# GFI for high-current 12 VDC systems



## jwarren (Sep 18, 2017)

I'm reposting this here, as the only decent answer I got on the EE stackexchange was "build it yourself," of which I am not capable. Any help is welcome; I'm also open to the answer that no such device exists at all (at least then I'll know to give up!).

https://electronics.stackexchange.c...ion-devices-available-for-high-current-12-vdc

Here is the text of the question:

I have a pickup truck with a 3kw AIMS inverter-charger. It is wired with 4/0 welding cable (for both positive and dedicated neutral back to the alternator), and I installed a block with a 300A fuse in the engine bay as close to the alternator as possible. My concern is that most of the conceivable events causing a short circuit would have a current path with resistance such that the short would be much less than 300A and therefore not blow the fuse.

What is the recommended practice for wiring such systems? I did a search for "DC ground fault protection" but none of the products that came up appeared to be appropriate for my application. If a 300A 12VDC GFCI device was available I would gladly purchase it. Thanks in advance.

Edit: I am talking about a live-to-ground short, not live-to-neutral. Suppose the insulation surrounding the live wire to the inverter (which is routed along the truck's frame) is somehow sliced open and the bare wire makes a weak short against the frame. This is the event I want to be protected from. To be clear, answers pertaining to the physical protection of the live wire are not appropriate here; I have already taken steps to make sure it is as far out of harm's way as possible.

Edit 2: These are the events I foresee possibly occurring as a result of a live-to-ground short (and want to prevent): 1) batteries being drained, 2) batteries exploding due to a short of up to 300A, 3) a fire being started due to the resistive heating from a short of up to 300A through components of the body or frame.

Edit 3: I emphasize that the inverter has a dedicated neutral that connects directly to the alternator housing. Therefore, ALL return current must pass through this wire unless there is a live-to-ground fault somewhere. In such an event there will be a difference in current between the live and neutral wires, which is the condition upon which I want my mystery device to trip and disconnect the live wire.


----------



## Nobr8ks (Sep 30, 2015)

You should try posting this on "Sprinter Source" https://sprinter-source.com/forum/

Sounds similar to my class B Roadtrek setup (Mercedes Sprinter) but it uses 6 volt house batteries. 3k inverter, solar, and an Onan.


----------



## Bondo (Dec 8, 2007)

Ayuh,.... Yer talkin' in tongues,.... There's *No Neutral* in 12v dc systems,...

There's Positive(+), 'n Ground(-),.....

A fuse near the source will protect the cables/ wirin',....

No such thing as a GFI for it,....


----------



## JasperST (Sep 7, 2012)

jwarren said:


> Edit 3: I emphasize that the inverter has a dedicated neutral that connects directly to the alternator housing. Therefore, ALL return current must pass through this wire unless there is a live-to-ground fault somewhere. In such an event there will be a difference in current between the live and neutral wires, which is the condition upon which I want my mystery device to trip and disconnect the live wire.


Your terminology is confusing but disconnecting hot leads are what fuses do. Sounds like you are looking for a device that opens the circuit if there's any drain. Why not use a relay that opens the circuit when you turn off the engine?


----------



## jwarren (Sep 18, 2017)

Bondo said:


> Ayuh,.... Yer talkin' in tongues,.... There's *No Neutral* in 12v dc systems,...
> 
> There's Positive(+), 'n Ground(-),.....
> 
> ...


That's not true. Usually, 12V systems are wired such that the ground is used for all return current. However -- as I stated in my post -- I ran a dedicated neutral wire from the alternator housing to the inverter to eliminate the possibility of any resistance in the body/frame/engine interfaces causing voltage drop and reducing the maximum power I can draw from the inverter. Therefore, since all return current must pass through this neutral wire, in principle it should be possible for the positive and neutral wires to be run through a GFI. That is, if there is *any* difference in current between those wires it indicates that there is a ground fault.

However, based on my discussions with the folks at the EE Stackexchange, it seems that such a device does not exist in off-the-shelf form and any sort of home-baked GFI for this purpose would be either exorbitantly expensive or require actual engineering and fabrication. The former is not acceptable and the latter are outside the scope of my capabilities and inclination. Guess I'll be rolling the dice.


