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Automotive electronics generally use the metal chassis as the negative ground connector for the DC circuits. Obviously this saves something on wiring. Is there an electrical reason for this approach?

(I am not asking why negative instead of positive, but why metal frame instead of wire.)

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    \$\begingroup\$ It's a huge conductive blob with probably equivalent of very very low impedance.. and yes, it can be used as saving lots of ground return wires \$\endgroup\$ – KyranF Oct 6 '14 at 19:39
  • \$\begingroup\$ Related: Automotive electrical system. How does everything work? \$\endgroup\$ – JYelton Oct 6 '14 at 19:40
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    \$\begingroup\$ And some cars are positive ground. Can't remember which : old VWs? \$\endgroup\$ – Brian Drummond Oct 6 '14 at 19:43
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    \$\begingroup\$ Ever had that annoying "loose ground wire" issue before, in breadboards etc? Well not with a car! Just bolt it right in there! She'll be right mate! \$\endgroup\$ – KyranF Oct 6 '14 at 19:49
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    \$\begingroup\$ @KyranF Have to disagree, unfortunately. There are scores (or more) of grounds in a car and when one of them gets loose it can cause all kinds of weird problems. Under the dash (many wires) and the ground strap attaching the engine block to the chassis are common ones in older cars. Bleh. So much pain, and it seems to happen much more often than problems with the other side. \$\endgroup\$ – Spehro Pefhany Oct 7 '14 at 8:27
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There's not an electrical reason, but instead a weight reason. By using the existing metal structure as a ground, it effectively reduces the number of wires by around half, and therefore saving a great deal of weight. (For example, otherwise each tail light would have to have two wires instead of one.)

Remember too, that some electrical loads in an automobile use a lot of current. A starter motor, for example, very commonly uses 0 AWG wire which weighs about 0.5 kg/m.

Interestingly, although not your question, the choice of negative versus positive is entirely arbitrary. In fact, back in the 1960s, Volkswagen used a 6V "positive ground" system for the Beetle up to around 1967 when they finally changed to the 12V negative ground system that is standard today.

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    \$\begingroup\$ There may be some benefit to galvanic corrosion protection to using the ground as negative, corrosion will occur on the body and not the positively charged wiring. Also having the chassis as ground there is almost no chance of ground bounce or ground loops because all systems have an almost single/star point ground connection. \$\endgroup\$ – KalleMP Oct 6 '14 at 19:52
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    \$\begingroup\$ @KalleMP, I think you got the corrosion protection backwards. I get all the corrosion on my positive battery terminal, not my car body. It's also called cathodic protection. This was my first hit, rroc.org.au/wiki/index.php?title=Technical:Polarity \$\endgroup\$ – George Herold Oct 6 '14 at 20:00
  • \$\begingroup\$ @KalleMP, but a positive chassis provides the same low-impedance common connection, so doesn't it have the same lack of "ground" loops as a negative chassis? I don't see how polarity affects ground loops. \$\endgroup\$ – cristoper Oct 6 '14 at 20:53
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    \$\begingroup\$ @cristoper Quite right, the ground part is irrelevant to the ground loop issues, it is the common part that helps with that part of it. A positive ground would supply the same benefits is all the acessories were also positie ground and could have their casing connected easily. \$\endgroup\$ – KalleMP Oct 27 '14 at 12:35
  • \$\begingroup\$ @GeorgeHerold I may indeed have got the corrosion thing backward but it may still relate to the reasons for the choice of Negative Ground. Whereever the corrosion occured it may have been decided that with a Positive Ground it was in the wrong place. See also the answer I made below for links to elaborate on the confusion \$\endgroup\$ – KalleMP Oct 27 '14 at 12:36
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The more wires you use the more wire you have that can fail. The more wires you have that can fail, the more likely it is that something is going to stop working. There's really no way (as far as I know) for the chassis to fail electronically that won't also make things stop working for other, more important reasons, so by using the chassis you decrease the number of possible ways your car, or some component thereof, can stop working.

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    \$\begingroup\$ Ironically, this came up because I had a chassis connection failure on my boat trailer. The trailer is a pivoting type and only a rustly bolt connects the front section to the back. The ground wire was on the front and the lights on the back. When being pulled, occassionally the connection was lost as the sections bounced, and the lights flickered. I since have attached a longer ground wire and moved the attachment point to the back section of the chassis. \$\endgroup\$ – bib Oct 6 '14 at 20:08
  • \$\begingroup\$ A whole host of annoying and difficult problems can occur with a missing or corroded ground strap between the engine and the chassis; everything from random misfires to dead battery to flickering tail lights. \$\endgroup\$ – Edward Oct 6 '14 at 20:11
  • \$\begingroup\$ Trailer wiring should be an example of how avoiding having wires is a benefit. It seems that every spring I discover some rodent has used half of my trailer wiring for a snack. \$\endgroup\$ – whatsisname Oct 6 '14 at 23:29
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While the cogent other answers are correct, I should add the really really obvious answer, which given this is an engineering focused site, should have been mentioned by now.

COST

The automotive industry is highly competitive, with razor thin margins and a demanding environment to operate in. And copper is very expensive.

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This is in reply to my comment above that would not fit in a comment.

It also tries to offer a link to a good writeup on the primary question and then some history to clarify the negative vs positive ground issue that I mistakenly thought was clearcut.

