Why Connect to Underwater Plate

[scherzoja]

I'm a little confused about why DC negative and AC grounding are connected to an underwater ground plate in some circumstances.

If the underwater metals are not bonded because they are electrically isolated (bronze through hulls on a fiberglass boat, synthetic drive saver on shaft) and an isolation transformer is installed, why would one connect the DC negative bus and AC ground (green wire) to an underwater plate?

In this scenario, the lightning 'grounding' system is independent and has its own underwater plate.

From what I understand, the only reason to connect the DC negative to the AC ground (green wire) is in case there is any AC leak onto the DC system, that AC current will find its way back to the AC grounding bus, then back to its source, correct?

[SailingHarmonie]

There is no need or reason to connect to an underwater grounding plate, except as a counterpoise for an SSB radio or as part of a lightning protection system. How and why that is done is a complex topic, and I don’t think it is what you are asking about.

Anyone saying it is required as part of a standard boat electrical system can be safely ignored.

[scherzoja]

You're right, the question is not related to lightning protection. I guess it's my misinterpretation of the oft-cited diagram from ABYC (attached) that shows the immersed plate (part of lightning protection) as the final stop for all negative/grounding conductors. When lightning protection is separate, I could not think of a reason for connecting grounding to immersed plate.

[Fishspearit]

Originally Posted by scherzoja

From what I understand, the only reason to connect the DC negative to the AC ground (green wire) is in case there is any AC leak onto the DC system, that AC current will find its way back to the AC grounding bus, then back to its source, correct?

Yes, that's correct. Not an uncommon thing for someone to take that black wire that they assume is DC and connect it to the DC negative bus, not realizing it's an ungrounded AC conductor. Or if your water heater shorts to the casing ground, which is often tied to the bonding/ground system it will allow the AC current to flow back to the source on shore through the water.

All of that is moot in your example of having an isolation transformer though, as the source is not on land, it just needs to get back to the transformer.

[s/v Jedi]

You can wait to be attacked on the forum for this, but in my (professional, albeit retired) opinion you are exactly right and doing the perfect setup.

- Isolation transformer: yes, double check that AC shore ground isn’t connected to AC ship ground. There can be pesky jumpers completely negating the function of the transformer!

- Ships AC ground only for AC ground. Connect all outlets, inverters, chargers and transformer ground pins to it but nothing related to DC, bonding, ground plates or anything else.

- Lighting strike protection: preventing a strike is sheer impossible, even diverting it to other targets has a so-so track record, so dealing with the strike is the #1 important thing to do. Having an underwater bonding plate, rated for lightning bonding (not just a SSB plate) and mounted strategically very close to the mast base, then a heavy AWG 6 connection between mast and plate which goes as straight down from mast to plate without sharp bends as possible is the way to go. You must make this path more attractive for the strike than ionizing air into plasma for travel straight through the hull, pounding a hole in it in the process.

- DC ground: I hate DC ground, use a DC negative conductor everywhere. The engine is where it gets difficult so consider the whole thing the appliance and try to isolate it’s block to all other metals including propshaft.
Another point to keep an eye on is the windlass. Is DC negative isolated from it’s metal housing? And the chain, not touching anchor rollers, toerails etc. that are connected to the DC negative?

[scherzoja]

Originally Posted by s/v Jedi

You can wait to be attacked on the forum for this (I ignore the attacks, most of the time. Often they are a result of misreading of what was typed), but in my (professional, albeit retired) opinion you are exactly right and doing the perfect setup.

- Isolation transformer: yes, double check that AC shore ground isn’t connected to AC ship ground...

I'll watch out for the ship/shore AC grounding jumper when I get the iso transformer. thanks.

- Ships AC ground only for AC ground...

Yes, all AC devices have their grounding wire connected to the AC grounding bus. AC is US 120 volt system and the white neutrals go to their own neutral bus.

- Lighting strike protection: ...

I'm working on that one, but my brain is suffering from too many inputs and I'll work on this after DC and AC are done. For now I have a 1/4" x 1.25" x 12' long copper bar bolted to the outside of the hull down in the turn of the bilge and the aluminum mast is connected to the closest 1/2" bronze through bolt with a 4/0 stranded copper cable (maybe solid copper or flat copper is better?). Once I finish DC and AC, I'll tackle lightning.

- DC ground: I hate DC ground, use a DC negative conductor everywhere. The engine is where it gets difficult so consider the whole thing the appliance and try to isolate it’s block to all other metals including prop shaft.

