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Go Back   SailNet Community > On Board > Gear & Maintenance > Electrical Systems
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  #1  
Old 03-02-2013
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Voltage on Pedestal

My pedestal and rudder are connected together but neither is bonded i.e. connected to my battery ground. I measure 0.5 volts between the pedestal and battery ground and do not understand where this voltage originates. Could it be that I am measuring the zinc voltage?
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Old 03-28-2013
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Re: Voltage on Pedestal

This is a tough question for several reasons. Electrics are PFM for most people, which makes communication difficult and time-consuming. Sometimes it is *very* onerous to find a connection causing a voltage when it isn't an obvious wire. And additional confusion arises because there are two extreme camps when it comes to bonding, bond everything vs bond as little as possible, both of which work, as well as the intermediate variations.

Starting with vocabulary, "ground" is earth, zero potential, the water your boat floats in. Your DC negative bus is your electrical system's common ground by being connected to your engine, or somehow connected to the water if you don't have an engine. "Bonding" is used for the system with zincs and green wires protecting your underwater metals, not the negative part of the DC electrical system (black wires).

Back to your rudder shaft, if it is stainless steel it absolutely must be bonded - as in connected to the zincs on your boat, and you DO NOT want it connected to your DC ground system. My rudder post is inaccessible at the moment so for comparison I just measured the voltage between my lightning ground, which I KNOW is 100% isolated from every system on my boat, and my stainless water tanks, which are not bonded, and I measured 0.26 V.

The next step is to check the resistance between the two, mine measured three mega ohms, which proves there is no connection between the two metals. So where does the voltage come from?? If you find out please let me know! If you have low resistance between your rudder shaft and DC ground system that means you do have a connection and you need to eliminate it. Additionally, your rudder shaft should be checked for corrosion - crevice and pitting (if it is stainless steel) from not being properly bonded, and stray current from being part of an unintended electrical circuit.

Your rudder shaft should be part of one circuit only: a green wire connecting it to the bonding system bus, which is connected to the zincs on the hull, then the water completes the connection from the zinc to the submerged part of the shaft. And confirming that there is only one connection can be difficult - as you noticed, the rudder shaft is connected to the steering system, which is connected to the pedestal, which over the past few years have more electronics mounted on them than the Apollo rockets of the 1960s.

On a related tangent, it is commonly said that bonding connections should have less than one ohm of resistance, but I think one ohm a lot of resistance for a bonding system. Every bonded metal in my boat has 0.1 ohm of resistance between it and the bonding bus. You need to haul your boat to check the outside connections between the hull zinc and the protected metals, and those should also be less than 1 ohm, preferably much less. Assuming a clean, properly connected bonding system, that measurement is actually the resistance between the bonding system bus and the zincs on the hull.
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Old 03-28-2013
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Re: Voltage on Pedestal

Quote:
Originally Posted by AnnapolisStar View Post
My pedestal and rudder are connected together but neither is bonded i.e. connected to my battery ground. I measure 0.5 volts between the pedestal and battery ground and do not understand where this voltage originates. Could it be that I am measuring the zinc voltage?
When you say pedestal, do you mean steering pedestal? Do you have any electronics installed on the pedestal? Even a light for the compass if you have a binnacle? Or are you talking about the actual rudder stock?
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Old 03-28-2013
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Re: Voltage on Pedestal

Quote:
Originally Posted by ShipShape View Post

"Bonding" is used for the system with zincs and green wires protecting your underwater metals, not the negative part of the DC electrical system (black wires).

Back to your rudder shaft, if it is stainless steel it absolutely must be bonded - as in connected to the zincs on your boat, and you DO NOT want it connected to your DC ground system.

If you have low resistance between your rudder shaft and DC ground system that means you do have a connection and you need to eliminate it. Additionally, your rudder shaft should be checked for corrosion - crevice and pitting (if it is stainless steel) from not being properly bonded, and stray current from being part of an unintended electrical circuit.

Your rudder shaft should be part of one circuit only: a green wire connecting it to the bonding system bus, which is connected to the zincs on the hull, then the water completes the connection from the zinc to the submerged part of the shaft. And confirming that there is only one connection can be difficult - as you noticed, the rudder shaft is connected to the steering system, which is connected to the pedestal, which over the past few years have more electronics mounted on them than the Apollo rockets of the 1960s.
The bonding circuit is connected to the DC negative bus but is not current carrying. The AC green is also connected to the DC negative bus, for safety and per ABYC.

Zincs will not eliminate crevice corrosion of stainless steel which does not require current to be present. Stagnant water without the presence of oxygen causes it.


As far as the voltage on the pedestal there must be a connection from a chafed wire or bad connection somewhere and it should not be hard to find.
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Last edited by mitiempo; 03-28-2013 at 10:38 PM.
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Old 03-29-2013
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Re: Voltage on Pedestal

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Originally Posted by mitiempo View Post
The bonding circuit is connected to the DC negative bus but is not current carrying. The AC green is also connected to the DC negative bus, for safety and per ABYC.

Zincs will not eliminate crevice corrosion of stainless steel which does not require current to be present. Stagnant water without the presence of oxygen causes it.


As far as the voltage on the pedestal there must be a connection from a chafed wire or bad connection somewhere and it should not be hard to find.
I agree with this, except for the "not hard to find" as it could well be a wire that is buried deep. Really depends on how many wires there are. I have found tracing wires can be real hit or miss, sometimes you pull off the covers and say ah ha, other times it can be so hidden that you pull your hair out (and I am lucky to still have some to be worried about) for days, months and have sometimes longer till it is found. Then again I drove British cars for years.

It is not like on a car though you do want to find the source, don't just live with it.
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Old 03-29-2013
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Re: Voltage on Pedestal

Any DC "chassis" grounding onboard is going to be connected to the engine block itself for the starter and alternator connection, the engine gauges, etc. And the engine usually will be raw-water cooled, so it is in turn connected to the salt water that...wait for it...the bonding system and thru-hulls are all connected to!

So no matter how you slice it, your propshaft and rudder shaft are connected to the DC ground. The only question is whether you have a low-resistance wire connecting them, or a higher resistance connection through the salt water. Where your zincs and galvanic currents are supposed to be controlling things, either way.

Separating or isolating bits and pieces has been a long and loud argument for many years, even between the top organizations and authors "in the business". But totally isolating everything? While it all sits in an electrolyte like salt water? That would be a good trick.
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