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Old 06-07-2011, 08:44   #1
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Isolated Electrical System ?

I'm in the process of ordering a new diesel and the question has come up on whether or not to have it electrically isolated. This is the first time I've ever run across said concept and need some guidance. Could someone a) explain the concept to me and B) list the pros and cons of the concept?

I've been looking at the electrical drawing the dealer supplied and can't make out head or tails. Partly because it shows + and - leads but no battery. A little clarification would be nice. How does it all work? About all I can figure out so far is that the negative battery lead goes via a solenoid switch to the engine and is only connected to the engine during start up. Once the engine is running, the negative is cut.


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Old 06-07-2011, 09:38   #2
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Re: Isolated electrical system?

In a isolated system the engine is not connected to the battery negative. This reduces the risk of corrosion problems particularly in metal boats. An easy practical way to achieve isolation is to use a solenoid to only connect the engine to the negative during starting. For the few seconds the starter is working the engine is no longer isolated, but corrosion problems are not a factor for this short period. This system allows for a normal (engine ground ) starter motor.
The senders for oil and water need to be 2 wire (the negative is not supplied by the engine block) and the alternator needs to isolated (many are anyway), but it easy to make these modifications. Many experts in corrosion advocate this system in fiberglass boats, but it is not common because it adds (slightly) to the cost of installation .

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Old 07-07-2011, 06:18   #3
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If you have a choice go for electrically isolated. But make sure you have isolated senders etc not just the alternator.

Fully isolated starters are available as well

Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! -
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Old 09-07-2011, 01:55   #4
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Re: Isolated electrical system?

As far as I know, the main reason for electrically isolating a diesel engine is to ensure that there is minimum electrical noise present on the DC system that can disturb the electronic control module (ECM; i.e., "brain") of the engine.

In theory, it is easy to do. In practice, as the vessel ages and more and more people work on it, you may find that the diesel will be included in the B- of the entire boat unless you are diligent.

Charlie Johnson
JTB Marine Corporation
"The Devil is in the details and so is salvation."
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electrical, electrical system

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