Each DC device's negative goes to a local negative bus/terminal strip or fuse panel, then home run to the main DC negative bus. The engine senders are grounded to the engine via their threads, but engine has its own negative cable running from one bell housing bolt to the alternator bolt, then to the main engine DC negative bus (Then to the main DC negative bus). Other than the prop shaft, there is no metallic device connected to the engine.

Another point to keep an eye on is the windlass. Is DC negative isolated from it’s metal housing? And the chain, not touching anchor rollers, toerails etc. that are connected to the DC negative?

Windlass is Maxwell VWC 1500 and I don't know if the negative is isolated from the case. I'll look at that. I installed a double pole switch (Cole Hersee 75912, see attached picture) on the windlass circuit that stays off unless I use the windlass or am underway and might need the windlass. This disconnects the positive and negative (the 4/0 cables that run to the bow). The solenoid control box negative is fastened to the negative lug on the windlass, so it also gets disconnected by this switch. Rollers are UHMW PE and chain stopper is not connected to any electrical circuits

Fishspearit,
All DC negatives are yellow, positives are red.
AC: hot = black, neutral = white, grounding = green.
All heatshrink color matches the cable color (or is clear in case of AC green and white)

There should be no confusion about AC hot or DC neg (except for the ignorant an unobservant)

Thanks y'all for the advice.

[PerfectPirate]

Originally Posted by scherzoja

Fishspearit,
All DC negatives are yellow, positives are red.
AC: hot = black, neutral = white, grounding = green.
All heatshrink color matches the cable color (or is clear in case of AC green and white)

There should be no confusion about AC hot or DC neg (except for the ignorant an unobservant)

Thanks y'all for the advice.

You are selectively quoting the current ABYC standards for wiring boats, which stipulate a yellow negative wire. However, many of us are floating around on boats which far predate the ABYC standard and have black negative wires, or may have been built in foreign jurisdictions. In forty years of boat ownership, none of my boats have had yellow negative wires. And when I install additional wiring, I stick with having black negative wires in order to avoid any future confusion.

Positive wires from the batteries are red, but when they leave the breaker panel they can be one of a multitude of colours. There is an extensive ABYC colour scheme for positive wires depending upon the purpose which they serve. And then, of course, there is whatever cabling previous owners or boatyards chose to use since your vessel left its builder's manufacturing plant.

So, implying that potential confusion is solely the province of the ignorant or unobservant is to do a great disservice to those who have recently acquired classic boats and are trying to keep them going. Some of us have been at this for a very long time and have watched these standards get developed and evolve. Back in the seventies, most boats did not even have marine-grade wiring, since only automotive wiring was available to the builders. A lot of those boats are still going strong.

[scherzoja]

Oh my. I certainly did not intend to paint everyone with the light of incompetence. My intended meaning was that anyone coming aboard my boat would be somewhat imperceptive of what lay before them, where all circuits are labeled and where all DC negatives are yellow and AC hots are black.

[PerfectPirate]

Ah well, glad we cleared that up. I may even be slightly envious of you having a boat with clear, modern, and rational wiring. Especially after some of the stuff which I have had to clean up in my old boats. After I finished pulling the old wiring out of my current boat, my remark to the Admiral was: "It is a marvel and mystery to me how this boat has survived 48 years without having an electrical fire." Sail on!

[s/v Jedi]

Originally Posted by scherzoja

Oh my. I certainly did not intend to paint everyone with the light of incompetence. My intended meaning was that anyone coming aboard my boat would be somewhat imperceptive of what lay before them, where all circuits are labeled and where all DC negatives are yellow and AC hots are black.

To make things more complicated… with US 240V, we have black for L1 and red for L2. I don’t see the problem, you’re supposed to know what wiring you’re working on and make sure breakers are off anyway.

[deblen]

Originally Posted by s/v Jedi

To make things more complicated… with US 240V, we have black for L1 and red for L2. I don’t see the problem, you’re supposed to know what wiring you’re working on and make sure breakers are off anyway.

Opinion warning!
IMHO there are too many "tinkerers" playing with their boat's wiring,but,it's your boat & your $$s,so.....


The simple old rule for DC wiring in N.America is(was?) that Black is always Negative is always Black.

The other color is positive.
Except on Rule bilge pumps.


I was not aware that ABYC has changed that "ancient" practice. Why??
Why yellow??



Len

[s/v Jedi]

Originally Posted by deblen

Opinion warning!
IMHO there are too many "tinkerers" playing with their boat's wiring,but,it's your boat & your $$s,so.....


The simple old rule for DC wiring in N.America is(was?) that Black is always Negative is always Black.

The other color is positive.
Except on Rule bilge pumps.


I was not aware that ABYC has changed that "ancient" practice. Why??
Why yellow??


Len

And AC live has always been black and L2 has always been red.