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Wiring Fundamentals: Common Paths

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Common Paths


00:00 - The key idea we need to understand to design a proper ground scheme is that of common paths.
00:06 This might seem somewhat counter intuitive, but what we want to accomplish is to avoid any circuits having common ground paths.
00:12 That is if the ground connection in a circuit fails in any way, we don't want the current in that circuit to be able to find another way of getting back to our start - finish line, ground.
00:23 The reason for this is to eliminate the chance of there being current in a wire that we haven't accounted for when we sized that wire.
00:30 A good example to illustrate how this can happen can be seen in starter motor circuit of an engine.
00:35 The starter motor draws a very large current from the battery from an appropriately sized power cable.
00:41 This current exits a starter motor into the block of an engine and then heads back to the battery via an also appropriately sized ground cable.
00:48 If the connection at either end of this ground cable gets corroded or vibrates loose, its resistance will increase.
00:54 This will cause the starter motor current to try and find an easier path back to the battery.
00:59 If we have for example connected the power grounds of our ECU to both the battery and the engine block, this will offer that lower resistance path.
01:08 The size wire we use for our ECU power grounds cannot handle the larger starter motor current and will be destroyed, possibly damaging the ECU as well.

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