grounding of ac dc systems question (few different scenarios)

pltek

New member
hello, i have searched on this, there is a lot of information to sort through but have not found an answer to my understanding; i appreciate feedback.

the questions build on themselves so i can hopefully understand the impact of each component on the grounding needs.

baseline scenario
my system is 2 lithium batteries in parallel for 200A bank, on (+) side from battery, i have a main shut off, then distribution block. On (-) side i have the vitron shunt, then distribution block. Fuse box is connected to both distribution block and all consumers have 2 wires each.

If there was nothing else in this system (BB charger, inverter) , would i need a ground?

add bb charger to above
add bb1260 sterling 60a charger, positive wire with 80a fuse to charger, then + and - negative wires to the distribution blocks?

If i did not need ground in baseline scenario, do i need one now? if so, can i simply run a cable from (-) distribution block to chassis and be done?

add inverter to above
add progressive dynamics pd1620 2000w inverter to + and - power distribution blocks.

If i did not need ground in baseline or with bb charger scenario, do i need one now? if so, can i simply run a cable from (-) distribution block to chassis and be done?

twist: introducing ac power here by adding ac in from shore power. the inverter has internal switch so it will use either dc battery power or ac power to feed the ac outlets. Does that change anything in terms of grounding meaning if i didn't need a ground at all so far do i need one because of the ac in?
 

marklg

Well-known member
hello, i have searched on this, there is a lot of information to sort through but have not found an answer to my understanding; i appreciate feedback.

the questions build on themselves so i can hopefully understand the impact of each component on the grounding needs.

baseline scenario
my system is 2 lithium batteries in parallel for 200A bank, on (+) side from battery, i have a main shut off, then distribution block. On (-) side i have the vitron shunt, then distribution block. Fuse box is connected to both distribution block and all consumers have 2 wires each.

If there was nothing else in this system (BB charger, inverter) , would i need a ground?

add bb charger to above
add bb1260 sterling 60a charger, positive wire with 80a fuse to charger, then + and - negative wires to the distribution blocks?

If i did not need ground in baseline scenario, do i need one now? if so, can i simply run a cable from (-) distribution block to chassis and be done?

add inverter to above
add progressive dynamics pd1620 2000w inverter to + and - power distribution blocks.

If i did not need ground in baseline or with bb charger scenario, do i need one now? if so, can i simply run a cable from (-) distribution block to chassis and be done?

twist: introducing ac power here by adding ac in from shore power. the inverter has internal switch so it will use either dc battery power or ac power to feed the ac outlets. Does that change anything in terms of grounding meaning if i didn't need a ground at all so far do i need one because of the ac in?
If you have a Sterling, you need a shared ground or it will not work. It only has one ground connection, for a shared ground.

In general, you should have a common ground for everything or else unsafe voltages could result from floating systems, even if they work.

If you have 120V systems, NFPA 70 requires them to be grounded to the vehicle chassis for safety, otherwise the ground wire in the electrical system could have lethal voltages, if the inverters and converters work at all.

Electrical current flows in a loop. It has to go somewhere. Think of it as a water hose. If water flows into a hose and the other end is not connected to a drain, the water goes somewhere else and you get wet. In case of electricity, you get electrocuted if you are where it goes.

The only floating systems I am aware of are designed to be fully floating. The output including "ground" are protected so current cannot flow to the input ground at all if you accidentally or deliberately touch them. That is not true for RV inverters, converters and appliances.

If you do not have a good understanding of this, i recommend for your own safety to find someone who does. Electrical and Propane systems for that matter are not to be trifled with.