----------



## JasperST (Sep 7, 2012)

The alternator housing would be grounded. Neutral wires are A/C. I don't see where the problem is, cut the juice when the engine is off. Done!


----------



## Bondo (Dec 8, 2007)

> I ran a *dedicated neutral wire *from the alternator housing to the inverter to eliminate the possibility of any resistance in the body/frame/engine interfaces causing voltage drop and reducing the maximum power I can draw from the inverter.


Ayuh,... There is *No Neutral* in 12v dc systems,....

You ran a dedicated _Ground_ directly to the alternator,....

My portable inverted also has a 3rd wire, 'n clip,.... still a _Ground_ wire,....

Gfi is an AC thing, not 12v dc,...


----------



## jwarren (Sep 18, 2017)

Bondo said:


> Ayuh,... There is *No Neutral* in 12v dc systems,....
> 
> You ran a dedicated _Ground_ directly to the alternator,....
> 
> ...


By your logic, there is no neutral in residential 120/240 VAC systems either, since neutral and ground are bonded together at the main panel -- exactly the way my inverter is wired. The fact that the other devices in the vehicle utilize ground for return current is completely irrelevant.

Anyway.

The question is how to protect this system from weak ground faults of relatively low current. Here is an example of how such a fault could occur:

"Edit 4: Here is a concrete example of a situation I want to prevent from which the fuse offers no protection. Imagine that the vehicle is involved in a collision and the body of the truck pierces the 12V wire insulation. The 12V wire will now have a weak (the body of the truck is painted and likely dirty) short against the truck body, resulting in finite current that could quite possibly be way less than 300A but still sufficient to cause a fire due to resistive heating and/or battery explosion (or even just drain the battery and make the truck unusable)."

Do you have a suggestion for how to mitigate the risks associated with such an event?


----------



## JasperST (Sep 7, 2012)

This is bizarre. Grounds and neutrals are not the same thing. Autos don't have neutrals. One could simply disconnect the circuit with a relay and not worry about what if a collision happened and dinged a wire. The EE guys musta had a blast.


----------



## jwarren (Sep 18, 2017)

JasperST said:


> This is bizarre. Grounds and neutrals are not the same thing. Autos don't have neutrals. One could simply disconnect the circuit with a relay and not worry about what if a collision happened and dinged a wire. The EE guys musta had a blast.


Again, vehicles are typically wired such that ground is used for all return current. As I have stated innumerable times, _that is not how this inverter is wired._ It has a "neutral" in the same sense that your house has one, and despite ground being used for other devices' return current, it is not used for _this_ device, and therefore from the inverter's standpoint ground and neutral are distinct.

Back to the question:

How do I protect the vehicle from a fault occurring while the inverter is being used? Here is a specific scenario:

"Edit 4: Here is a concrete example of a situation I want to prevent from which the fuse offers no protection. Imagine that the vehicle is involved in a collision and the body of the truck pierces the 12V wire insulation. The 12V wire will now have a weak (the body of the truck is painted and likely dirty) short against the truck body, resulting in finite current that could quite possibly be way less than 300A but still sufficient to cause a fire due to resistive heating and/or battery explosion (or even just drain the battery and make the truck unusable)."

On that note, (most of) the EEs responded with precisely the same amount of schoolgirl-like giggling until they realized the validity of the concern. It's worthwhile to fully understand what is being asked before responding so dismissively.


----------



## Shadow99 (Nov 3, 2017)

jwarren said:


> Again, vehicles are typically wired such that ground is used for all return current. As I have stated innumerable times, _that is not how this inverter is wired._ It has a "neutral" in the same sense that your house has one, and despite ground being used for other devices' return current, it is not used for _this_ device, and therefore from the inverter's standpoint ground and neutral are distinct.
> 
> Back to the question:
> 
> ...


I laughed at your post too !

So, you want to install a device that can draw up to 300A , yet your worried that it might start a fire or explode your battery :vs_worry:

If you're not going to load it up to 300A, then reduce your fuse size.
And install a relay interlock to the ignition as others said.

p.s. Bondo and others are correct. There is NO neutral on your truck, just a parallel ground for the wire you added.


