Electrical systems How many grounds?

babell2 · February 20, 2011, 10:37:37 PM

In design mode. MCI-9 24 Volt bus with chassis ground.

I will require a 12 Volt system to run the things I can't find or get in 24 Volt. (mostly lighting)

I will also need a 120 Volt system for Stuff. May also need 240 depending on systems.

As I see it I need at least 3 separate ground systems. Bus frame for 24, independent isolated ground system for 12 Volt and another isolated for alternating current. Am I off on this?

How do I isolate my gen set from the bus to ground the 120/240 AC system?

JackConrad · February 21, 2011, 04:45:22 AM

12 and 24 volt can both be grounded to the bus frame. AC is a little different. grounds are connected to bus frame, HOWEVER neutrals must be isolated and only combined at the source (ie. geneator, shoreline, or inverter). This is why you need a transfer switch that also switches the neutral and a relay at the inverter for the same reason. Jack

bevans6 · February 21, 2011, 05:12:56 AM

Ground is ground, there is no such thing as an "isolated ground". Floating negative, isolated common connection point maybe, but not isolated ground, except by accident. You need to tie the ground for your 12 volt back to the bus chassis, the 24 volt the same, and the AC the same. As noted, neutral in the 120VAC is isolated from ground except for the ground-neutral bond at the source.

Now, you certainly can do what I call "home-run" the 12 vdc grounds, which is bring them back to a common point or buss bar, which is itself grounded to the bus chassis. I do that a lot. Same with high current 24 VDC wiring for the inverter, you can run that ground back to a common point with the batteries. AC wiring is all home-run, it's not usual to have many separate ground connections but rather to connect the bare ground wire back to the main ground point at the main distribution panel.

Brian

Len Silva · February 21, 2011, 06:16:43 AM

I would add that if I were building from scratch, I would have a large (perhaps 2/0) copper ground from the batteries to each electrical panel and use that as the return path for 12 and 24 volt loads rather than the chassis. It is just a better conductor and more reliable connections than using the steel frame of the bus.

My 4104 was wired like that from the factory, I don't know if other buses do that today.

Heavy loads like the inverter should be grounded directly to the battery, and not use the chassis at all for the return.

TedsBUSted · February 21, 2011, 07:10:29 AM

However, with series connected batteries, common ground connections can not be shared between batteries.

For example, if two 12v batteries are series connected to make a 24v battery bank, then both batteries can not share the same chassis ground, because that would essentially short the #2 battery's terminals together. Barring some "smart" electronic isolator, such a system would require that either any center-tapped 12v loads would be isolated, or that 24v loads would be isolated.

Sean · February 21, 2011, 08:08:15 AM

Quote from: TedsBUSted on February 21, 2011, 07:10:29 AM
However, with series connected batteries, common ground connections can not be shared between batteries.

For example, if two 12v batteries are series connected to make a 24v battery bank, then both batteries can not share the same chassis ground, because that would essentially short the #2 battery's terminals together. Barring some "smart" electronic isolator, such a system would require that either any center-tapped 12v loads would be isolated, or that 24v loads would be isolated.

That's not how it works. The negative post of the upper battery has no "ground" -- it's just a negative post. It sits at +12v with respect to ground.

12-volt loads and 24-volt loads also share a common ground, which is the same ground tied to the chassis and also the ground for the 120-volt AC system. Only the hot sides of these circuits are isolated from each other.

Except in very specific engineered circumstances (such as the headlights on MCIs), no loads can be connected to just the upper batteries of a 24-volt bank, precisely due to this common ground issue. This is why battery equalizers exist -- to keep the upper and lower halves of the bank in balance since all the 12volt loads will be on the lower half.

-Sean
http://OurOdyssey.BlogSpot.com

TedsBUSted · February 21, 2011, 08:56:20 AM

Quote from: Sean on February 21, 2011, 08:08:15 AM
Quote from: TedsBUSted on February 21, 2011, 07:10:29 AM
However, with series connected batteries, common ground connections can not be shared between batteries.

For example, if two 12v batteries are series connected to make a 24v battery bank, then both batteries can not share the same chassis ground, because that would essentially short the #2 battery's terminals together. Barring some "smart" electronic isolator, such a system would require that either any center-tapped 12v loads would be isolated, or that 24v loads would be isolated.
That's not how it works. The negative post of the upper battery has no "ground" -- it's just a negative post. It sits at +12v with respect to ground. . . .

Sean we're on the same page about (not) grounding the center link, maybe my reply wasn't so clear.

I possibly misinterpreted this thread's first posts to mean that what was being contemplated was to connect both the #1 battery and the #2 battery, of a series connected set, to chassis ground at each battery's neg terminal. Which of course would not be workable.

FWIW, in addition to the unique MCI light circuit, another odd-ball exception can be found with a 24v isolated starter motors.

Ted

babell2 · February 21, 2011, 09:48:10 AM

OK
Now that I have some of the terminology correct let me go British with this to make it more clear.

All systems share Earth with each other.

12Volt will be tapped out not with a center tap but with a converter to prevent the short to center tap 12volt and 24volt mix.

120/240 share earth but return is isolated.

How do I split out the return 120 from the 240 leg of the system?

In all the house wiring I have done (3 wires) 110 hot/ 110 hot / return. combine one 110 to return = 110.

Combine both through load with return = 220. Earth and return are tied at the panel.

Brice

HB of CJ · February 21, 2011, 10:22:27 AM

Even though it's not an electrical system per say, current does flow through the drivetrain and chassis just going down the highway.

Running a separate, dedicated grounding strap from the engine components, tranny, radiator, and rear end is a good idea to stop electrolysis.

An extreme example may be a engine that continues to eat headgaskets or a radiator that rots; caused by a small amount of discharge current.

Engine head gaskets sometimes do unwanted double duty insulating the engine components. Grounding straps pervent this. HB of CJ (old coot)

bevans6 · February 21, 2011, 02:29:09 PM

Brice, you're going to be farther ahead to use customary terms for ground, neutral, and hot. People understand them and can understand what you are trying to say. In your example, you suggest that in a house you have neutral and ground bonded in the main panel. That's often true, and beat to death here recently

But - in a bus it's critical to never bond neutral and ground anywhere but at a source of electricity. On a bus, that can be at the generator, at the inverter, or at a pedestal or other outlet you may plug in to. You always make the source of the electricity the source of the bond, and you switch the neutral through to it.

Brian