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NOWHERE13 10-12-2021 07:00 PM

Often times we confuse "grounding" with "bonding". Check out Article 551.56 a-d

Rwnielsen 10-12-2021 08:53 PM

Quote:

Originally Posted by NOWHERE13 (Post 455846)
Hello Rwnielsen, check out Article 551.56 A-D.

I did, what's your point?

Rucker 10-12-2021 09:25 PM

Quote:

Originally Posted by Rwnielsen (Post 455843)
Yes
Single point ground, at the source, either generator, shore power, wind, solar.... The NEC does not allow multiple grounding. I've tried to explain, to deaf ears, too many times to get involved.

Either I'm misunderstanding your answer here or we disagree completely on what's correct.

In this context I'm pretty sure when you say ground, you mean in a more technical sense 'common reference voltage', where the neutral wire and the ground wire are connected together, or bonded. Let me know if I've got that wrong. And although I don't have access to the references Nowhere13 mentions I'm guessing it states pretty much what you and I understand: nowhere in the rig should the neutral (typically white) wire and the green or exposed ground wire be connected together. The only place where those two wires should have a normal connection is all the way back to the panel, where the neutral wire and ground wire are bonded and there's a rod stuck in the ground. If using a generator, the neutral and ground MAY be bonded at the generator or not, but that is the only place where they should be connected, and nowhere in the rig should the neutral and green wire connect together.

There. Got that out of the way.

None of this though relates to grounding of devices, nor to connecting the ground wire of an AC circuit to the chassis of the bus. An AC device ground is essential for providing a path for current under fault conditions and, if not connected, poses a huge risk to you.
All AC devices in your rig should have a grounding plug, and that is the key to not getting electrocuted where a fault condition exists with the device. If there is a short circuit in the device and you touch a conductive part of that ungrounded device it you will get a zap somewhere between a tickle and a fatal to the degree that you conduct to ground. Very bad scenario.

If that device is grounded, you will only get a zap to the degree you form a better circuit to ground than the ground wire does. Much better scenario for you.

But if you have not grounded the chassis of your rig to that ground wire going back to shore power, and if the short is to a conductive surface on the rig, like for instance where a grommet has failed and the black wire is chafed through on a hat channel, you are right back up there in the very bad scenario. You will you will get a zap somewhere between a tickle and a fatal to the degree that you conduct to ground, just as you would with that ungrounded device. I suggest that if you believe in grounding your devices, you should also believe in grounding the chassis of your bus.

One more time: grounding the chassis of the rig to the AC ground circuit is essential for providing a path for current under fault conditions where the AC hot wire has come into contact with a conductive surface of the bus itself, because even though you potentially form a better circuit to ground than the ground wire does, without that chassis ground to negative you form the best circuit.

Importantly, for the DC lines, the return path to the shore power bonded ground exists, but only as an open circuit. It does not complete back to the negative terminal of the battery. Likewise, touching the negative terminal of the battery does not complete any circuit on the AC lines.

So to Cadillackid's question, the answer should be no: none of this precludes using the rig chassis for the return path of a DC circuit.


I note that his question was asked like a bazillion years ago, but getting the answer right, especially with a new crop of forum members, is pretty important.

NOWHERE13 10-12-2021 09:56 PM

It is not my point. Just providing information. Safe travels and stops!

DeMac 10-12-2021 10:05 PM

I concur
 
Rwnielsen & Rucker interpret all of the sections together. I concur with you both, different terminology. Hard to explain shortly, right?

The first four Chapters of the NEC apply to all installations. Chapters 5, 6, and 7 are the "special" chapters, covering special: occupancies, equipment, and conditions (in that order).

Article 551 amends the Chapter Three wiring methods; it does not replace them. Where Article 551 is silent, Chapter 3 requirements apply [551.47(A)].

Do not ground interior electrical equipment. Where 551.55 speaks of "grounding," it means "bonding." If we review these terms in Article 100, we will see the terminology error when reading the requirements. Make sure you bond, not ground, interior equipment.

Bonding conductors for non-current carrying metal parts must be at least 8AWG [551.56(C)]. Number 8 eliminates differences in potential. If reliability is the concern, use bonding jumpers around raceway connections instead of simply upsizing bonding conductors.

Don't use the grounded conductor (neutral) as an equipment bonding conductor [551.76(D)]. This applies to all circuits, not just RV ones. The reason is the neutral (which is, at these voltage levels, the grounded conductor) carries the unbalanced current back to the source. It does not provide a low impedance path for eliminating differences of potential. Confusion here can result in a lethal touch shock.

Don't connect the grounded conductor to a grounding electrode on the load side [551.76(E)]. Actually, don't connect anything on the load side to any ground electrode; such a connection serves no electrical purpose.

What's the difference between bonding and grounding? Grounding electrode or grounded conductor?
Look in the NEC, Article 100.
Does the NEC refer to grounding incorrectly in any of its articles? Yes! So apply the Article 100 definitions. Don't ground where you should bond.

