Multiple voltage ground loops on a Listeroid generator

[veggie]

I need some guidance on an electrical charging system.
My Listeroid has a combo starter/generator which runs on 12 volts.
It is connected to a 12 VDC starting battery and once running, the generator reverses flow and charges the battery.
It works well.
https://www.youtube.com/watch?v=qbcbuygYJdM

I just added an additional alternator to the base which is driven from an Auxiliary pulley on the crank.
The alternator is a 24 VDV unit and will be used to charge a 24 volt battery bank which drives a Grid tie inverter to feed into the house mains.

Just as I began preparing the 24vdv wiring to the alternator I realized...

HEY ! ..... Wait a minute ! .... Both of these DC generators use ground as the return path from the battery.

Can I use the engine frame as the ground return for TWO alternators with differing voltages ?

Below is a sketch of what I am trying to illustrate. <click the picture to enlarge>

To compound the question....the system also has a 3kw AC generator head which I believe is also grounded to the frame via it's chassis.

Cheers,
Veggie

[BruceM]

It's good to think carefully about grounding, and to avoid multipoint grounding of power systems so that current isn't flowing through non-wire paths such as pipes and earth.  Safety grounding systems should NOT be carrying current. Your situation does require some thinking and planning to be safe and code compliant.

The point of the safety grounding system is to keep metal boxes/cases and chassis at earth potential (no shock) even with a big load, and to provide a current path to clear hot to ground faults capable of tripping the offending circuit breaker. 

The US NEC does a good job of this, even though the early 1900's era designed Wye power distribution grounding system does not.

There is no problem with having a common ground for the two DC systems.   It is also OK for the generator AC to be connected to the same ground, as long as that's the only place that happens.  IF the AC is going to the home, which is grid tied, then WHOA. we need to look at a more complete diagram.

If feeding a home via a transfer switch, normally you'd have to run the safety ground from the main or sub panel, all the way to the generator ground/chassis, which would also become tied to your DC ground.

If the 7KW ac generator neutral is internally bonded to the case, and you ARE connecting this generator to feed a grid tied home then you should isolate that connection, or isolate the mount.  Otherwise, the safety ground will effectively be tied to neutral at both the generator end and at the house power panel, and thus the safety ground will carry neutral current all the time the generator is feeding the house.  If it's an off grid home and the 24V inverter is the only AC source besides the generator AC, then we should look closely at the inverter wiring/grounding diagram to see if you can leave the neutral to ground bond connected in the generator head. 

If you show us a diagram and or tell us exactly what is going on, then I can try to be more specific. 

[Lloyd]

For efficiency use a separate ground cable from each alternator to it's own battery. All the better if the alternators have an isolated ground. Same for the  AC gen, use a separate cable bolted to the gen back to the AC ground bus. Now make a cable from the gen frame to an earthing ground rod driven at 6 ft as close to the gen as possible.

Lloyd

[veggie]

BruceM,
The AC Generator Ground in not bonded to the generator Neutral (Floating neutral) so that I can power the house through a transfer switch.
For this reason, the Generator chassis is not grounded with a ground rod to earth because the ground for the AC generator is actually in my house Mains panel.
The 1.2kw Grid Tie inverter is a very basic unit which accepts 24vd at two terminals and has a standard 15 amp grounded wall plug to feed 110AC power into the house.
The inverter has "Island Protection" which prevents operation if the grid goes down, so backfeeding into a dead grid is not an issue.
Hmmm.... When the AC gen is not being used with the transfer switch, the system has no ground.
I may have to drive a ground rod into the earth and connect it to the generator base through a switch which can be opened whenever I use engage the transfer switch.
Thereby switching my ground pathways.

Lloyd,
I like your idea of delivering the (-) line back to the Alternators with dedicated cables rather than using the chassis like an automobile.
I will make up some cables and connect the (-) battery posts directly to the case of the alternators.

Thanks guys,
veggie

[BruceM]

Sounds like you're in good shape, Veggie since your 7KW head neutral and ground are isolated.

Your transfer switch should not, by code,  interrupt the safety ground connection.  You don't need a ground rod at the generator as your separate safety ground wire from the transfer switch to your generator grounding point/chassis will provide that function.  Adding a ground rod at the generator is not required by code and would be considered a violation in some jurisdictions, just like adding a ground rod to a sub panel.  The ground from the main panel/transfer switch is required.

Adding a ground rod at the generator , connected to the ground wire from the main/transfer switch, in the case of your 7KW head with isolated N and G,  will not cause your own generator current on the safety ground wire, but will cause additional net neutral current to earth if you're connected to a Wye power distribution system (most of the US and Canada), since the typical Wye grounding practice does not provide transformer isolation (an engineering abomination courtesy of the early 1900s).  Adding earthing conductors to a home with Wye power will cause elevated net neutral and magnetic field problems.  The worst offenders are metallic water and gas mains (which should be isolated, with the house pipes bonded to the safety ground within the home, per code) and water well pumps (which I feel should be run ungrounded on GFI breakers, or whose safety ground should be connected only through a 24V TVSS unit to eliminate net neutral current to earth).

[veggie]

BruceM,

My 3KW head is only connected to the house mains during emergencies when I plug the 4 prong (L1,L2,N,G) connector into the transfer switch.
Until I plug the genny  into the transfer switch (rarely) the system has no ground.
That's why a "seperately derived" ground using a grounding rod was proposed for all the other times when I use the generator.

PS: My UL approved transfer switch does not switch the ground wire. Only the hot leads.

veggie

[BruceM]

Thanks for clarifying that Veggie.  I'd try to find a way to have the house ground connected to the genny all the time, myself, as then you're totally code compliant and have no added net neutral current, but the additional earth grounding at the genny won't cause any overt safety problems at all.

[Dualfuel]

The only thing relevant that I have to add to this, is a cautionary tale of placing those alternator or generator ground leads on some portion of the frame where they are not getting vibrated. I have had some trouble with the cable ends developing resistance because they corrode. The corrosion isn't readily apparent, until one sees the small cracks in the insulation or sealant at the cable ends...in my case, I thought it strange that one of my cables was warm. Then I discovered that it was carrying all the ground current.