Re: Ground Behind Nav Station

amelliahona <no_reply@...>

Hi Mark:

I visited your boat web site, very cool.  Reading the "Frustration" post made me laugh out loud.  Sooooo… familiar.

As to the bonding system and seeing voltage on the green wires, I will offer an opinion.  The bonding system on our Amel's is there to protect various metal parts that are (or may be in some way) in contact with seawater from stray current.  The theory is that IF all metal objects are "bonded" (i.e. connected together with a wire) there will be no difference in voltage between any of the items.  Where there is no difference in voltage there can be no current flow (voltage is EMF or electromotive force and it takes a force to move something, in this case it takes a force to move electrons thru a wire. Electrons moving thru the wire is the definition of current (measured in amperes miliamperes).  So with all metal objects that are in contact with seawater "equipotential" due to bonding then no current will flow.  In actuality there is always small amounts of current flowing just due to leakage current and also to galvanic cell current due to dissimilar metals etc.  That is why the bonding system is tied to a sacrificial anode, so that any stray current is conducted to the anode and it corrodes (sacrificially) and prevents more nobel metals from corroding.  The bonding wire provides a low resistance path for the stray current to reach the anodes rather than finding a pathway to the seawater via other metals, for example Kent's C-drive prop shaft.  

Generally there should not be voltage measurable on the case of an electrical appliance (Furuno Radar or other). 

To check this, 
1.  disconnect the green wire from the case (stud) of the Furuno and other item (I can't remember what it was), 
2.  then, with power disconnected from the appliance measure the continuity (ohms set on a low ohm setting) between the grounding stud and the negative pin of the power cord (AGAIN, WITH THE POWER REMOVED FROM THE APPLIANCE).  
3.  then, with the appliance ON measure for significant voltage between the power supply positive and the ground stud of the appliance (WITH THE GREEN WIRE DISCONNECTED, SO YOU ARE JUST LOOKING AT THE CASE OF THE APPLIANCE and not some other sneak circuit) 

There should be high resistance (nearly infinite) between the power supply ground and the grounding stud.  If there is low resistance OR if there is voltage measured between the stud and the positive power lead,  then the appliance ground stud SHOULD NOT be connected to the bonding system (in my opinion).  The above finding would indicate that the grounding stud is connected to the appliance power ground (perhaps intentionally by the manufacturer perhaps for electrical hazard or lightning damage protection) or there is a faulty connection within the appliance.  Either way, it is my opinion, that you do not want line voltage (24 VDC in this case) to be present on the bonding system.  That would create an EMF on the bonding circuit, causing current to flow, and at the very least, depleting your sacrificial anodes. 

Every time I launch into one of these explanations I fear that I will create more confusion than I solve, so if something isn't clear here let me know. 


Gary Silver
s/v Liahona  - USVI

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