There is a similar discussion in this forum:


Regards,

Mark
 

pltek

New member
thanks Mark, when you say shared ground, to me it means the ground I can create by connecting the negative power block to chassis since all power generating devicess (b2b charger, shore power charger) as well as consumers ( fuse box with fan, lights, usb ports, fridge as well as inverter) connect back to it. is that it?

i am asking because on sterling, the negative on the b2b charger is supposed to go to starter battery and from there to the battery bank (house battery). Couldnt I just connect the negative on sterling directy to the negative power block if its grounded to chassis?
 

marklg

Well-known member
thanks Mark, when you say shared ground, to me it means the ground I can create by connecting the negative power block to chassis since all power generating devicess (b2b charger, shore power charger) as well as consumers ( fuse box with fan, lights, usb ports, fridge as well as inverter) connect back to it. is that it?

i am asking because on sterling, the negative on the b2b charger is supposed to go to starter battery and from there to the battery bank (house battery). Couldnt I just connect the negative on sterling directy to the negative power block if its grounded to chassis?
I have a battery disconnect on the negative of the starter battery. It disconnects the chassis from the negative terminal of the starter battery. The negative cable from the Sterling is connected to the chassis side of the battery disconnect. The negative side of my house batteries and inverter are also connected to the chassis in the back, so there are really two paths for the Sterling to ground. If the Sterling negative wire was connected to the starter battery minus side of the disconnect, the disconnect would not be effective as there still would be a fairly poor connection from the battery to chassis via the long wire to the Sterling.

Regards,

Mark
 

Graphite Dave

Dave Orton
My house electrical system is not grounded to the chassis. I do not directly charge the house battery from the vehicle 12 volt system. I have two inverters. I can charge the house battery from the vehicle by using a second inverter powered by the vehicle 12 volt system. The inverter 120 volt AC output can power the shore power charger.

The advantage of the system is having 120 volt AC available with engine running and always charging the house battery with a proper three stage charge profile. Both solar controller and the house charger provide three stage charge profiles.

System has two selector switches. First switch selects either real shore power or "shore power" from the vehicle powered inverter. Second switch selects power to the house inverter/charger/transfer switch or power to heat shower water or power to a electric air heater in back of the van.

The loads powered by the vehicle powered inverter are all less than 1000 watts so that limits the load on the alternator.

Electrical | Orton Travel Transit (ortontransit.info)
 

pltek

New member
GDave,

your post and another build summary i came across are very similar so you are not alone in that 1%, so that is why i question necessity of grounding a standalone 12v dc system if all connections have + and - wires running to and from and are fused.

 

Graphite Dave

Dave Orton
GDave,

your post and another build summary i came across are very similar so you are not alone in that 1%, so that is why i question necessity of grounding a standalone 12v dc system if all connections have + and - wires running to and from and are fused.

Have had two conversions with the ungrounded to van steel system. No issues so far in 5 years with each conversion.

All DC loads have two wires and each load is fused. SOOW cords used for wiring with no splices between the fuse and the load.

The discussion about a ungrounded system centered around the fact you can have a short from a positive wire to the van steel and that will not blow a fuse. It takes a second failure of the negative wire contacting the van steel before the fuse will blow. That risk is real but not very probable IMO. SOOW cords are very well insulated so not much opportunity for a short.

I did the ungrounded system because I was afraid of interconnecting with the Sprinter electrical system. Ungrounded system works if you do not directly connect the vehicle battery to the house battery for charging. I use a pure sine inverter powered by the vehicle 12 volt system to provide 120 volt AC power that can be used to power the shore power charger. My house battery is always charges with a proper 3 stage charge profile.
 

avanti

2014 GWV Legend 3500 I4
Dave's summary of the (rather thorough) previous discussion is essentially accurate: Bonded systems are safer, but it requires two failures before it matters.

Dave is also correct that bonding is in no way necessary in order for a fault-free circuit to operate. The best summary of this I have encountered is the following:
[D]oes a circuit have to have an earth ground in order to work? Absolutely not. Many circuits don't have a physical and direct connection to the earth. Electronic devices that pose a safety hazard normally do have one for extra protection, but it isn't an a requirement for a circuit to work.

[Yes, I know that a van chassis is not a true "earth" ground. However, its relationships are precisely analogous.]

It is worth noting that, as far as I know, ALL professionally-designed systems use a common ground for power-carrying circuits. The talk of separate grounds appear to all come from DIY enthusiasts.

I did the ungrounded system because I was afraid of interconnecting with the Sprinter electrical system.
The logic of this is incorrect. A common ground DOES NOT constitute "interconnecting with the Sprinter electrical system." For current to flow, there needs to be a circuit, which requires TWO connections, and a common ground is only one. Circuits with common grounds do not effect each other in any way. As I have pointed out before, almost all terrestrial power circuits are interconnected through earth ground.