In this first link the author clearly explains the reasons and benefits of using the chassis as a common ground point and also how best to use it and when not to. To put it simply the chassis is a good low resistance common voltage reference point. http://www.w8ji.com/negative_lead_to_battery.htm

Many sites seem to indicate that the positive ground was in use to prevent corrosion of the wires in the days of less effective inulation. http://lajagclub.com/the-argument-for-positive-ground/ http://www.jalopyjournal.com/forum/threads/why-neg-ground-vs-pos-ground.497935/

Wikipedia has a tiny snipped of discussion relating to the choice of polarity mentioned on a talk page but the radiator sludge information is no longer in the automotive battery page as it may have been unverifiable. https://en.wikipedia.org/wiki/Wikipedia:Reference_desk/Archives/Science/2010_October_4#Six_volt_automotive_systems

Many more vague references indicate corrosion as having been a concern but no consensus as to which one realy was worse or if the chassis or the wiring was being protected. http://www.yesterdaystractors.com/cgi-bin/viewit.cgi?bd=nboard&th=848277

The move to standarisation may have been driven from the accessory market. Increasing use of semiconductor components (voltage regulators) meant that it mattered which way parts were connected. Cars were often converted to negative earth to support radio equipment that was made to electronic design conventions. The need to pick a standard 'any standard' for cars may have been external to the desires of the car electricity needs and driven by commercial convenience. The quality of the spark is mentioned occasionally but the polarity of the chassis would not have forced it to be one or the other as the coil could be connected as required.

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    \$\begingroup\$ The first solid state car radios used PNP Ge transistors everywhere because that was all that was available at the time .This made a positive earth more convenient and may have contributed to the british cars being late to go negative ground . \$\endgroup\$ – Autistic Oct 15 '15 at 3:37
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Bib, To answer your Question: Yes there is an electrical reason to using the vehicle chassis as a ground. Here's an example: You require an electrical outlet (Anderson plug, say) at the rear of your vehicle to power a device that draws 50 amps & the length of cable required to reach the plug is 7 metres. Now to calculate the cable size you have to take in the length of the return (negative) cable as well. Using the example requirements above with a twin cable installation i.e. positive & negative cables run to the plug from the battery & based on a battery voltage of 13.8 & a voltage drop of ~3% giving a working voltage of ~13.4 volts at the plug. You would need to run 14 metres of 4/0 AWG (B&S) cable. Whereas using the vehicle Chassis as the ground you would only have to run a 7m length of 5 AWG (B&S) plus a short length from the negative terminal of the plug to the chassis, say ~1 metre, bringing the total length to 8 metres of 5 AWG as against 14 metres of 4/0 AWG, a huge saving. A good idea is to upgrade the body & chassis ground cables from the negative terminal of the battery as the original cables are sized to the standard electrical load of the vehicle & may not be heavy enough to handle an increased load on the chassis ground.

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  • \$\begingroup\$ Of course that's a 7 metre length of 5 AWG (B & S) cable from the positive battery terminal \$\endgroup\$ – Grey Nomad Nov 11 '15 at 16:44
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I'm not an electrical engineer, but it seems to me if the chassis is indeed a suitable ground (whether or not the negative side of the battery is connected), then the usual explanation of why we should always disconnect the NEGATIVE terminal prior to working on our car is bogus (see link below). As long as the positive cable is connected, we run the risk of shorting out any number of circuits in the system. I've blown enough fuses to learn the hard way.

Disconnecting the Power Supply

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  • \$\begingroup\$ It's not bogus. The problem is that if you disconnect the + first you run the risk of bridging the + to chassis while doing the task. Disconnecting - first doesn't suffer that problem. Similarly, when attaching jump leads you connect + first so that if it slips and hits chassis it won't matter because there is no complete circuit. \$\endgroup\$ – Transistor May 16 '16 at 13:07
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All wrong answers!

Since the spark plug has only one port for positive the negative comes from the engine block. Grouding engine block means grounding chassis. Period.

Long time ago when I was wiring up a cart that we built decided to wire up all the grounds directly to the battery that way I foolishly thought i would not have to ground the chassis. You know when you just want to do something for no good reason. Yeah but the engine wouldn't spark and after breaking my head over it I realized that it needed ground. Soon as I put a jumper cables from the negative terminal to the engine block she fired up.

I think manufacturers could easily come up with 2 ports on a spark plug today but there's no use for that. Chassis ground also help greatly in reducing electrical spikes caused by ignition coils.

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To have a additional security level: If any cable connections gets lose, falls onto the chassis and sets it under voltage a current would flow through the body of some one touching it. This is a high risk to people. To avoid this it get set to ground.

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  • \$\begingroup\$ I think that you'll find that most automotive wiring is 12VDC, so a human can quite safely touch both terminals of a car battery. The only part that's high voltage (in a non-electric, non-hybrid car anyway) is the secondary side of the ignition circuit -- the spark plug wires. \$\endgroup\$ – Edward Oct 6 '14 at 21:14
  • \$\begingroup\$ True, generally I just would rather stay on the safe side and keep it grounded. Another reason is that different power supplies potentially could by exceeded in their abilities if a current flows from one to device to the next via the chassis. This increase a risk of failure. \$\endgroup\$ – spekulatius Oct 6 '14 at 23:39
  • \$\begingroup\$ If the chassis were isolated and it came in contact with a wire, a person would have to contact both the chassis and a return path to the voltage source in order to be subjected to any current. \$\endgroup\$ – gbarry Oct 7 '14 at 4:20

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