Electric system feedback and grounding questions

[farok]

I'm looking for advise before I wrap up my rudimentary "phase 1" electrical system in the attachment. Please see my diagram for the short term setup - any comments? One obvious omission is grounding to the chassis/frame... see below.

I've read a number of posts here that say the AC circuit should be grounded to the frame of the bus to prevent "hot skin" condition. This makes sense, since any such condition would then be grounded, and should trip a breaker. However, I've seen a few other posts (http://www.skoolie.net/forums/f10/mi...html#post81574 is one example) that indicate a completely floating AC system works as well, I assume by "making sure" there's no way for the AC wires to contact the bus frame. While I don't intend to leave any possible way to energize the skin or frame, I don't see why not grounding is any safer, and plan to run a ground wire from the power center ground bus bar to the frame. Any reasons I shouldn't just ground to the frame?

As for DC grounding to the frame, I'm less clear on this. The house system is isolated from the starting system, and at some point I'd like to install a DC-DC charger or an isolator solenoid for the house battery to charge while driving. Anyway, it sounds like it's still a good practice to connect my negative bus bar to the frame as a chassis ground, just in case something like an antenna or some other item is installed which could make its own ground to the frame otherwise. Again, any reason not to run a ground wire from my negative bus bar (or from the negative battery post) to the vehicle chassis?

I may have missed it, but I did try to read through the stickies in this forum to find answers, but didn't get much beyond that already noted above.

Thanks!
Chris

[Jsneeb]

Ground the negative of your DC system.

This will allow you to avoid running two strands of large gauge cable to your devices or to your inverter.

Also grounding the DC system will avoid "ground loops" forming, like what you described with an antenna. If you have a short to the chasis or bus body from the +12 ( or 24V ) of your DC system, and the main negative bus bar is not grounded, then the current will find the next possible path to complete the circuit. This could result in damaged devices, or increase the risk of a fire because cables not intended for high current will all of a sudden be carrying it.

As for the AC system. IDK. Mine is ungrounded to the body for what it's worth. I plan to run dual inverters in a split phase configuration and may ground the center neutral to the body at that time. Currently with just single phase 110AC I have not done this.

[farok]

Thanks! In doing some more digging outside of skoolie.net, the following brief article by Mike Sokol is enough justification for me to run a ground wire from the power center to the frame:
https://www.rvtravel.com/proper-rv-c...kin-condition/

As for the DC ground, thanks for the confirmation! Is there any real difference between running a ground from the negative bus bar as opposed to straight from the battery's negative post? The negative post is much closer to the frame and should be easier to attach, and should prevent the shunt from taking a large spike should a ground out occur. Then again, it's a 100-amp shunt, and I don't plan on running more than 40 amps through it for now.

Any other comments or concerns with my diagram?

Chris

[family wagon]

Mike Sokol does a great job explaining the practice, the theory, and the rationale for electrical safety techniques.


As to the DC grounding: the purpose of the shunt is to measure the current flowing into or out of the battery. If there's any path for current to flow around the shunt then the accuracy of the battery monitor will be compromised (it'll quickly become useless). Probably the right thing to do is wire from battery post to shunt, then from shunt to frame. Make sure the shunt-to-frame connection is good enough for the current it'll have to carry. The negative bus bar connects somewhere on the same node as the shunt-to-frame connection. Whether you have two cables from the shunt (one to the bus bar and one to the frame), or whether the path is shunt to bus bar, then bus bar to frame, is mostly a matter of convenience and aesthetics.



The shunt is just a resistor. It is rated for a specific amount of continuous current at some particular temperature, but it can handle much higher currents for shorter times (ie the time it takes for a fuse to blow) without damage.


It wouldn't be a bad idea to have a main fuse of 100A or whatever in series with the battery connections.

[farok]

Thank you very much for the feedback! Good to know about the shunt and what it will take and why placement is so important. I'll wire the ground from the negative bus bar.

Two follow-up questions...

1. I'd like to poke only one hole in the floor of the bus. Any concerns with running the battery connections (positive and negative) as well as the AC grounding wire in one conduit? The AC ground isn't expected to have any power in it, all things being normal, and is just a safety, so I would assume this is fine?

2. If I have a 40-amp circuit breaker directly off the positive post of the battery, as that's the most the system is designed to pull -- the only thing connected is the power center -- is there a need to also have a fuse? If I were to add other appliances other than the power center, I can see needing something, but question the necessity in the current configuration.

Thanks again for the help on this!

Chris

[Jsneeb]

1: Should be fine to run both in one conduit.

