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Old 07-15-2016, 09:36 AM   #1
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Engine block as ground bus?

Looking into adding an ACR to my boat I noticed that all wiring diagrams assume all grounds are connected to a common bus that connects to one point on the engine block.
I have one start battery and two house batteries and other ground circuits all grounded to various points on my gas engine block; a total of 14 grounds grounded to 6 different spots on the engine block.
Is this a potential problem? Continuity between the 6 grounding spots on the block is less than 1 ohm, most are 0.2 to 0.4 ohm.
I can spent the time and money to connect them all to a common bus that connects to the block at just one point, but am wondering if this is going to do anything for me other than just tidying things up?
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Old 07-15-2016, 09:48 AM   #2
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Is your systems tied to an external grounding plate or anode? While you're probably ok now. Having all the systems tied to a common ground bus will yield less corrosion issues in the future. A metal that rusts (cast iron engine block) would not be my choice for a low voltage ground bus.

Ted
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Old 07-15-2016, 09:52 AM   #3
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Originally Posted by O C Diver View Post
Is your systems tied to an external grounding plate or anode? While you're probably ok now. Having all the systems tied to a common ground bus will yield less corrosion issues in the future. A metal that rusts (cast iron engine block) would not be my choice for a low voltage ground bus.

Ted
Yes, I have a bonding wire that connects the engine block, skeg etc to a sacrificial anode.
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Old 07-15-2016, 02:07 PM   #4
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To answer your question, yes it is a potential problem. In a perfect world it would be ok, but corrosion being what it is....

1 ohm is a lot when you only have 12v and some devices can draw 15-20 amps.

There should be only one main ground point and it should be made of plated copper for reliable connection. All other grounds should radiate from there. The engine block should be one of those. And there should never be multiple paths for any single ground to get back to the main ground.


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Old 07-15-2016, 02:37 PM   #5
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Ken is absolutely right. A copper bus bar is the right way to ground many circuits, not the engine block.


But it may be a PITA to fix it. The six different places on the block mean that there probably isn't enough wire to get to one common point to install a ground buss. That means replacing wire.


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Old 07-15-2016, 08:38 PM   #6
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Ken is absolutely right. A copper bus bar is the right way to ground many circuits, not the engine block.


But it may be a PITA to fix it. The six different places on the block mean that there probably isn't enough wire to get to one common point to install a ground buss. That means replacing wire.


David
Thanks, I'll end up adding cable to collect those 14 grounds via power posts, in turn connected to a common bus, using proper sized cable of course.
Thanks for your replies - I guess I was wondering if there was a good reason for the way my set up is, apparently not.
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Old 07-16-2016, 01:14 PM   #7
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"I was wondering if there was a good reason for the way my set up is, apparently not."

It was easy over the years and did not require any purchased parts.
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Old 07-16-2016, 02:14 PM   #8
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YEs, this is a potential problem. The ABYC standards recommend that all grounds come to one DC Main Negative Bus. That includes the ground wire from our distribution panel and any other sub panels. The AC ground bus, bonding system bus, accessory negative ground bus and the negative ground from the big DC loads such a house bank, a thruster, Windlass and DC dinghy crane. That buss is then connected to the same bolt as the battery negative. In this manner all the dialectical circuits are tied together at one point. This brings all DC circuits to the same potential and it eliminates the potential for ground loops. Copper is actually not a good material for this ground bus because it will corrode and that creates resistance and poor connections. The ground bus(s) should all be tin plated copper, such as the ones by Blue Seas. Rather than extending the individual wires. it would make sense to install a ground buss on each side of the bot where you can bring all the grounds to a single bus and then tie that bus to the DC Min Negative Bus. In DC wiring wire length is your enemy due to voltage drop. Voltage drop is a function of the wire length and its diameter As you clean up the wiring make all the runs (both positive and negative) as short as you can to reduce the effect of voltage drop.
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