grounding 12 v system

[ozskipper]

Originally Posted by Quique

hahaha!
what will i drink then

Cheap wine for a fortnight! Then back to beer after that ;-)

[osirissail]

We all had to start somewhere, sometime ago and once you do some comprehensive reading in the books like the "12V Boat Doctor" and others (which are normally available in libraries and book swap places in marinas) you get the basics and can learn the names of purposes of each part of the electrical system.

But the 12VDC (or 24VDC) side of boats is not all that critical and quite tolerant of early errors - so long as you don't direct short your battery(ies). And learning how and why the system is arranged goes a long way to saving lots of money later on when some supposed professional starts feeding B.S. to you about this or that and how the cost is going to be big bucks.

But the first step is getting the names of things correct so that you can communicate effectively what you want to do and others can respond accordingly. After that it is really just a matter of following the guidelines in the books and keeping everything neat and orderly so you can "see" how things work and the power flows from one place to the other.

[goboatingnow]

Just to paint a view here,

I would not personally recommend using the engine as a central ground point, use the battery negative terminal or a nearby Bus bar.

I see no reason to connect DC ground to seawater. in fact I see advantages in having a isolated DC system and that includes the engine ( but this is awkward to arrange).

Protect AC into the boat with a whole boat GFI or better yet an isolating transformer, hence youll have both an isolated DC and AC system. if you remove earth return paths you are much safer. ( ie with an isolated AC) . Protect the inventor output with a dedicated GFI ( RCD)

I do not personally support binding the DC negative and AC protective earth together, but I know ABYC are wedded to it, even if the RCD isnt. ( this is really a GFI issue IMHO)

If you have a rats nest, dont just poke about , redo the whole lot, rats nests start do to incremental additions and bodges.

The cases of DC systems are very rarely connected to DC negative in pure DC devices.

Dave

[osirissail]

Generally all very excellent views that I share, but to illuminate a little more on some of them - -

Neatness goes a very long way towards a safe and efficient systems. That is, a single battery sized cable to the engine block from either the "load-side" terminal of the shunt or the engine start battery itself is better than lots of negative (ground) cables from here, there and everywhere.

In order for the engine starter and the alternator and the engine instruments to function the engine block needs to be attached to the boat's DC negative (ground) system.

And getting rid of the "rat's nest" by either starting over with proper marine-rated electrical wire or at least sorting out and arranging existing wires into logical and neat groups is desirable.

Whether the boat's DC system is or is not connected (grounded) to seawater is usually a moot question as the design of the engine propulsion system in the vast majority of boats results in metal to metal attachments that end up with the propeller which normally is immersed in water. However, a few boats have propulsion systems that are "isolated" electrically to reduce or prevent electrolytic deterioration of the drive system. Again, the voltages in an average boat's DC system are not high enough to pose a hazard to humans so sea water grounding is not a major consideration compared to a boat's AC system.

It is rare these days to find DC powered instruments/loads that have metal cases, most are plastic. But in the ancient DC systems (my old boat had quite a few) metal cases were indeed bonded to the DC negative terminal of the instrument/load so that a positive lead short to the metal case would trip the fuse/circuit breaker rather than pose a spark or fire hazard.

GFI's are now (in the USA, at least) a part of the ABYC guidelines and without proper bonding between genset or inverter neutrals and safety grounds, the GFI's would constantly trip or refuse to stay set. AC "leakage" was rampant in the old equipment and a real pain to correct.

I suspect it was primarily a cost issue for boat builders to simply bond the AC safety ground to the DC negative (ground) system rather than installing a separate sea water ground plate for the AC system. Here again, the voltages in a normal boat's AC system can do serious damage to humans so having the system be able to trip the circuit breaker or GFI when a sufficiently large AC leakage developed was necessary.

Personally, I prefer keeping the boat's DC systems separate and apart from the boat's AC system. In AC to DC bonded systems frequently AC "leakage" would cause havoc with sensitive instruments not to mention that "tingly" feeling whenever you touched a metal part of the boat especially after climbing back onboard after a swim in the ocean.

[goboatingnow]

GFI's are now (in the USA, at least) a part of the ABYC guidelines and without proper bonding between genset or inverter neutrals and safety grounds, the GFI's would constantly trip or refuse to stay set. AC "leakage" was rampant in the old equipment and a real pain to correct.

I suspect it was primarily a cost issue for boat builders to simply bond the AC safety ground to the DC negative (ground) system rather than installing a separate sea water ground plate for the AC system. Here again, the voltages in a normal boat's AC system can do serious damage to humans so having the system be able to trip the circuit breaker or GFI when a sufficiently large AC leakage developed was necessary.

I dont follow the first paragraph

Any nuisance GFI/RCD ( 30ma types) trips on AC on board need to followed up and eliminated. They point to leakage currents flowing outside the designed return path. I have a 30 year old boat with inverters chargers, generators, shore systems and 20 AC breakers, I don't get false RCD trips. There is a reason you get them.


In the 2nd paragraph, Ive have never seen AC safety ground specifically bonded to sea water. Why on earth would you do that, particularly in this GFI(RCD) era. in fact most "code" seen suggests that the only source back should be true the neutral or in a short via the protective earth wire, you do not establish another ground ( not to mention the issue of ground loops!).