2: I would add a 100 amp main fuse right on the battery, then your 40 amp breaker. That way the breaker, would be protected from ever seeing anything over 100 amps.
Others can chime in, keep in mind if you are using a thermal breaker ( you probably are ) that in warm conditions it will trip at lower amperage. So if you anticipate 40A max load, size the breaker for 50A. Also make sure that any wiring post breaker is rated for 40A ( or 50A if you choose to do what I said above ) and that anywhere you have a junction box or splice from the main wire to smaller gauge you fuse that section of wire for the appropriate rating of the wire.

[PNW_Steve]

Originally Posted by farok

Thank you very much for the feedback! Good to know about the shunt and what it will take and why placement is so important. I'll wire the ground from the negative bus bar.

Two follow-up questions...

1. I'd like to poke only one hole in the floor of the bus. Any concerns with running the battery connections (positive and negative) as well as the AC grounding wire in one conduit? The AC ground isn't expected to have any power in it, all things being normal, and is just a safety, so I would assume this is fine?

2. If I have a 40-amp circuit breaker directly off the positive post of the battery, as that's the most the system is designed to pull -- the only thing connected is the power center -- is there a need to also have a fuse? If I were to add other appliances other than the power center, I can see needing something, but question the necessity in the current configuration.

Thanks again for the help on this!

Chris

Be careful as to who you take electrical advice from. Bad advice can be fatal.

As a former electrician, I would not recommend running your low voltage DC in the same conduit as 120v AC.

I would also recommend that you look at allowable conduit fill.

Good luck.

[farok]

Originally Posted by PNW_Steve

Be careful as to who you take electrical advice from. Bad advice can be fatal.

As a former electrician, I would not recommend running your low voltage DC in the same conduit as 120v AC.

I would also recommend that you look at allowable conduit fill.

Good luck.

Thanks for the note. I've looked at conduit fill already, and am well within limits there.

I know AC with DC in the same conduit is bad for a number of reasons, but wasn't sure if the AC ground wire made a difference, hence the question (I hadn't seen anything on the subject). If you believe it's a bad idea, I can definitely run it separately, and will plan to do so. That's why I'm checking!

Thanks,
Chris

[farok]

Originally Posted by Jsneeb

2: I would add a 100 amp main fuse right on the battery, then your 40 amp breaker. That way the breaker, would be protected from ever seeing anything over 100 amps.
Others can chime in, keep in mind if you are using a thermal breaker ( you probably are ) that in warm conditions it will trip at lower amperage. So if you anticipate 40A max load, size the breaker for 50A. Also make sure that any wiring post breaker is rated for 40A ( or 50A if you choose to do what I said above ) and that anywhere you have a junction box or splice from the main wire to smaller gauge you fuse that section of wire for the appropriate rating of the wire.

Thanks. It's a thermal breaker, and I hadn't considered temperature fluctuations. I may elect to put in a 60-amp fuse as a backup precaution instead of a 100-amp fuse, as that's what the wire is good for at 3% loss given the temp rating and distance I have to run it. I oversized a little, but not too much.

Chris

[BlackJohn]

Think of your setup electrically as if you are building it for resale purposes, which will eventually happen. Nobody is going to buy anything thrown together haphazardly that can't be easily traced and understood. They would be a fool not to understand what they are buying from that viewpoint so do it right and ask questions as you are.
Not following your a ground conversation at all. Just what part of the system are you grounding? You do know that grounding is provided in the shore power able connection, do you not? That is all you need. Pics might help of what you have or intend cuz words an be confusing in this case. Perhaps you are grounding the ac panel enclosure itself?

This is not as clear as mud to me, your intentions thus far.


John

[kazetsukai]

Is there a point to binding ground to chassis without GFCIs?



I already have DC side negative bound to the chassis.

[BlackJohn]

Originally Posted by kazetsukai

Is there a point to binding ground to chassis without GFCIs?



I already have DC side negative bound to the chassis.

All of your ac grounding of receptacles, boxes, gfci's, light fixtures etc should be interconneted all the way back to the ac panel ground bus. Continuity everywhere if connections are made correctly.
I would keep ac and dc grounding totally separate. Why? An a ground spike could do damage a lot of electronics, cause a fire, battery explosion possibly.
This is an area that terrifies me about our bus building, people don't respect electricity and some will learn the hard way I'm afraid.


John

ps, kaz, your work is impressive,

[farok]

Hopefully this helps. This is what I've installed so far. On one side of the wall is the front of the power center, and the main battery disconnect/circuit breaker.

