminal, I will take it as well. If we have enough manpower, we'll assign people to work on completely clearing the battery. If we are short, we'll go about the rescue with just one terminal disconnected.
Orrville (OH) Fire Dept.
[This message has been edited by MetalMedic (edited July 09, 2000).]
07-10-2000, 12:37 AM #4Neil MurphyFirehouse.com Guest
It is better to disconnect the negative first, however as Metalmedic said you may only have access to one terminal and it is not always the negative.
Rely on your training, assessment of the incident and available manpower.
Good luck and stay safe!!
07-21-2000, 06:42 AM #5J.HardcastleFirehouse.com Guest
Rather than say "NEGATIVE FIRST" - I think we should point out that it should be "EARTH" strap first. In almost every case this is negative, but there is always a chance that there is a positively earthed vehicle, i.e. custom cars. I beleive the reason they stopped using positive earhts was becuase it encouraged rusting......
I myself have seen it happen when the live terminal has been approached first - touch any other metal part with the spanner and you get sparks! I ALWAYS advise earth strap first - the wire that connects terminal to car body - Then disconnect the other terminal. I ALWAYS advise disconnecting both. I always make the reason for this approach clear so that if it is impossible to take the earth out first, then everybody knows to be extra careful about battery disconnection and spanner placement.
07-21-2000, 01:41 PM #6N2DFireFirehouse.com Guest
O.K. - Before I begin, let me post the disclaimer that not every idea works in every situation & Certanly I agree 100% with the unbolting process on a vehicle with little to no dammage. But . . .
If your disconnecting the battery in a vehicle with significant dammage - why are you trying to unbolt the terminal clamp/contact ?? Is it really going to help to save a couple bucks worth of wire and parts on a car that needs a new hood already?
Why not invest in a good set of INSULATED side (diagonal) cutters ?? Kline tools makes a nice set - insulated to 1000 Volts (Not that I'd ever want to cut something that hot with short handled cutters, but you get the point)
Simply reach in as best you can - cut the wire you have the best access to (ground still prefered whether positive or negative) and then throw a little duct tape (or electrical tape if you prefer) over the exposed ends and presto - isolated battery.
Just my .02 cents worth - I'm not knocking the way anyone else does things - just offering a different method that (IMHO) is faster, easier, and safer.
I do welcome any comments and feedback (pro and con) from anyone who uses / has used this method
Take Care - Stay Safe
07-21-2000, 04:21 PM #7E229LtFirehouse.com Guest
I'm not an electrical engineer, but, my understanding is, Direct Current (DC) will not arc to it's ground but will arc to an amperage draw on the positive side. If the vehicle involved had every electrical device turned off and you disconnected the positive first you would have no arc. However, if you were drawing power for a fan, stereo, or other device an arc is likely.
On the other hand, the removal of the ground cable first should negate or, at least, minimize any arc.
Also, as stated from others, the touching of a tool from the negative to the car's chassis will not create any arc while cutting or removing the cable.
07-21-2000, 05:46 PM #8J.HardcastleFirehouse.com Guest
CUTTING vs UNBOLTING!
I was always taught to unbolt rather than cut primarily to assist the police in their accident investigation. OK, thats secondary to the casualty - but if at all possible its still the way I do it and advocate doing it.But - thats England for you!
The other advantage is if you decide at a later time that you want the battery reconnecting at any stage - OK, highly unlikely that it will be absolutely necessary (possibly even a dangerous measure!) but in todays ever increasing gadget filled world - you never know!
In general though - I go along with the sentiment - if urgent - cut! BUT, I still think EARTH first....... Just to be safe!
07-21-2000, 06:39 PM #9J AlmonFirehouse.com Guest
Most of the metal parts of a car are tied to the negative terminal. If you are using a cable cutter to cut the negative cable before the positive, there is a good chance you will contact other metal parts. Since they are at the same potential as the negative terminal, there is no arc, and your tools don't get hot. If a positive cable is cut first and contact is made with the tool against metal, there is a bright arc, a possible ignition point and the tool gets hot. Just short across a car battery's terminals sometime with a ratchet and see what sort of fireworks you get!
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