Bonding & Grounding

[grady]

I am in the process of completing my complete rewire. I was wondering if bonding my two gas tank together and tying them into my grounding circuits. ( basiclly -12v )

They are Isolalated in seperate compartment, and only have rubber hoses for connections (although the fuel fill has wire reinforcements)

My stainless fuel fill deck mounts have to be bonded but do I bring them into the ground (-12v) circuit and if I do should I bond the tanks with them.

I'am concerned with stray current. I don't have a external dynaplate bonding system.

I do have a single metal below the water line thru hull.

anyway please advise as to the best way approach this task.

P.S. I really didn't want to take on the expense of a seperate bonding system.

Thanks Tony.

[CDK]

Bond anything you don't want to have on a higher potential than the water around you. The tanks, the engine(s), the filler plumbing and ultimately some metal surface in the water. That is your "ground".
Connect it to the negative terminals of your batteries, but do NOT connect it to anything on the shore because nobody can guarantee you that their ground is the same as yours.
If your metal object below the waterline can be eaten away by electrolysis, protect it with a large piece of zinc.

[pamarine]

I am a bit unclear on what you are referring to as your "grounding system."

Are you referring to an AC ground or your DC negative circuit?

[gonzo]

You are required to bond your tanks to ground. The shaft and propeller should provide some grounding.

[baeckmo]

CDK's answer covers what to do. In addition, if you use a charger from shore, make shure its trafo has primary and secondary windings completely separated. Some cheap crap comes with one end in common, which will let any floating ground potential from shore net into your system, causing corrosion.

[BTPost]

In talking about "Grounding & Bonding" on vessels that float in Conductive Fluids, (Salt Water) one really needs to specify just WHAT you're asking. There are MANY different situations covered by these Generic Terms. Are we speaking of DC Power, Ac Power, Lightning, RF, or just what? Some of the foregoing can be covered by the same discussion, and some are so different in approach, and function, that they require a completely different design criteria. Most Marine Electricians can handle AC & Dc Power. RF, especially LF/MF/HF Systems, require a totally different View of the vessel, and Lightning is, again, one of those esoteric areas, not well understood, even by the BEST Practitioners in the Business.

[jonr]

When you say -12V, I assume you mean your ground is 0V and your positive is +12V wrt ground?

Are galvanic isolators (diodes) enough to safely connect a shore ground to the onboard ground (ie, when bringing in 120VAC from shore)?

[CDK]

Originally Posted by jonr

When you say -12V, I assume you mean your ground is 0V and your positive is +12V wrt ground?

Are galvanic isolators (diodes) enough to safely connect a shore ground to the onboard ground (ie, when bringing in 120VAC from shore)?

The post you are referring to is from May last year!

And no, you do not use diodes in an AC circuit. Shore ground should stay on the shore, the boat has its own ground, obtained by submerged metal parts. The actual voltage potential will be different between boats because it depends on the hull material and zinc anode surface. Even connecting two boat ground leads can result in galvanic corrosion unless the boats are identical.

[jonr]

Parallel and series diodes are used with AC in galvanic isolators - not because they are rectifying anything, but because that want an ~2V voltage drop to block ground circuit flow. Are they 100% effective - that's my question.

[CDK]

A back-to-front diode pair allows approx. 600 mV before current starts to flow. That would give protection against ground fault currents if the shore ground has electrodes near or in the water.

Unfortunately there are numerous examples where ground electrodes are far away and the ground wire is connected to the 0 wire in a 3-phase grid. In such cases the "shore ground" has a large offset, determined by the amount of unbalanced current drawn from the grid. I witnessed cases where there was a fluctuating voltage differential with peaks over 10 volts.
Under such circumstances diodes do not help.

The only correct albeit more expensive way is the use of an isolation transformer.

[jonr]

Getting from 600 mv to 2V is why they also use diodes in series. But I agree, a 2V galvanic isolator will not work if you have a 10V differential.