The first circuit breaker is the main AC breaker. The second is to run the power center's converter (all wiring remains internal to the power center), and the third is for an outlet I've temporarily wired in the bus. I can always add duplex breakers down the road if I decide I want one or two more AC lines run from shore power.

On the DC side, the first fuse is the battery reverse polarity fuse. The second is lights and the third is the roof fan.


On the back, you have the battery positive lead from the power center (one of the red wires) running to the circuit breaker. I've yet to install the battery, but plan to run a wire from the circuit breaker to a 60-amp ANL fuse and then to the battery positive terminal. The white wire is the negative lead, which is currently connected to a bus bar. From there, I plan to put the battery monitor shunt, after which I'll run to the battery negative terminal.

The other six wires (one red and six black) are the leads for my DC loads. I've run them to a terminal block to make attaching the wiring easier.

The two DC circuits are, as mentioned, the temporary lighting and roof vent fan.

On the AC side, the 3-conductor wire through the wall is my 5-15P shore power connection (oversized to 10 gauge), which lands inside the AC side of the power center. The second 3-conductor wire is connected to the last circuit breaker as noted, and runs to a temporary outlet box.

Inside the power center, the hot wires each go through a circuit breaker. All neutrals are bonded, and all grounds are bonded, but the two are NOT bonded together.


My AC circuits are all "floating" (as I understand the term), and are grounded through the shore power connection. However, from Mike Sokol's articles, the way to avoid the dreaded "hot skin" condition is to connect a ground wire from the ground bus bar inside the AC side of the power converter to the bus's frame, and so I plan to do this.

On the DC side, my system is again "floating", not connected to the bus frame or the existing DC system. However, as others have pointed out, it sounds like a good idea to run a ground wire from the negative bus bar also to the bus frame, and so I'm planning to do this as well.

Therefore, I still need to run the DC positive and negative from what's pictured down through the floor and into the battery compartment. I also need to run a DC ground wire from the bus bar to the frame. Thirdly, I need to run an AC ground wire from the inside of the power center to the frame. I believe that will complete my rudimentary power system for now.

Comments? Is it hopefully more clear? Am I still missing anything major? Here's a marked up copy with the missing components (and existing components) labeled.


Chris

[farok]

Originally Posted by BlackJohn

I would keep ac and dc grounding totally separate. Why? An a ground spike could do damage a lot of electronics, cause a fire, battery explosion possibly.

Interesting. Maybe it doesn't make sense to run a ground wire from the DC system to the bus frame and instead keep it "floating"? The hot skin condition I read about really makes me at least want to ground the AC side just in the off chance that it gets energized by something, anything, at some point down the road. It's not like I plan to do anything that would bond the DC side to the frame, and I plan to run two wires to all my DC appliances rather than use the frame for the return/negative connection.

Nothing is energized yet (other than was for a short test of the DC appliances), so now's the time for me to get it right!

Chris

[family wagon]

Originally Posted by kazetsukai

Is there a point to binding ground to chassis without GFCIs?

Having the ground connected to the chassis, and a ground-neutral bond in the upstream shore power service or in the generator/inverter, provides a fault path. If a hot wire should happen to contact the chassis then the ground/neutral connections provide a path with high current capability, and this results in a circuit breaker tripping. That prevents the chassis from remaining energized.


GFCIs look for ac current that is returning to its source by any path other than the neutral wire. In some sense the ground/neutral connections to the chassis actually enable the kind of fault that GFCIs protect against -- but it's still a net improvement to safety to be able to trip a breaker and also to have GFCI protection.

Originally Posted by BlackJohn

I would keep ac and dc grounding totally separate. Why? An a ground spike could do damage a lot of electronics, cause a fire, battery explosion possibly.

In some sense this is true. For example, if the ac and dc grounding is tied together and an ac hot wire happens to hit one of the dc circuits, fireworks will ensue. Some electronics might be damaged, but if breakers/fusing are done right, the damage shouldn't be any more severe than that. Had the ac and dc grounds not been interconnected maybe nothing would have happened -- but that is its own kind of danger. With electricity people often assume that if "nothing happened" then "all is well." Then the same fault - an ac hot wire touching dc positive - could endure indefinitely. I suggest that this condition is even more dangerous, and that if bonding ac and dc grounding can cause fuses to blow or breakers to trip and call attention to the problem, then that's a good thing.


