Unusual bonding system VOA meter readings?

[GaylesFaerie]

This is beginning to feel like those full body scans some doctors recommend one not have because you may find plenty of anomalies to worry about and you may want treated but they are actually benign and if you never had the scan all would be fine...

As part of Spring prep I was testing the resistance for various items in the bonding system after adding a transom anode. Long story short, I'm showing Ohm readings that gradually go up the farther away from the batteries I measure, to the point I'm seeing 2.9 Ohms on the underwater metals (all 3 seacocks, the rudder and new transom anode). I secured the red lead of the meter to the negative battery post of one of the 3 house batteries and probed down stream with the black lead. Here are the readings all of which produced a tone on the meter as one gets farther from the battery:

Neg. battery post to the lug of the neg. cable that is attached: 0.0 Ohms
Neg. battery post to the lug on the other end of the neg. cable attached to the shunt: 0.2 Ohms
Neg. battery post to the lug on the other side of the shunt: 0.7
Neg. battery post to the post on the neg. bus bar receiving the neg. cable from the shunt: 1.8 Ohms
Neg. battery post to the bonding bus bar which is connected to the neg. bus bar: 1.8 Ohms
Neg. battery post to the Rudder or thru-hulls: 2.9

I also found a green bonding wire on the House PDP and it read 0.7 Ohms

Temperature was 45 degrees.
All batteries are fully charged.
All connections are tight down the test line.
No visible corrosion anywhere except the rudder post woven strap.
No extensions used on the VOA leads.

BB Marine has noted that bonding system readings for all components should be 1 Ohm or less which is what I was hoping for. Do my readings mean I have bad wires in need of replacement? I don't fully understand what I'm seeing or the implications. Comments welcome. Gary

 

[BB marine]

Neg. battery post to the lug on the other side of the shunt: 0.7

This .7 ohm is the resistance between the battery post and the shunt. That is not part of the bonding system it is part of your DC grounding.


Bonding is closely related to cathodic protection and simply refers to the electrical interconnection of metallic objects (such as shafts, rudder stocks, struts, etc) that remain in common contact with water to protect against galvanic and stray current corrosion. Bonded metals are connected to an anode, which is a source of cathodic protection. Electrical resistance between an anode and bonded metals receiving protection from a cathodic protection system should be kept to a minimum and not exceed one Ohm.


When testing your bonding system attach your meter lead to the bonding Bus bar and the anode check the resistance between it and all the components that have the 8 gauge bonding wire connected to them. You want to make sure that the resistance is below 1 ohm. The object is to make sure that you have a connection between all underwater components and the new transom anode that you installed. This insures the anode is protecting the metals. Basically all the testing should be where there is a green bonding wire connection not a yellow wire.

Yes you want the DC ground to be connected to the bonding system and the resistance should be small but that is not what you are checking when you are testing the bonding of under water metals and the anode protecting them.

 

[GaylesFaerie]

Ah, thanks Brian. I must have mis-remembered your post. I'll rerun the tests from the grounding bus bar. Also, given what you just mentioned does the following logic / math make sense:

Rudder to batt. neg Ohms minus bonding bus bar to batt. neg. Ohms = Rudder to bonding bus bar Ohms
3.0 - 1.8 = 1.2 (over desired resistance)

Seems with that corroded braid strap that would make sense. I'll still redo the tests, but want to make sure I understand the issue more completely. Thanks, Gary

 

[GaylesFaerie]

Retested with red lead on bonding bus bar and all readings 0.2 Ohms or less. Thanks again Brian.

 

[BB marine]

Retested with red lead on bonding bus bar and all readings 0.2 Ohms or less. Thanks again Brian.

It is a good idea to do that inspection. Replacing anodes is the step that most do. Checking to make sure the anode is able to protect all the metals under the water is just as important. Looks like you are in good shape!

 

[GaylesFaerie]

It is a good idea to do that inspection. Replacing anodes is the step that most do. Checking to make sure the anode is able to protect all the metals under the water is just as important.

And given how quick and easy it is to do those tests, there is really no excuse not to do them, especially considering the pain and costs it can save one from. Gary