----------



## jwarren (Sep 18, 2017)

Shadow99 said:


> I laughed at your post too !
> 
> So, you want to install a device that can draw up to 300A , yet your worried that it might start a fire or explode your battery :vs_worry:
> 
> ...


I'd be laughing at yours too, but I happen to find the potentially lethal combination of overconfidence and incompetence tragic rather than comical. Maybe I should lighten up.


----------



## JasperST (Sep 7, 2012)

The school girl responses are all yours. The a/c is on the opposite end of the power conversion, why you keep want to hook the neutral to chassis ground makes no sense. If it did the electronic guys could have helped. You invented a problem that doesn't exist. 

On the DC side it would be handled like any other automotive electric concern.


----------



## JasperST (Sep 7, 2012)

jwarren said:


> I'd be laughing at yours too, but I happen to find the potentially lethal combination of overconfidence and incompetence tragic rather than comical. Maybe I should lighten up.


Why don't you just get a generator?


----------



## Bondo (Dec 8, 2007)

Ayuh,... Some people just don't understand electricity,....

I'm done here,....


----------



## lenaitch (Feb 10, 2014)

I've followed this thread hoping I might learn something new, but doubt I have. I think the OP confused the issue by using the term 'neutral' to describe what sounds to be simply a dedicated DC return path, one that is ultimately connected to the chassis-tied return path, not earth ground. GFCIs, in the normal household sense that most of us consider, are designed to protect us, not equipment, by preventing a fault current from going through us on its way back to an earth-ground tied 'source'. In an automotive situation, a person is only at risk if they are in contact with the grounded side of the circuit, like the chassis.

I did some Googling and it seems as though there are such animals as Direct Current GFCIs but appear to be specialty items for industrial and marine applications. I started reading some sites but quickly realized the issue is above my mental paygrade. Regardless, I doubt such a beast is reasonably available for an automotive application, certainly not at the current being considered. I would think simply turning off the inverter circuit when not in use would be the simplest solution. There might be some kind of isolation equipment available for situations where an inverter is required during vehicle travel - maybe ambulances, but I really have no idea.


----------



## jwarren (Sep 18, 2017)

lenaitch said:


> I've followed this thread hoping I might learn something new, but doubt I have. I think the OP confused the issue by using the term 'neutral' to describe what sounds to be simply a dedicated DC return path, one that is ultimately connected to the chassis-tied return path, not earth ground. GFCIs, in the normal household sense that most of us consider, are designed to protect us, not equipment, by preventing a fault current from going through us on its way back to an earth-ground tied 'source'. In an automotive situation, a person is only at risk if they are in contact with the grounded side of the circuit, like the chassis.
> 
> I did some Googling and it seems as though there are such animals as Direct Current GFCIs but appear to be specialty items for industrial and marine applications. I started reading some sites but quickly realized the issue is above my mental paygrade. Regardless, I doubt such a beast is reasonably available for an automotive application, certainly not at the current being considered. I would think simply turning off the inverter circuit when not in use would be the simplest solution. There might be some kind of isolation equipment available for situations where an inverter is required during vehicle travel - maybe ambulances, but I really have no idea.


Well -- strictly speaking -- a neutral _is_ just a dedicated current return path. That's really all the neutral line in residential electrical systems is; a current return path that's insulated and kept separate from anything people are likely to come into contact with. The idea is that if there was any resistance (or a loose connection) downstream of the load on the neutral line, the portion of the line upstream of that resistance would be at nonzero potential and pose a shock hazard. If residential systems used ground for return current, a loose ground connection downstream of the load would result in exposed hardware having nonzero potential, which is unacceptable. At 120V that's a major electrocution hazard; at 12V it hardly matters, hence vehicles' use of ground for return current. In my case, the dedicated neutral isn't for shock hazard prevention, but rather for resistance reduction.

Regarding GFIs, I know that in 120V land they are used to protect people from current less than the fuse/breaker rating finding a return path through them. But since what they _do_ is cut power to the load whenever a difference in supply and return current is detected, such a device _in principle_ could be used to protect a system such as mine from a situation in which the 12V cable makes a weak (sub-fuse-rating) short / ground fault against components of the body or frame, because all return current runs through the dedicated neutral. Thus, if there were any difference in current between the 12V and neutral wires, there must be a ground fault.

