Hi fitunderground,
I haven't seen a flood of public responses to your question yet, and I feel like I might be able to help shed some light on your (our?) situation. Let's not sugarcoat it though, electricity is not necessarily a trivial topic in the first place.
Often, a particular question comes up:
What's all this grounding and isolation business anyhow?
Heads up! Abridged version below.
In the beginning, there was a 'Goesinta' and a 'Goesouta.' Kirchoff said that's all you need. Two wires, one source, one load, four connections, isolated. Houses here in the US were wired in this fashion...that is, not _locally_ grounded...for the first 50 years or so. Back from the days when cars were black and phones had crank handles. But there were very often some particular safety problems with this arrangement, and most electrical codes began requiring that "_high_" voltage electrical services >50V must add a non-current carrying 'grounding conductor' tied to the Earth through an electrode. All user accessible conductive components must be connected to this grounding conductor.
48Vdc was picked as a standard early on for many commercial applications to generally remain under this particular threshold. Wiring insulation ratings and requirements are standardized in conjunction as well. 'Low Voltage' circuits usually _still_ require ground _referencing_. You can't float things safely if lightning strikes nearby. Ground referencing is done with what is called a 'grounded' conductor. Intended to carry current, but also bonded to the earth at one, and only one location in the system, typically near the service entrance, transformer secondary, generator, etc. This conductor is called Neutral in an AC system, or should be the negative most terminal in a DC system.
The short story: On a docked boat's electrical system, there's an incoming 'Earth' from (proper) shore power, when connected. This can be a source of stray current flow into the water, for any number of reasons. That current can 'lyse' (dissolve) the boat's 'electrode' or even, (shudders,) her prop. Generally, this phenomenon is a much bigger problem in saltwater than fresh water because of respective conductivities.
A _completely_ galvanically isolated prop-shaft simply can't develop a problem like that, however, the rest of the universe exists. I'm personally skeptical of the 'belt drives,' as they resemble a certain famous static electric generator configuration. That's the apparent current preference for 'DIY' inboards though, but...in any case...you want to make sure all of it is grounded with grounding conductors, connected, to a single point, then to the boat's grounding (& zinc) electrode(s). If there is any current wanting to flow, it will prefer the zinc automatically because: chemistry.
The problem: If the Earth reference for the shore power is somewhat distant from the boat, which is very likely, a large 'ground loop' can easily be formed between the conductors, conduit, Earth, and/or water. Any time-varying magnetic field that passes through a conductive loop _will_ cause current to flow. Per Faraday and Maxwell. Tesla figured how to make practical transformers using this phenomenon, and this in particular is very useful for galvanic isolation. The bigger the loop area, the more flux captured in the loop, the bigger the currents developed.
The solution: 'Galvanically isolating' shore power. Coupling power transfer through an electric or magnetic field, like thru a transformer, instead of a direct conductor. This effectively solves the problem by 'breaking the loop' formed by the two 'grounded' conductors, shore and shipside. The two green wire 'grounding conductors,' shore and shipside, can come near at a single point, namely the transformer, but must not connect. There's a diagram linked below which shows it as 'optional.' Disregard that. :-)
The boat's 'secondary side' of the transformer is ground referenced to the _boat's_ grounding electrode. Boat world and shore world. Shore world doesn't come on the boat, except up to the tranny.
Here's the ABYC recommendations.
Figure 18 on page 54 has an example of a multiple battery setup, similar to the one you are attempting, and shows the recommended interconnection of the grounds. There's a number of other diagrams for the AC power sections, but the AC _grounding_ conductor's connection to the DC systems' is shown in Fig 18. They want everything ground referenced, BTW, but don't necessarily require a 'grounding' conductor under 50V. So, a 'grounded' conductor _and_ a 'grounding' conductor for the AC, but grounding the negative sides of the batteries is both necessary and sufficient for the DC stuff. Overcurrent protection is required on the 'ungrounded' conductors.
If you're using a local (Zinc) grounding electrode, which you should, you can 'transformer couple' the hot and neutral incoming shore power and get _excellent_ isolation. Nothing else really beats transformer coupling, done properly, in this regard. That incoming shore power green wire only connects to the transformer's shield (if any), and that ground current path terminates there. All onboard grounding is thru the onboard electrode. See Diagram 6 and 7 pages 38 & 39 for examples of this. NO DIRECT CONNECTION from the incoming green wire to your boats 'local' grounding is allowed. Everything onboard is grounded thru the Zinc. Then you're isolated from shore power _and_ grounded.
tldr;
1) First and foremost, you need to determine if you have a gasoline system (engine and/or tank) on board. Electrical safety requirements and certifications get remarkably stringent in spaces with gasoline.
2) You need ground referencing for lightning. Full stop.
Every point in all systems should have one and only one reference to the Earth. This is the simplest check to determine if you've done things safely. Multiple ground paths form 'conductive loops' and you _don't_ want that.
3) In practice, we often see conduit and distant enclosures they're electrically connected to _both_ bonded to earth, effectively in 'parallel' with the green wire.
Best practice is to designate one 'main panel,' where the bonding of earth and conduit (and neutral) happens, for each complete system. All other enclosures are 'subpanels' where the green wire is kept separate from neutral. We don't like loops, but we don't like relying on galvanized or aluminum on steel electrical connections for safety either. There are plastic bushings for conduit entry into subpanels, if you like, that remedy this. Then you ground the 'subpanel' (but again, _not_ the neutral) with the incoming green wire and you're good to go. All circuits' grounding emanating from the subpanel is connected back thru it's incoming green wire.
One and only one path to the Earth from any point, OK?
4) For watercraft, ABYC likes DC systems to be 'two wire' and 'ground referenced.' In particular, they want all sources connected electrically to the water through the most negative terminal in that system. They also generally require overcurrent protection on the positive side(s), except for starting motors. In practice, the negative most terminal on each battery bank should have a single connection directly to the 'grounding electrode.' Like in Fig 18.
5) Direct current wires that may create magnetic fields (that is, higher current circuits) should be run in twisted pairs. Your compass, and therefore your navigator will thank you. Steel conduit can't hurt there either. AC current carrying conductors can be run in a twisted pair fashion as well.
6) ABYC apparently does not _require_ you have a grounding system for DC yet they require that the negative of the system is _grounded_. If you do have a DC grounding system, there are some rules to follow, outlined in sec 11.16 on page 31 of the 'e-11' document. They basically work the same as AC grounding. If you have 120 or 208/240, grounding for those circuits is always required.
So, that's what you should do. And WHY.
Best of luck!
Dave
On Thursday, May 5, 2022, 12:53:56 PM CDT, fitunderground via groups.io <fitunderground=yahoo.com@groups.io> wrote:
Hi all 👋🏼 does anyone know of or could recommend someone experienced in installing electric sail drives or an ABYC specialist that has existence with electric drive systems? I'm open to doing online or phone, although I'd prefer someone local (Southern California), that could help me finalize my electrical plan and figure out what else I need for safety.
The previous threads have confused me more than they've helped (mostly due to my lack of knowledge and experience) but I'm starting to struggle with the final pieces for grounding, galvanic isolation and if I should keep my 12v (house) and 48v (motor) banks floating. I would greatly appreciate any and all recommendations or advice. Thanks!
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