Tie 12v & 24v grounds together?

[OldYachtie]

I am designing an electrical system for a 65' catamaran that will use 2 Yanmar 6BY260 engines. Yanmar is supplying these only with 12 volt systems, and a lot of the innards of these new Tier 2 engines use electricity--the injectors are "magnetic," which I assume means electric injection, a computer controls everything, and who knows what else besides sensors is electrical. I think the reason Yanmar won't supply a 24 volt version is that it is a marinized car engine from Germany.

I want to run a 24 volt boat, with two 24 volt alternators bolted to the engines in addition to the stock 12 volt alternators, so I want to completely separate the house bank from the starting bank. All I want to connect to the system running on 12 volts are the engines, starting batteries, and their harnesses.

All of the sample circuit drawings show one connected system, so I don't have much to go on. Do I need to connect the grounds of the two systems for safety? Does it matter that they are working at different voltages?

I don't have any use for a 220 volt AC system, as this boat will live on a mooring buoy, and I don't intend to operate any equipment that uses AC power. I'll operate the bridge's electronic stuff that doesn't come in 24 volts from a little 24v DC to 12v DC converter.

(Answers to FAQ-It's a big boat, and needs 24v power for big loads with long runs; yes, you can get 24v in almost any piece of equipment; no, it's not very complicated if you start out as a 24v boat as opposed to adding 24v to a 12v boat.)

[Maddog]

Call Peter at Jack Rabbit Marine. He should be able to point you in the right direction.

[never monday]

This is a non fact based answer.

For the sake of the engines and posible warranty situation with mechanincs who might not understand your system. How about leaving them seperate.

[OldYachtie]

Hi, Pat. With your inlandmarinediesel.com signature, you should know. On the other hand, the guy who makes WH autopilots says to tie them together. Nigel Calder's book shows everything-all negative busbars, lightning conductors, etc. attached to the ground, which it seems to me would have the effect of connecting the two different voltage negative busbars together. What error might a mechanic make if he didn't understand the system, and they were connected together?

Thanks,

Tim Dunn
near Seattle

[Alan Wheeler]

All Earths are to be tied together and to one common point. It doesn't matter what voltage Earths are married to. All Earths are....or need to be at zero volts. It's why they all need to be connected at a common point, so as no current is created between points.

I think the reason Yanmar won't supply a 24 volt version is that it is a marinized car engine from Germany.

I am not sure why this is should be a reason. But I do presume it is possible that a 24V model has not been created. This really has nothing to do with where an engine is built.
Plus, who told you it was a German built car engine????

[never monday]

Like I stead, I'd build it so the apperance was they are seperate.
I wouldn't want some shop on a remote island giving me grief about the install IF there were a warranty issue. Or have you wiring diagram/ABYC standards on board.

Wheels,
The BY are a BMW joint venture. The 4BY is a 2L and the 6BY is a 3L BMW production engine.

[GordMay]

All the DC Negative Grounding Conductors, irrespective of Voltage (12 or 24VDC) should terminate at the main DC Ground Bus, thence to the Immersed Ground Plate.
You might install two DC Gnd Bus' (1 ea voltage), thence to Gnd Plate.
Sea: Basic DC Neg & Gnd Wiring: http://www.cruisersforum.com/gallery...age.php?i=1391

[never monday]

I like Gords sugestion

[BBill]

OldYachtie...you have a pm.

BB

[OldYachtie]

Thanks, guys

Connect the grounds it is. I'll use separate 12 volt and 24 volt negative busbars and tie them together, and to the grounding busbar. Yanmar had a press release stating that the BY engines were part of a joint venture with BMW. My thinking is that the electronics, or at least some of them, are also used on the car, and that cars don't use 24 volts because their loads have short runs, especially the big loads. I read a rumor online that the move to higher voltages in autos was aborted because all of the switches would be too expensive.

Tim

[EngNate]

Ditto Gordie and others, all (-) busses of all DC systems are to be connected.

[hellosailor]

In theory, yes, all "ground" busses would be tied together. But in practice, this creates a new hazard on the boat. By tying the 12 and 24 volt grounds together, you now create the hazard that you can have a short or other unintentional connection between the 12 V+ and 24V+ systems, putting 24V on the 12V system or creating a 12V loop in the 24V side.

That might be a small hazard and worth ignoring in favor of having one common ground--but I'd check out the system with that thought in mind, and make doubly sure the two system positives were kept isolated from each other to make sure it couldn't happen. Perhaps by using special connectors on the 12V system, to make Real Damn Sure that someone couldn't accidentally tie it to the 24V system, or at least putting permanent warning tags on the 12V and 24V hot cables.

[EngNate]

All wiring circuits are to have overcurrent protection properly sized for the wire. The idea is that in the event of a short from one system to another the circuit protection will be activated. There are other standards covering installation practice on the live ends.

Similarly, we use an AC safety ground even though grounding the system means you only have to touch one side of the circuit to get a shock. In ungrounded AC system, you would have to contact both conductors at the same time. Overall, the grounded system is safer.