My understanding at this point is that no suitable device exists and that this is just a hazard I have to accept. Thanks anyway for a thoughtful response.


----------



## JasperST (Sep 7, 2012)

jwarren said:


> Well -- strictly speaking -- a neutral _is_ just a dedicated current return path. That's really all the neutral line in residential electrical systems is; a current return path that's insulated and kept separate from anything people are likely to come into contact with.


You don't know how alternating current works.


----------



## jwarren (Sep 18, 2017)

JasperST said:


> You don't know how alternating current works.


Thank you for your helpful and informative post. I must also commend your brevity.

Edit: I could have gone on about how the current doesn't really have "direction" in AC systems, so the concept of "return current" doesn't really apply, and instead referred to the neutral as a "dedicated zero potential" connection. But I don't think that would have been helpful. If residential systems used 120 VDC I am pretty sure they'd be configured similarly to how they are now. 120 VDC and 120 VAC RMS are both dangerous.


----------



## JasperST (Sep 7, 2012)

jwarren said:


> Thank you for your helpful and informative post. I must also commend your brevity.


It was to the point. I'm not going to explain a/c when there must be a million places you could go.


----------



## jwarren (Sep 18, 2017)

JasperST said:


> It was to the point. I'm not going to explain a/c when there must be a million places you could go.


A reasonable position; thank you for sparing me such a diatribe. Especially in light of the difficulty you faced with regard to such esoteric concepts as DC return current paths and short circuits through high-resistance interfaces.


----------



## JasperST (Sep 7, 2012)

jwarren said:


> A reasonable position; thank you for sparing me such a diatribe. Especially in light of the difficulty you faced with regard to such esoteric concepts as DC return current paths and short circuits through high-resistance interfaces.


spare me the passive aggressive routine. I was an electrician in the military and went to school for four months and couldn't put what I remember in a post. The bottom line is that a/c is completing a circuit with a neutral, electrons go both ways. 60 times a second in this country. That isn't how d/c works, that battle happened long ago for the electrical grid.

But why not just get a generator? That's what most people would do.


----------



## Brainbucket (Mar 30, 2015)

DC is automobiles. AC is house or business. Now the only AC component on a vehicle is the alternator but it is converted to DC before it come out of the alternator. By comparing 120/240 to auto electric systems is apples to oranges. I can't get what your wanting to do. I haven't had my first cup of coffee yet. I did residence and industrial electrical work as a journeyman but when I seen 40,000 volts hit a guy, I went back to being a mechanic. Now the DIS (Distributorless Ignition Systems) have more voltage than that and they carrie amps which is what kills you. If you are wanting a 300 amp source, your gonna have to install either a giant alternator or multiple alternators like the fellas do on these car stereo systems. I seen a Hummer with 6 300 amp alternators. May I ask why you want a GFI= ground fault interrupter? By the way AC has a hot, neutral, and a ground. DC has hot and ground.:vs_cool:


----------



## jwarren (Sep 18, 2017)

JasperST said:


> spare me the passive aggressive routine. I was an electrician in the military and went to school for four months and couldn't put what I remember in a post. The bottom line is that a/c is completing a circuit with a neutral, electrons go both ways. 60 times a second in this country. That isn't how d/c works, that battle happened long ago for the electrical grid.
> 
> But why not just get a generator? That's what most people would do.


Look man, you fired the first shot by saying that you _laughed_ at my post, which was an honest and good-faith attempt to get some input for a project that I've put quite a bit of work into. At that point you gave up any claim to civility, so my _aggressive-aggressive_ replies were totally reasonable given the tone of this conversation. If you want to elevate this dialogue back to something useful that would be great, but don't act like I'm victimizing you. To use a tired idiom: it takes two to tango.

Anyway.

I was (am) also in the military (Army), but I'm in the infantry and was previously an 88M (truck driver). My only formal training in anything electricity-related is a single class in electrical engineering as an undergraduate.

As for the generator idea, yeah, that could also work, but there are a few applications for which it wouldn't be practical. For example, I'd like to be able to provide climate control to the truck bed (with a cap installed) so my pets can ride back there on long trips if the cab is full of humans. I can easily drive a 10k BTU/hr air conditioner with the inverter, as well as a space heater and heating pads. A generator that can do that would be a 150 lb. chunk of crap that I can't easily haul in and out of the truck bed by myself, or I'd need to get a set of those parallel 2k generators and connect them together. Either way it's a lot more noise and headache (plus refueling annoyance) than I want to deal with. 

