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  1. #1
    MembersZone Subscriber rmoore's Avatar
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    Default Can you "ground" a Hybrid?

    Received a question from a Colorado fire training officer who was given confusing information from a local electrical engineer in their community. Here's the original question and my reply;

    "As an Electrical Engineer, who also is*a Hybrid owner, I would*recommend outfitting your extraction teams with a "Jesus Rod," that is, a ground-rod (spike) connected to a 12-2 (at least) insulated cable that is made up to*a large*"Alligator Clip" with insulated handles that can be securely attached to the car's frame.* The area around where the rod is driven into the ground should be*roped off to warn everybody not to*stand in the water or the moist ground around it.* Remember that the human body is about*600-ohm resistor,*and that*electricity seeks the path of lease resistance to get to a lower potential.**Like water, electricity flows down hill-- and can*ruin your whole day."

    My reply as a non-electrical engineer...
    I do not endorse or advocate even considering the use of a grounding rod when responders are dealing with a gasoline-electric hybrid vehicle. *Hybrids and even the new plug-in electric vehicles from Chevy(VOLT) and Nissan (LEAF) have two separate electrical systems. *Their high-voltage systems are completely isolated from the chassis or body of the vehicle itself. *These higher voltage systems are also separate from the high-voltage system on the car; different wiring, different battery, etc. *Only the standard and familiar 12-volt electrical system is actually grounded to the vehicle.

    This is why the suggestion of a grounding rod is not advisable. *All manufacturers of hybrids and plug-ins offer ERGs, Emergency Response Guides. *I have pdf files of all of them. *NONE of these ERGs, straight from the manufacturers themselves, recommend or even mention anything about a grounding rod because they know for a fact that the rod, connected to the body or chassis of the damaged vehicle, would only be grounding the 12-volt system. *

    Where I believe the confusion is coming from is the failure of your electrical person to fully understand the design of these vehicles and their unique electrical systems. *If an electrical engineer or an electrician were just talking about working on or around a potentially energized source it would be expected that they would recommend grounding as part of their preparation work. *

    With the high voltage system 'floating' on the vehicle, only grounded back to itself, use of a grounding rod would be a waste of time and on-scene resources. *The intelligence of these high-voltage systems is such that the electrical fault continuity is constantly monitored internally. If a crash, a fire, or even a rescuer causes the high-voltage circuit to detect a fault, the system shuts down.
    Ron Moore, Forum Moderator
    www.universityofextrication.com


  2. #2
    Forum Member Rescue101's Avatar
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    AND, there are SO many variables to grounding and bonding(My Hazmat's showing). Time consuming,often multiple attempts to find a GOOD ground,no thanks. We have had to soak the ground before to get a good ground path (tanker transfer) . Good points ALL. T.C.
    Last edited by rmoore; 10-29-2010 at 05:40 PM.

  3. #3
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    Question

    Ron if one cable's insulation breaks and contacts the body will this shut down the high voltage? If its grounded to itself and insulated from the body how can it blow the fuse (trip the protection)?

  4. #4
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    I am also an electrical engineer and I can see no reason to connect the vehicle electrical system to earth ground. All this would do is reference the electrical system to ground, allowing someone to be shocked if they touch a hot wire on the vehicle and are standing on the ground. Without the ground rod they would need to touch both a hot wire and a neutral point on the vehicle.

    I think using the "electricity runs down hill, like water" comparison could be dangerous if someone took it literally. Electricity follows the path of least resistance, however it usually takes multiple paths, and even the amount of current flowing in one of the higher resistance paths could still kill you.

    The resistance of the human body is a hard number to pin down. I have measured the resistance from one hand to the other with dry skin and gotten several hundred thousand ohms ~200k ohms. However, the harder you grab the two test points and the more sweat there is on your skin, the lower the resistance goes. Wet hands bring it down further, and if you puncture the skin with a conductor and a return line then you get into the hundreds of ohms range where a 12V system can be dangerous.

    Rescue101's point about grounding during HAZMAT operations is a different animal. In his case, you want the chassis of both vehicles to be at earth ground potential so that the chance of sparks from static electricity between vehicles or to the ground are reduced. Especially when moving material (loading/unloading product) there can be a lot of static electricity produced (ever get shocked by a CO2 extinguisher?)

    My recommendations with any MVA are to follow the manufacturer's recommendations for disabling the electrical system. Making up your own steps without a full knowledge of the system you are dealing with can cause major problems at a scene and if you ever had to defend your actions in court.