Once away from the marina, all onboard AC is ground isolated and should be kept that way. creating a path to earth only provides additional shock hazards. ( think double isolated drills as an example)

dave

[Quique]

thanks for the advice.
the inverter had a metal case. but i removed it anyway
it is very old. and i guess maybe a bit dangerous
i have a wire now from the - bus to the engine block.
iam not messing with any of the boats ac.

i have a 1750 inverter for sale that gives a tingle along with ac power

[osirissail]

In order to align definitions, etc., AC systems on this side of the Atlantic use two return systems - neutral and earth ground. The "hot" AC wire comes from the power source and is mated with the "neutral" which is, after the load, also directly routed back to the power source to complete the circuit. A second return is the "earth ground" which rather than using a wire that returns directly to the power source is routed via the actual "Earth" as its path back to the power source. This is why homes and land based AC systems have very long "grounding rods" driven into the "Earth" which are connected to the AC system's "earth ground." We don't have "dirt" when floating on a boat to which to connect the "earth ground." So old boat's have the AC earth (or safety) ground typically connected to the conveniently "grounded" engine/propeller DC ground.

Because of the lethal nature of the AC system it was deemed necessary to provide two ways to enable a shorted or leaking AC load to trip a fuse or circuit breaker. First via the normal "neutral" return wire or if that part of the circuit is interrupted then through the "safety ground." With a broken neutral return path, if a human contacted the live "hot" wire path, he could provide a better return path with lethal results before the fuse/circuit breaker would interrupt. But the "safety ground" was not all that efficient and so GFI's were invented to detect rather small amounts of leakage by comparing the return (neutral) to the safety ground and if leakage was detected in the safety ground line the GFI tripped interrupting the supply "hot" wire.

However, before the use of GFI's were recommended for use on boats the only way to provide that second "earth ground" was to bond the earth ground to the water. That insured that a boat with a broken neutral and shorted load when connected to shore power would trip the circuit breaker.

The introduction of inverters and gensets into the small boat arena added a problem of how to detect shorts or leakages with a compromised neutral and GFI's helped. However, salt air, salt crystals, and other conductive debris can build up over time and result in leakages in an old boat's AC system. Retrofitting GFI's into old boats usually resulted in numerous GFI trips until the system was cleaned and stray leakages eliminated.

Since, I believe, most CF contributors have or end up owning "old" boats and not brand new ones, understanding how to deal with leaky AC systems is relevant. A lot of folks simply accept a certain amount of AC leakage rather than invest money in redoing their whole AC system to "clean it up." Installing GFI's typically forces them to clean up their systems or they just do not use GFI's and live with the leakage.

Bonding your boat's AC safety ground system to water is merely a back up way of protecting against shorts/leakage problems when using shore power. Not everybody has the "new and/or latest" in their boat's electrical systems. And different folks have different tolerances to how much "tingle" they can take before their curly hair straightens and starts to smoke.

[Quique]

good advice. i was negligent in continuing the use of the inverter even when i was geting the tingle.

that problem fixed for now. lesson learned. thanks!

[Zil]

Having the negative terminals of the 12 volt systems connected to the engine block is not a bad thing.
I bet your old inverter was the cause of your shock?

[goboatingnow]

I will stand to be corrected here, but I do not beleive that there is any requirement to bond AC protective to seawater in ABYC. They suggest using the engine as a common grounding point, but there is no requirement for conductivity to seawater.

Note that GFIs trip on am imbalance in the Live (Hot) and Neutral lines , not by detecting fault currents in the AC protective ground.

[irwin37]

I would never connect an AC ground to the sea water via any device of any kind..

Picture this... My boats AC is grounded to the engine or some other connection to sea water. I am plugged into a shore power connection at the dock which is a little iffy, the docks ground is broken or faulty. I have a short from a hot lead to ground on the boat, light fixture maybe, the drill, battery charger, who knows. Now I jump over the side (Salt Water) I or the next boats kids could electrocuted as the short circuit current is flowing thought the water and me. As the current might not be enough to trip a breaker it would be a very exciting swim at best or the end of me and the kids life.

The only ground should be back to the source, the dock shore power in this case. Using a isolation transformer would be the best insurance against shock hazard on a boat, but they are heavy and expensive. A galvanic isolator will provide some protection from electrolysis but not from shock hazards. GFI will work on a boat with or without a good ground as it measures the current in the hot and neutral wires and trips if there is more than 30ma difference. If you have leakage current in you AC system that trips your GFI then you need to fix it. It could bite you when you least expect it if you don't.

[goboatingnow]

I agree +10 with Irwin. If you get RCD ( GFI ) nuisance trips then fix it. There no reason on an old or new boat to be subjected to such trips ( from 30ma GFIs )

Dave

[Zil]

Back to AC? The white neutral wire should not "ground" to water except on the dock main panel. Never on the vessel. Water and the green wire all connect to earth. Neutral, the white wire, sometimes has potential to earth, the green wire.
It should not hurt the vessel to have it's green wire bonded to water. It can hurt the vessel to have it's white wire bonded to water.
DC is different. DC has two wires, plus and negative. It will not hurt the vessel to connect DC negative to the engine blocks.
My deff: vessel = the boat, ship, raft, and all the people and parts on-board.

this is only MHO. and how I wire my ship.

[goboatingnow]

Back to AC? The white neutral wire should not "ground" to water except on the dock main panel. Never on the vessel. Water and the green wire all connect to earth. Neutral, the white wire, sometimes has potential to earth, the green wire.
It should not hurt the vessel to have it's green wire bonded to water. It can hurt the vessel to have it's white wire bonded to water.

I dont beleive anyone ever suggested connecting Neutral to seawater. The discussion has focused on the earth wire.

dave

[Zil]

No argument here, Dave. Just trying to grasp all the differing ground/neutral/earth terms as used on this forum, it can get confusing for us amateurs. Just wanted to be clear about white to green.