In addition to that, maintaining ac and dc systems truly isolated is actually hard. There are many devices that span both systems such as TVs, laptops. They're ac powered but have lots of connectors on their low voltage/dc side. Some have a metal housing/chassis that is connected to ac ground or dc negative. It's not uncommon for the shell of a USB connector to have an electrical path back to the ground pin of the ac power cord, for instance. Thus there's likely to be many connections between ac ground and dc ground through various equipment. I'd much rather have the fault path described above go through intentional wiring connections than have it go through my laptop's power inlet and USB, for example!

[farok]

Originally Posted by family wagon

GFCIs look for ac current that is returning to its source by any path other than the neutral wire. In some sense the ground/neutral connections to the chassis actually enable the kind of fault that GFCIs protect against -- but it's still a net improvement to safety to be able to trip a breaker and also to have GFCI protection.

Thanks for the comments! My outlets are GFCI, and I wish my power center supported arc-fault breakers, but it's too cramped.

Originally Posted by family wagon

In some sense this is true. For example, if the ac and dc grounding is tied together and an ac hot wire happens to hit one of the dc circuits, fireworks will ensue. Some electronics might be damaged, but if breakers/fusing are done right, the damage shouldn't be any more severe than that. Had the ac and dc grounds not been interconnected maybe nothing would have happened -- but that is its own kind of danger. With electricity people often assume that if "nothing happened" then "all is well." Then the same fault - an ac hot wire touching dc positive - could endure indefinitely. I suggest that this condition is even more dangerous, and that if bonding ac and dc grounding can cause fuses to blow or breakers to trip and call attention to the problem, then that's a good thing.


In addition to that, maintaining ac and dc systems truly isolated is actually hard. There are many devices that span both systems such as TVs, laptops. They're ac powered but have lots of connectors on their low voltage/dc side. Some have a metal housing/chassis that is connected to ac ground or dc negative. It's not uncommon for the shell of a USB connector to have an electrical path back to the ground pin of the ac power cord, for instance. Thus there's likely to be many connections between ac ground and dc ground through various equipment. I'd much rather have the fault path described above go through intentional wiring connections than have it go through my laptop's power inlet and USB, for example!

Good points. My AC side is grounded to the bus chassis. I am still debating grounding the DC side from the bus bar to the chassis, and it sounds like it's a good idea just in case something from the AC side causes an issue. That said, the only things I plan to have on the AC side are some kitchen appliances (and maybe a couple extra lights) for the time being. Anyway, much to think about, and much appreciated!

Chris

[wireguy]

For starters, exactly what FAMILY WAGON said.
I'm a multi licensed master electrician with 35 years of electrical service work behind me. That means I worked on every conceivable system under every conceivable circumstance. Because I performed diagnose and repair as a routine part of my work I had to know how these different kinds of systems actually work. I had to be able to see them in my mind.

That said, I don't think you can go wrong looking at a bus as a mobile home. The code lays out the service parameters for mobile homes, which are simply metal skinned and/or framed buildings. The basic service requirements are the same for mobile homes and metal framed/skinned buildings, which is that any metal parts must be grounded. The idea that you are going to wire in such a way that it is impossible for a ground fault to occur isn't realistic. Ground faults happen in buildings that are just sitting there, and you think you can't have a ground fault in a "building" that is jiggling and jostling down the road? Why not just ground the alternating current system to the frame and skin of your bus and then if you get a ground fault there will be a low resistance conductor path back to ground that is for all practical purposes a short circuit and will trip a breaker or at least make it far less likely that metal housing will be hotted up enough to create ground path through the comparably high resistance of a human body?
As for the 12 volt DC system, the AC and DC systems have no interest in each other because they derive from entirely different sources. The electrons flowing in the DC system have no "potential" to the AC system, and vice versa, so having the two systems grounded to the frame doesn't create any kind of conflict. The electrons, or current, flowing in either system only has potential back to that system so something has to be very wrong for either system to "see" the other system and get crossed up with it.
Keep in mind for you people who have electronics built into the bus that it is critical all your connections be solid so things like 120 volt AC doesn't see your electronics as an alternative conductor path.

[Native]

Well put! I like your description very much. I am an electrical engineer and to me, you and Family Wagon did the topic justice! God going , men!

[farok]

Yes, thanks all for the feedback! This has been tremendously helpful! I've been waiting to finish the wiring until I have some good detail from folks much smarter than myself, which I now have. Skoolie.net expertise comes through again!

I will be grounding both circuits to the chassis, and already have circuit breakers and fuses on all AC and DC circuits, respectively. All my AC outlets are going through an initial GFCI outlet to provide that extra protection as well.

Chris