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Inverter case grounded to chassis

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5.6K views 5 replies 5 participants last post by  Roach711  
#1 ·
This has been discussed many times all over the internet, and never has any good reason been put forth. So I want to walk this through and ask you all what the reason is, and whether my reasoning is correct.

My reasoning is that it is assumed that one will hardwire the AC output including grounding the chassis to the AC ground for safety, and assuming the inverter might be bolted directly to metal.

An inverter usually has neutral and ground internally bonded, and you don't want another bond anywhere. So the manufacturer can't ground the case internally or else you would get a neutral-ground loop (internal ground-case-chassis-output ground). Therefore, the only way to protect against a short to the inverter case would be an external ground to chassis, which is in turn grounded to the inverter output.

Does this line of reasoning make sense? If one uses an extension cord and doesn't ground the chassis to AC output, then it seems that grounding the inverter case to the chassis accomplishes nothing.
 
#2 ·
Mike Holt runs a no shock zone forum that has quite extensive discussions about this topic.

In general there are two approaches, each correct if done 100% one way or the other.

The weak link is that not all inverters are identical and it also depends on if an inverter is a combo inverter / charger or just an inverter.

I don't have the energy to go into a deep discussion on this topic. :)
 
#3 ·
I'm afraid the logic is above my pay grade, but here's the relevant section from my inverter-charger. It sounds like it's needed regardless of how the AC side is grounded. Elsewhere it calls for 6AWG for RVs. Following their instructions, I connected it to my main negative bus bar, which is grounded to the van chassis.
Image
 
#4 ·
Hi,
I'm not understanding your 2n paragraph. If you ground the inverter case to the RV chassis, how is this creating two places where the AC neutral is bonded to ground? It seems to me that the AC neutral is bonded to ground in the inverter (and only there), and the inverter case is grounded to the chassis - how does that create two neutral to ground bonding points?

But, the DC side of the inverter also has grounding requirements.
If you think about the DC side of the inverter grounding requirements, the inverter case needs to be grounded to the chassis for the case of the 12 volt DC faulting to the inverter case - this has to be done no matter what is happening on the AC side. According to the Samlex inverter manual, the wire used to bond the inverter case to the chassis should be large enough to blow the inverter fuse - quite a large conductor for most inverters.

From the Samlex manual - note that they also provide links to two white papers on grounding, which should make for some great bedtime reading.

The excerpt just below is from the Samplex 1212F inverter/charger manual: https://samlexamerica.com/wp-content/uploads/2021/01/11027-EVO-1212F-1224F-0220_Hrez.pdf

