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What Is a Chassis Ground?

In a vehicle, the chassis ground point is typically close to the battery.
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  • Written By: Paul Scott
  • Edited By: R. Halprin
  • Last Modified Date: 14 September 2014
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Chassis ground is a term that applies to electrical circuits and refers to a ground connection on the casing of an appliance or the bodywork of an automobile. This connection serves an essential purpose in circuits that do not have a physical earth ground connection, including the provision of a zero potential voltage reference and a dissipation point for interference, transient voltages, and static. In this way, a good ground provides protection for the circuit and those working on it. It typically consists of a stud or lug on the metal casing or vehicle body to which the grounding or battery negative lead is attached.

Generally, a ground point is a physical connection to earth, either by means of a ground spike or via an electrical grid ground system. Grounding an electric circuit is important for a number of reasons: it allows for measurable zero potential or referential voltages to be established, and it also acts as a dissipation path for voltage surges caused by circuit malfunctions or lightning strikes. Transient voltages and static buildup will also be eliminated via the ground protecting the circuit components and anyone working on them.

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Electronic equipment and appliances with metal cases typically feature a lug or stud built into the casing which serves as a chassis ground. In the case of electronic circuits, a lead is run from the ground track on the circuit board to this point. Appliances have a similar lead, which runs from the motor to the chassis. In the case of mains equipment, all ground points should ideally be connected to a grid ground point. This allows for the ground leakage protection system to cut out in the case of a short or electric shock.

The electrical systems in automobiles are typically negative earth circuits, which means that the negative battery terminal is connected to the body of the vehicle. The chassis ground point on a vehicle is typically a large, integral stud or bolt close to the battery, and connected directly to the negative battery terminal. The body of the vehicle, therefore, serves as the return path for the entire electrical system of the vehicle as well as a dissipation path for static, RF interference on sound systems, and short circuits. For this reason, care should be taken when connecting battery terminals in a vehicle not to short the positive terminal to the body with the wrench because serious burns or battery damage may result.

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Fiorite
Post 7

@Aplenty- I wanted to add a little of my own knowledge to this grounding and bonding discussion. I operate heavy machinery for logging and land clearing. One of the things I was taught when I was first learning to operate the machinery is to stay in your vehicle during a thunderstorm.

The chassis of the heavy equipment that has a poll cage system is made of solid steel, and acts like a shielding cage from electricity. Standing outside of the vehicle, even though it is taller than you, is much more dangerous than remaining inside. When exiting a vehicle, you create dangerous step and touch voltage systems where your body creates an alternate path to the ground. This will cause the lightning to pass right through you.

If lightning starts striking around you, remain in the cab, close the door, and sit with your hands on your lap until the lightning passes.

aplenty
Post 6

@GlassAxe- Your assumption about a vehicle being safe can be both true and false. It depends on the type of vehicle, the material the vehicle is made of, and how the person is interacting with the vehicle.

Metal vehicles can act as Faraday cage and protect an occupant from lightning, but it is no fun. I would assume being in a car struck by lightning would be like someone lobbing a flashbang grenade in front of someone. It would completely stun someone's senses.

The problem with your assumption is when you have a convertible or a vehicle made of fiberglass. These vehicles offer much less electrical shielding, resulting in possible electrocution of passengers if the vehicle is struck on the open road. If you are in one of these vehicles, it is best to exit the vehicle near a building (something naturally grounded, and wait out the storm indoors.

If you do need to wait out a storm in a vehicle, follow the basic lap rule, and sit in your vehicle with your hands on your lap. This will help ensure you do not come in contact with the the shielding surface (your vehicle's exterior).

Comparables
Post 5

@Georgesplane- I can give you pointers on selecting the proper ground strap for your project. When you are trying to determine the proper gauge for your ground and power straps, you need to determine the current demands of your electrical system. Essentially, you need to find your total number of watts your system can output, convert them to amperes, and install the appropriate gauge wire.

To convert watts to amps, add the RMS (root mean square or "average") watts of your components. The RMS wattage is often labeled on the device, the packaging, or the spec sheet. Next, you must double the total RMS wattage. Finally, divide the number by 13.8 to determine the approximate amperage.

Once you have this number, perform an internet search for car stereo cable guide. This guide will allow you to see what gauge power and ground wires you can use based on the amperes and length of wire. For example, a 50-amp system can use a four-foot section of 10-gauge wire as a ground.

GlassAxe
Post 4

Is it true that a person is safe from lightning inside of an automobile? Is a person riding in a car grounded in the event of a lightning strike? Or do the chassis grounds in the vehicle protect the person from electrocution?

This discussion came up last night, and most of us agreed that you were safe inside of a car in the event it was struck by lightning. However, we could not explain why that person would be safe. Can anyone provide a layperson's scientific explanation?

Alchemy
Post 3

@Georgesplane- Pelestears got most of it right, but I would like to add a few tips to help ensure you install your ground wire properly.

1) If you are drilling your own holes for your ground wire, you should aim for areas on the side or top of your trunk. Make sure you are not drilling through an area that breaks the weather seal of your vehicle. Also, be sure not to drill though gas lines, your gas tank, or brake lines (this is why it is best to drill on the sides or top of your trunk.

2) If you are installing multiple ground wires to a single point, make sure you install the most current-hungry ground wire closest to the chassis and the least current hungry (probably your crossover) at the top of the "pile". I personally would recommend installing each ground wire individually with at least a half inch of spacing between the ground points.

I work at a stereo store, and these are our standard grounding techniques we use. These techniques reduce liability and increase functionality of the stereo equipment. Grounding is easy, but make sure you do it right.

PelesTears
Post 2

@Georgesplane- It sounds like you have completed all of the difficult steps in installing your new car stereo system. Grounding your components is easy, and depending on the type of vehicle you have, will not require any modification. At most, you might need to use a thread cutting machine screw, coarse sandpaper, rubbing alcohol, and a drill to finish your project.

Look around the area you have installed your box, amp, and components and see if you can find any screws or bolts already cut into the metal body panels of your vehicle. You can use these points to ground your stereo system.

Once you find an appropriate bolt, unfasten the bolt and clean the area with rubbing alcohol. Once you have cleaned the area, sand away any paint or finish with steel wool or sand paper. Clean again with alcohol, thread the ring clamps of the ground wire through the bolt, attach a star washer at the base, and then attach the bolt back to your vehicle's chassis.

If you do not have a suitable bolt, find an out of the way area and create your own bolt-hole using your drill and a bolt head, thread cutting machine screw. Prepare the site just the same as if the bolt already existed and fasten your ground wires. Voila! Your stereo is ready to shake the paint off the pavement.

Georgesplane
Post 1

I am trying my first install of a car stereo system, and the directions dictate attaching the ground wire to the chassis of the vehicle. Where should I attach the ground wire? How do I go about attaching the ground wire? What size ground wire should I use?

I believe that I have everything hooked up correctly besides attaching the ground wires. I need to ground two amps and an electric crossover. Can I ground all of my components together or should I have a separate ground for each?

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