How you ground things is application/design-dependent. In some designs the metal chassis might be the lowest impedance ground. Automotive is a great example of that. Using the chassis as ground saves a LOT of $$$ and weight vs. running additional thick copper cables everywhere. And (for the most part) in a car, you don't mind if there is some noise on the ground. A little noise on the DC won't matter to the headlights, horns, starter motor, windsheild wipers, etc.

One big automotive exception is car audio. A lot of you may have heard alternator whine on a car stereo at some point. That is because the pulsating rectified DC current from the alternator is causing the ground at different points (as the current flows through the chassis from the alternator back to the battery's ground) to have different voltages. Since a ground, even a chassis ground, has a non-zero impedance, current flowing through it (particularly the 10s of amps you see from an alternator), you will create a voltage (signal) across two different points of the ground. V = IR, where I = alternator current and R = resistance between two points that the alternator current is flowing through.

If one part of your audio system (maybe the receiver's ground lead) is tied to ground A, and another part (maybe the power amp or the receiver's chassis) is tied to ground B, then if there is alternator current between the two grounds, the audio system's power supply will have V(AB) (= alternator noise) across it at some point. And that signal often gets leaked into the audio you hear through various mechanisms. So for an audio system (particularly one in a car), a star ground is almost always the best choice.

A car is a good example of both grounding methodologies - you can ground anywhere on the chassis for a lot of the high-current stuff (alternator, starter motor, headlamps, etc.), but should only use one point for ground for the more sensitive circuits (audio, sensors, maybe microcontrollers).

How you ground things is application/design-dependent. In some designs the metal chassis might be the lowest impedance ground. Automotive is a great example of that. Using the chassis as ground saves a LOT of $$$ and weight vs. running additional thick copper cables everywhere. And (for the most part) in a car, you don't mind if there is some noise on the ground. A little noise on the DC won't matter to the headlights, horns, starter motor, windshield wipers, etc.

One big automotive exception is car audio. A lot of you may have heard alternator whine on a car stereo at some point. That is because the pulsating rectified DC current from the alternator is causing the ground at different points (as the current flows through the chassis from the alternator back to the battery's ground) to have different voltages. Since a ground, even a chassis ground, has a non-zero impedance, current flowing through it (particularly the 10s of amps you see from an alternator), you will create a voltage (signal) across two different points of the ground. V = IR, where I = alternator current and R = resistance between two points that the alternator current is flowing through.

If one part of your audio system (maybe the receiver's ground lead) is tied to ground A, and another part (maybe the power amp or the receiver's chassis) is tied to ground B, then if there is alternator current between the two grounds, the audio system's power supply will have V(AB) (= alternator noise) across it at some point. And that signal often gets leaked into the audio you hear through various mechanisms. So for an audio system (particularly one in a car), a star ground is almost always the best choice.

A car is a good example of both grounding methodologies - you can ground anywhere on the chassis for a lot of the high-current stuff (alternator, starter motor, headlamps, etc.), but should only use one point for ground for the more sensitive circuits (audio, sensors, maybe microcontrollers).