Finally, there is one other reason for bonding which, to my knowledge, has not been discussed here, and that is static charge dissipation. The bonding of two systems guarantees that any static charges that might build up in one is equalized in the other. Keeping the vehicle electrical system isolated from the house system risks large differential static buildups, which is not good. This is why in vehicles, even the fenders need to be bonded.
 

autostaretx

Erratic Member
Adding to the usual confusion is the mish-mash of terms.
The negative DC connection isn't really a "ground", that's just an all-too-common misuse of the word (which i do, too).

On the AC side, the US standard is "hot"," neutral" and "ground".
The loads' currents flow from hot to neutral, and the ground shouldn't see anything.
(in a 120 vac inverter system, even "neutral" is incorrect, since the "neutral" really only applies to a full 240 volt system (two "hots" and the "neutral" half-way between). In an inverter system you'd do well to consider both of the 120vac wires to be "hot", and neither should allow/provide current flow to the (let's say) metal box that holds the inverter electronics. That box will probably have a small stud to connect to a "real" earth ground system. ("system" meaning wires or other metal surfaces).

The "ground" in the AC system is a (hopeful) "bonding" (i.e. good connection) between all the metal parts to a real earthing ground connection.
If you're an RV plugged into shore power, that's what the 3rd (or ground in a 50 amp connection) pin is supposed to provide.

The killer in RV set-ups is when the metal of your vehicle becomes "hot" due to a wire or equipment failure and (especially with shore power involved) you step out on to the wet grass and your body becomes the "hot to earth" conductor.

--dick
 

Graphite Dave

Dave Orton
Adding to the usual confusion is the mish-mash of terms.
The negative DC connection isn't really a "ground", that's just an all-too-common misuse of the word (which i do, too).

On the AC side, the US standard is "hot"," neutral" and "ground".
The loads' currents flow from hot to neutral, and the ground shouldn't see anything.
(in a 120 vac inverter system, even "neutral" is incorrect, since the "neutral" really only applies to a full 240 volt system (two "hots" and the "neutral" half-way between). In an inverter system you'd do well to consider both of the 120vac wires to be "hot", and neither should allow/provide current flow to the (let's say) metal box that holds the inverter electronics. That box will probably have a small stud to connect to a "real" earth ground system. ("system" meaning wires or other metal surfaces).

The "ground" in the AC system is a (hopeful) "bonding" (i.e. good connection) between all the metal parts to a real earthing ground connection.
If you're an RV plugged into shore power, that's what the 3rd (or ground in a 50 amp connection) pin is supposed to provide.

The killer in RV set-ups is when the metal of your vehicle becomes "hot" due to a wire or equipment failure and (especially with shore power involved) you step out on to the wet grass and your body becomes the "hot to earth" conductor.

--dick
My 120 volt AC is grounded to the van steel. My DC is not. DC is a "floating" system that is self contained that just happens to be in a van.

I did use two pole circuit breakers on the AC system.

My understanding is the stud on the aluminum inverter enclosure is provided to ground the case. Have not had anyone explain that there are any connections between the stud and the inverter wiring inside the case. Does anyone know if grounding stud has any function besides grounding the case? Curious.
 

Aqua Puttana

Poly - Thread Finder
Nice overview. :thumbup:


Your neutral definition is a bit off.

...
(in a 120 vac inverter system, even "neutral" is incorrect, since the "neutral" really only applies to a full 240 volt system (two "hots" and the "neutral" half-way between). ...
A neutral is basically a current carrying conductor which is intentionally bonded to ground aka earth. (The official definition can be found in the NEC.) A 120 volt system that has a current carrying conductor which is bonded to ground has a neutral. 480v and 208v wye systems often include a neutral.

vic
 

avanti

2014 GWV Legend 3500 I4
My understanding is the stud on the aluminum inverter enclosure is provided to ground the case. Have not had anyone explain that there are any connections between the stud and the inverter wiring inside the case. Does anyone know if grounding stud has any function besides grounding the case? Curious.
This is what the installation manual of my Outback inverter says:
AC AND DC GROUNDING REQUIREMENTS
  • Connect only to a grounded, permanent wiring system. Ensure there is only one neutral-ground connection in the system at any time. Some codes require this connection be made at the main panel only.
  • Some generators have their own neutral ground connection. If a generator is used, its neutral- ground connection will need to be disengaged for proper system operation.
  • For all installations, the negative battery conductor should be bonded to the grounding system at one (and only one) point in the system.
  • OutBack products are not designed for use in a positive grounded system. Please contact OutBack Technical Support for further information.
 