The other reason is that I wanted to get familiar with inverter-charges and dealing with such huge amounts of DC current because there's another project (http://www.diychatroom.com/f18/converting-120vac-line-ground-split-phase-240vac-545698/) that I want to undertake eventually and it will require an even larger inverter-charger and a rather hefty bank of batteries. We're talking about $5k worth of stuff and I want to make sure I know what I'm doing before I start piecing it together. This pickup truck project wasn't cheap but it was much less of a commitment than the RV project, and I have an immediate use for it.


----------



## Brainbucket (Mar 30, 2015)

Motor home systems would probably be your best bet on solving your problem and the cheapest. Just a thought.:vs_cool:


----------



## JasperST (Sep 7, 2012)

jwarren said:


> Look man, you fired the first shot by saying that you _laughed_ at my post,


That's a lie. Go find the post. You're a drama queen, maybe trolling.


----------



## jwarren (Sep 18, 2017)

JasperST said:


> That's a lie. Go find the post. You're a drama queen, maybe trolling.


I apologize; I confused your reply with that of Shadow99. My bad. What you said was:



JasperST said:


> This is bizarre. Grounds and neutrals are not the same thing. Autos don't have neutrals. One could simply disconnect the circuit with a relay and not worry about what if a collision happened and dinged a wire. The EE guys musta had a blast.


The last part of which is equally disrespectful. 

I'm perfectly capable of throwing rocks if that's what you want, but be a man and admit it: I asked an honest question and you responded with dismissive and derisive bull****. Maybe then we can both move on to a higher plane of existence.


----------



## JasperST (Sep 7, 2012)

jwarren said:


> I apologize; I confused your reply with that of Shadow99. My bad. What you said was:
> 
> 
> 
> ...


You lied about me and want to make it my fault?

Yep, you're a drama queen!


----------



## jwarren (Sep 18, 2017)

JasperST said:


> You lied about me...


I mistook someone else's reply for yours, as I pointed out (and apologized for). Why don't you calm down about all this? None of this is worth getting angry over.

In case there's any question of my trolling, here's proof that I'm not:

https://photos.app.goo.gl/yp9G1YgjmxqWcQ0j1


----------



## JasperST (Sep 7, 2012)

jwarren said:


> I mistook someone else's reply for yours, as I pointed out (and apologized for). Why don't you calm down about all this? None of this is worth getting angry over.
> 
> In case there's any question of my trolling, here's proof that I'm not:


Now you're lying and saying I'm not calm. See above.

unsubscribed


----------



## Brainbucket (Mar 30, 2015)

I thought about it, kinda slow here, and an alternator/starter shop would be your best bet as they do this for other applications on all kinds of vehicles.:vs_cool:


----------



## jwarren (Sep 18, 2017)

Brainbucket said:


> I thought about it, kinda slow here, and an alternator/starter shop would be your best bet as they do this for other applications on all kinds of vehicles.:vs_cool:


Thanks. I guess I'll also see if any of the car audio places have a way of dealing with this, but at this point I'm pretty sure it's just a risk I have to accept. I've got the right size fuse and wire loom around all the positive cabling so it would take a pretty significant event for something to cut through all that and cause it to short against the frame or body.


----------



## Nobr8ks (Sep 30, 2015)

jwarren said:


> Thanks. I guess I'll also see if any of the car audio places have a way of dealing with this, but at this point I'm pretty sure it's just a risk I have to accept. I've got the right size fuse and wire loom around all the positive cabling so it would take a pretty significant event for something to cut through all that and cause it to short against the frame or body.


If that Sprinter site doesn't pan out, try "Expedition Portal" too.

http://forum.expeditionportal.com/index.php


----------



## jwarren (Sep 18, 2017)

Nobr8ks said:


> If that Sprinter site doesn't pan out, try "Expedition Portal" too.
> 
> http://forum.expeditionportal.com/index.php


Looks like a really cool site. Thanks!


----------