See the sentence I bolded ...
"
3.11 GROUNDING TO EARTH OR TO OTHER DESIGNATED GROUND
i
INFO
Please read following on-line White Papers for complete understanding of Grounding at www.
samlexamerica.com (Home > Support > White Papers):
• “Grounded Electrical Power Distribution"
• “Grounding System and Lightning / Ground Fault Protection”
SECTION 3 | Installation
SAMLEX AMERICA INC. | 57
Grounding means connecting (bonding) to Earth Ground or to the other designated Ground. For example, in a
motorhome / caravan, the metal frame of the motorhome / caravan is normally designated as the Negative DC Ground
/ RV Ground. Similarly, all metal portions of boats and marine craft are bonded together and called Boat Ground.
Grounding is required for (i) protection against damage due to lightning strike and (ii) protection against electric shock
due to “Ground Fault”. In case of EVO™, “Ground Fault” may occur due to inadvertent contact between an energized
ungrounded current carrying conductor and exposed metal surface resulting in voltage getting fed to (i) the metal
chassis of the EVO™ or (ii) to the metal chassis of the devices connected to EVO™ or (iii) to the metal frame/ chassis
in an RV / motorhome / caravan. When this energized exposed surface is touched, the voltage will drive current
through the body to Earth Ground producing electric shock. When properly grounded to Earth Ground (or Frame /
Chassis Ground in motorhome or caravan), the Leakage Current Protection Device (like RCD, GFCI etc.) or Over Current
Protection Device (like Circuit Breaker or Fuse) will trip and interrupt the circuit feeding power from the AC source
(EVO™ / AC Input) or the DC source (12V / 24V battery). Proper grounding will ensure that all exposed metal surfaces
will have equal potential and will be bonded to (i) a single common Earth Ground point i.e. the Ground Rod / buried
metallic water / gas pipe at the premises or (ii) the Frame / Chassis Ground in a motorhome / caravan.
3.12 GROUNDING ARRANGEMENT
Internally, EVO™ consists of DC and AC Section that are isolated through a transformer (See these sections in Figs 3.12
and 3.13). Both these sections are required to be grounded appropriately.
For wiring details for appropriate grounding, refer to Figs 3.12 and 3.13, and associated explanation under Section 3.13
and 3.14.
When using a generator instead of Grid, please ensure that the Neutral of the generator is bonded to the metal frame
of the generator and the metal frame of the generator is bonded to Earth Ground through the Grounding Electrode
(GE) i.e. the Ground Rod. Refer to Section 3.14.1 for additional details.
3.13 DC SIDE GROUNDING
Please refer to Figs 3.12 and 3.13.
DC side grounding involves bonding of the metal frame/chassis of EVO™, the metal chassis of the DC Electrical Panel
and the Battery Negative Terminal to Earth Ground in shore based installation (Fig 3.12) or to the metal frame / “Chassis
” of the motorhome / caravan (Fig 3.13). This ensures that in case of a ground fault in the +12V / +24V circuit, the
fuse in the +Battery line blows to clear the fault
. This fuse in the +Battery line has Ampere capacity matching the rated
DC input current of the EVO™ in Inverter Mode. The wire size used for DC side grounding should be minimum AWG
#6 or of the same size as the battery cable, whichever is thicker (Battery cable size should have minimum Ampacity ≥
the Ampere rating of this battery fuse depending upon the model of the EVO™ being used). This recommendation
on sizing of the DC Side Grounding Wire will be superseded by the National / Local Electrical Codes."

It goes on after this to talk about grounding the AC side of the inverter.

I do agree its a very confusing subject.

Gary
 
#5 ·
If the inverter case is grounded to DC ground, and also bonded to AC neutral which is bonded to AC ground, would that not mean that the vehicle chassis is therefore grounded?

Yet in my Renogy inverter, the vehicle chassis was clearly not grounded until I connected AC ground to DC ground. Yet the instructions say nothing about grounding vehicle chassis to AC ground, nor whether the inverter case is bonded to AC neutral.

It appears very clear in that Samlex manual that grounding of the inverter case is for DC protection. Yet if you don't do this and don't ground vehicle chassis to AC ground either, than an AC fault to the inverter case would be an AC danger, not a DC danger. But if the inverter case is grounded to DC ground and ALSO vehicle chassis is grounded to AC ground, then that would form a loop (it seems). So either way, in that case, grounding the inverter case is an AC protection as well as DC protection, no?

I never considered grounding metal for DC faults. I assumed that grounding the case was for AC faults. So I probably have my ground wire significantly undersized. I think I used 12awg assuming it's an AC wire. That's not large enough to blow my inverter fuse.
 
#6 ·
"Ground" is a different concept in AC and DC circuits.

DC circuits have a hot and a ground conductor (more accurately called a positive and negative conductor) while AC circuits have hot, neutral conductors and a ground wire. This is confusing because the return wire in a DC circuit is called a ground wire but in an AC circuit the return is the neutral. The ground wire in AC circuits is only a safety feature and will only carry current when there is a short in the circuit. The fact that DC grounds carry current and AC grounds don't explains why you can (and should) ground your AC system to the body.

Without an AC ground to the steel body the body can become a 120 volt conductor if a loose AC hot wire were to come in contact with the steel body (search on "RV hot skin"). The ground wire enables the AC breaker to sense the short and trip the breaker. With no AC ground connection a short won't trip the breaker and anyone outside touching the bus could get a nasty shock if they were standing on wet dirt or pavement.