Graphite Dave

Dave Orton
This is what the installation manual of my Outback inverter says:
The feet of both my aluminum housed inverters are bolted to the 80/20 aluminum structure. The aluminum structure is grounded to the van steel in one location. I did not use the ground the ground terminal of each inverter. What I want to know is if the feet serve the same purpose as the ground lug. That would be true if the ground lug is only attached to the inverter housings without any wire connections inside the inverter housing.

I did ask the tech at Magnum and was told it is a housing ground. Does anyone know for sure?
 

Aqua Puttana

Poly - Thread Finder
...
I did ask the tech at Magnum and was told it is a housing ground. Does anyone know for sure?
:idunno:

If the stud is directly electrically bonded Eg. - welded, cast into, permanently bolted to the equipment metal housing... it is 99.9999% that the function is for electrical bonding aka grounding.

vic
 

Graphite Dave

Dave Orton
:idunno:

If the stud is directly electrically bonded Eg. - welded, cast into, permanently bolted to the equipment metal housing... it is 99.9999% that the function is for electrical bonding aka grounding.

vic
I know it is for grounding. What I want to know is if the feet on the aluminum case provide the same function.
 

autostaretx

Erratic Member
I know it is for grounding. What I want to know is if the feet on the aluminum case provide the same function.
Places that care want a "secure permanent" connection between the case and the earth ground.
Depending upon simple metal-to-metal contact by "feet" doesn't count.
Securely bolting it down to grounded metal via the holes for the feet would usually satisfy the requirements.

In residential house wiring, depending upon a duplex outlet's mounting screws to serve as the "ground" to an otherwise grounded metal box is no longer acceptable... you're still supposed to have a ground wire from the outlet's ground screw.

--dick
 

Graphite Dave

Dave Orton
Places that care want a "secure permanent" connection between the case and the earth ground.
Depending upon simple metal-to-metal contact by "feet" doesn't count.
Securely bolting it down to grounded metal via the holes for the feet would usually satisfy the requirements.

In residential house wiring, depending upon a duplex outlet's mounting screws to serve as the "ground" to an otherwise grounded metal box is no longer acceptable... you're still supposed to have a ground wire from the outlet's ground screw.

--dick
All four cast aluminum feet of each inverter are securely bolted metal to metal to the 80/20 framework. Framework is grounded to the vehicle steel. If the ground terminal is only connected to the aluminum cases then suspect that serves the case ground function.
 

Aqua Puttana

Poly - Thread Finder
All four cast aluminum feet of each inverter are securely bolted metal to metal to the 80/20 framework. Framework is grounded to the vehicle steel. If the ground terminal is only connected to the aluminum cases then suspect that serves the case ground function.
That sounds fine.

vic
 

pltek

New member
so if i am grounding the negative bus bar, would all devices connected to it, consumers (dc fuse box, inverter) as well as b2b charger (perhaps aside for shore power charger) then be properly grounded?
 

autostaretx

Erratic Member
so if i am grounding the negative bus bar, would all devices connected to it, consumers (dc fuse box, inverter) as well as b2b charger (perhaps aside for shore power charger) then be properly grounded?
No. (but, then: what's "proper" in your eyes?)

The inverter's metal box is *not* connected to the DC negative wiring.
If part of the inverter's 120 volts AC section had a spectacular (or simply Murphy's Law) failure, it could make the inverter's metal box "hot".

If the box was not "grounded" (by either the "bolt the feet down" or "dedicated wire" method) then anyone (or anything) touching the box would also now be "hot". (which, inside the typical RV, would probably still be fairly safe).

--dick
 

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