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As an emergency responder we learn how to cut open cars and extricate injured or trapped people. As a part of that, we are taught to always cut the battery cables to prevent stray current flow.

My question: which side is the safer to cut; positive or ground (negative)? Or does it make any difference?

I have looked for answers on this site within the numerous q & a's about car batteries, but did not see one to help me with this question.

Thanks!!

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  • \$\begingroup\$ I think emergency responder training is missing crucial details if you have to ask here. Please be aware that isolating electric or hybrid vehicle high voltage batteries is different from regular 12V leaded batteries. See Tesla's page for reference. \$\endgroup\$ – Jeroen3 Dec 8 '17 at 8:32
  • \$\begingroup\$ Jeroen3 - you are correct. I am a volunteer and my training is indeed lacking, thus my question. We are volunteers in Nevada, far from any city with thorough classes. We try to get the best training we can, sometimes just from youtube. We have a couple formally trained volunteers who do their best to pass on their training. This question became a point of friendly argument in our practice the other night so I found this forum. Also, we are embarking on getting training on hybrid & electric vehicles; you are quite right about that also. \$\endgroup\$ – Zog Dec 9 '17 at 1:40
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I would prefer to cut the negative (ground) lead - that way your tool will be at ground potential, and won't cause any additional sparks/fire hazard/excitement.

If you cut the positive lead, and your tool touches the car body, it will make a short circuit between battery voltage and ground.

Even when disconnecting the battery in a non-emergency situation, disconnecting the grounded side first is recommended - disconnecting the positive side first may turn it into an emergency situation if the wrench happens to hit the car body!!

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  • \$\begingroup\$ Actually your tool will be at 0V relative to chassis ground regardless of the terminal disconnected. So technically false. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 8 '17 at 1:13
  • \$\begingroup\$ @TonyStewart.EEsince'75: No - while cutting the Hot wire, the tool will be Hot, and may cause an arc to Ground (even after the cut is complete, the tool may still be in contact with the battery positive terminal, so will be a possible hazard). \$\endgroup\$ – Peter Bennett Dec 8 '17 at 1:32
  • \$\begingroup\$ OK in that case, yes \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 8 '17 at 1:43
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  1. Cutting either wire "safely" removes any current flowing thru the car.

  2. In general if there is any risk of the +ve battery terminal coming in contact with chassis ground by crumpled hood or the cutting tool, then cutting the ground strap is the safest method.

  3. Cutting the +ve cable carelessly can cause sparks to chassis.
  4. Cutting any battery terminal cable does not reduce the risk of both terminals being shorted by a tool or crumpled hood.
  5. Cutting the battery wire while the battery was crushed or showing signs of boiling electrolyte from an internal short, released H2 is a risk by generating a spark near the battery if any current is flowing from fans or inductive load.

  6. To protect across the battery terminals , they must be covered with a plastic cup, double layer tape or any suitable insulation.

  7. If the battery is outgassing rapidly from the collision, it must be isolated quickly from sources of heat or sparks. 4% H2 is the threshold for ignition and exploding acid is the worst case risk. A plastic tube from ear near battery caps can hear if it is boiling or the caps may have popped off.
  8. If the battery is shorted internally and boiling electrolyte, cutting the + cable or ground strap only increases the risk of exploding boiled acid (H2 + other) unless there is fuel leaking near a running Rad fan with commutating brushes arcing. but then it will be running much slower due to the internal short and reduced voltage.
  9. leaving the cut ground wire carelessly exposed to making contact with the chassis, is a risk for conduction sparks.
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The car body is connected to the negative terminal of the battery : the chassis / body is the earth or ground or return for all circuits (unless it has a fibreglass body).

So, always cut the negative cable and to avoid the ends touching : make two cuts and remove a chunk or length.

A few older cars MAY still exist that have positive earth, so if the car in the accident is really old make sure you cut the cable connected to the body.

However, auto-electricians spent a lot of time converting cars from pos to neg earth so radios could be fitted.

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