  5. #5
    Forum Member dragonfyre's Avatar
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    Ron: I'm not sure of any other manufacturer but if any GM hybrid is involved in an accident with airbag deployment the hybrid system is shut down.
    Steve Dragon
    FFII, Fire Instructor II, Fire Officer I, Fire Appartus Driver Operator Certified
    Volunteers are never "off duty".
    http://www.bufd7.org

  6. #6
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    Quote Originally Posted by dragonfyre View Post
    Ron: I'm not sure of any other manufacturer but if any GM hybrid is involved in an accident with airbag deployment the hybrid system is shut down.
    an accident doesn't necessarily mean a airbag is going to be deployed, GM disabled bags in the event of a roll over in the past
    Am I being effective in my efforts or am I merely showing up in my fireman costume to watch a house burn down?Ē (Joe Brown, www.justlookingbusy.wordpress.com)

  7. #7
    Forum Member dragonfyre's Avatar
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    Quote Originally Posted by Capt387 View Post
    an accident doesn't necessarily mean a airbag is going to be deployed, GM disabled bags in the event of a roll over in the past
    Side air bags will deploy in a rollover so the system will still shut off.
    Steve Dragon
    FFII, Fire Instructor II, Fire Officer I, Fire Appartus Driver Operator Certified
    Volunteers are never "off duty".
    http://www.bufd7.org

  8. #8
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    Quote Originally Posted by dragonfyre View Post
    Ron: I'm not sure of any other manufacturer but if any GM hybrid is involved in an accident with airbag deployment the hybrid system is shut down.
    Always remember that in an accident, safety features such as this may not function correctly when the car is damaged. If the car is supposed to disconnect the batteries from the hybrid drive system the connection could always be made again when the car is reduced from 14ft long to 9ft by a head on with a semi. Good thing to keep in the back of your head.

  9. #9
    Forum Member dragonfyre's Avatar
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    Quote Originally Posted by KB1OEV View Post
    Always remember that in an accident, safety features such as this may not function correctly when the car is damaged. If the car is supposed to disconnect the batteries from the hybrid drive system the connection could always be made again when the car is reduced from 14ft long to 9ft by a head on with a semi. Good thing to keep in the back of your head.
    I'm sorry I work for a GM dealer and believe their propaganda!!!! The system is either shut down by the airbag deployment or by Onstar once they are notified of the crash.

    https://www.gmstc.com/FirstResponder.aspx
    Last edited by dragonfyre; 12-29-2010 at 04:40 PM. Reason: Spelling
    Steve Dragon
    FFII, Fire Instructor II, Fire Officer I, Fire Appartus Driver Operator Certified
    Volunteers are never "off duty".
    http://www.bufd7.org

  10. #10
    MembersZone Subscriber rmoore's Avatar
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    Hybrid high-voltage cables can be described as either being like a "coax" cable or being like "kermantle rope". What I mean is that they have a core that carries the current. That core is wrapped in a plastic layer.

    Surrounding this inner layer is the most important item; a thin metal mesh sleeve. Surrounding this mesh is the outer layer of plastic that you actually see.

    The thin steel mesh layer is the element of the HV cable that the brains of the hybrid's HV electrical system is monitoring. When, for any reason, this inner mesh layer is exposed and damaged, cut, burned, disconnected at a connector, etc, the HV system defaults to shutdown.

    It isn't 'grounding' that the system is monitoring as much as it is what's called fault continuity. When the HV system mesh does not send back the same continuity as it had a moment ago, the brain shuts the HV system down until it can be told that things are OK.

    It's quite a design and actually, it works.
    Ron Moore, Forum Moderator
    www.universityofextrication.com

  11. #11
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    Quote Originally Posted by rmoore View Post
    Hybrid high-voltage cables can be described as either being like a "coax" cable or being like "kermantle rope". What I mean is that they have a core that carries the current. That core is wrapped in a plastic layer.

    Surrounding this inner layer is the most important item; a thin metal mesh sleeve. Surrounding this mesh is the outer layer of plastic that you actually see.

    The thin steel mesh layer is the element of the HV cable that the brains of the hybrid's HV electrical system is monitoring. When, for any reason, this inner mesh layer is exposed and damaged, cut, burned, disconnected at a connector, etc, the HV system defaults to shutdown.

    It isn't 'grounding' that the system is monitoring as much as it is what's called fault continuity. When the HV system mesh does not send back the same continuity as it had a moment ago, the brain shuts the HV system down until it can be told that things are OK.

    It's quite a design and actually, it works.
    Thanks, I haven't had alot of training on the hybrids. This sounds like an excellent system of monitoring the HV cables.

    Just curious though, how fast does it shut down? Would the continuity break by an accidental cut with extrication tools allow for shut down before electrocution. This assuming improper disarming procedures weren't followed prior to cutting.

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