Rwnielsen 10-13-2021 02:50 AM

Quote:

But if you have not grounded the chassis of your rig to that ground wire going back to shore power, and if the short is to a conductive surface on the rig, like for instance where a grommet has failed and the black wire is chafed through on a hat channel, you are right back up there in the very bad scenario. You will you will get a zap somewhere between a tickle and a fatal to the degree that you conduct to ground, just as you would with that ungrounded device. I suggest that if you believe in grounding your devices, you should also believe in grounding the chassis of your bus.
I wasn't referring to a bad shore power connection. All bets are off if you're hooked up to a faulty electrical system.

This makes sense until you think about where your second point of contact is. I regularly handle hot wires but I'm very much aware of what I'm touching. Your bus (except when plugged into a grounded source) is an isolated electrical system. Ground in the normally accepted sense doesn't exist. Your neutral and ground are connected at the source. Most panel enclosures are bonded through the ground buss. Isolated ground busses exist but are generally supplemented with a standard ground buss and used in 'clean power' applications.
You should never hook up to shore power with an open ground.

Rwnielsen 10-13-2021 02:56 AM

I'm about out of gas on this subject. Someone in an earlier post said to ground your panel's ground buss to the chassis. This is not correct.

Rucker 10-13-2021 12:05 PM

Quote:

Originally Posted by Rwnielsen (Post 455897)
I'm about out of gas on this subject. Someone in an earlier post said to ground your panel's ground buss to the chassis. This is not correct.

I can't say I don't disagree.

Soaringcook 10-13-2021 12:12 PM

Easy
 
If people understood "path of least resistance and gfci breakers better, this wouldn't be so confusing.

Rucker 10-13-2021 03:46 PM

Quote:

Originally Posted by Rwnielsen (Post 455896)
I wasn't referring to a bad shore power connection. All bets are off if you're hooked up to a faulty electrical system.

This makes sense until you think about where your second point of contact is. I regularly handle hot wires but I'm very much aware of what I'm touching. Your bus (except when plugged into a grounded source) is an isolated electrical system. Ground in the normally accepted sense doesn't exist. Your neutral and ground are connected at the source. Most panel enclosures are bonded through the ground buss. Isolated ground busses exist but are generally supplemented with a standard ground buss and used in 'clean power' applications.
You should never hook up to shore power with an open ground.

Thanks for this. This is a super important topic.

If I understand your comment, the concern is when a bus is NOT connected to shore power, where there is no apparent ground (your words: 'an isolated electrical system'). I had trouble making sense of the second part of your comment 'most panel enclosures are bonded through the ground bus'. They are not--you need to put in the green screw to bond them, or take it out if it's screwed in. Maybe that's what you meant.

Here's my understanding: ground does exist. If your bus has an inverter (an independent 120 VAC-producing device) the inverter itself should have neutral and ground bonded together when running independent of shore power. That's what I understand is meant by 'at the source', and that's why I say house power via inverter is (or should be) grounded.

When inverting from battery power, the bond should be complete, or connected, so there is a return path for any short. When connected to shore power (or when a charger/inverter is powered by shore power) the bond should be open so that the source (shore power) is the single point of ground.

It appears that many charger/inverters out there don't clearly indicate how they handle ground and ground/neutral bonding. Without clear specifications you need to verify your own inverter's operation to ensure proper grounding under all circumstances.

Here's my setup: I have a straight-up inverter with a permanently bonded neutral/ground. I don't need to worry about how the ground and neutral are handled because when I plug in to shore power my inverter hot and neutral route from the inverter ONLY to the GoPower switcher and are disconnected in the presence of shore power. This is one more reason why I like discrete devices instead of combo units. If I had a charger/inverter I'd need to understand how that thing is handling ground. I charge with a separate Doco charger.

The reason for having that bond between neutral and ground on an inverter is the same as for shore power: the inverter is the 'source'.

(It's the same principle with generators-you need to ensure neutral and ground are bonded at the generator itself. I'm no expert and don't understand a lot of the discussion in the forums about floating ground and special circumstances, and GFCI tripping failures and the like. I hope to get more educated in this forum and through my own research.)

It's also why inverters, like all other electrical devices, should be grounded to the bus chassis. If a fault in the circuit electrifies the bus chassis, you stand a better chance of being less conductive than the ground wires running back to the inverter.

And, since there is no 'earth ground', you don't complete any circuit by standing in a mud puddle and touching a conductive surface on the bus due to a fault in the inverter power-again, when running solely on house batteries driving the inverter.

(It's a different thing if you are running a generator with a frame ground --that is more like a shore power scenario and why you'd want every device to ground to the chassis, and the chassis back to the generator).

This is my understanding to date. If I've gotten something wrong though I'm all ears!

Rwnielsen 10-13-2021 05:31 PM

Quote:

If I understand your comment, the concern is when a bus is NOT connected to shore power, where there is no apparent ground (your words: 'an isolated electrical system'). I had trouble making sense of the second part of your comment 'most panel enclosures are bonded through the ground bus'. They are not--you need to put in the green screw to bond them, or take it out if it's screwed in. Maybe that's what you meant.
Sorry, I wasn't more clear. The ground buss in the panel is bonded to the enclosure. The green screw is used to bond the neutral to ground and is only used at service entry. If your panel is mounted to the framing of your bus then it (the frame) becomes incidentally bonded (grounded) as well.

The rest of your post, I agree with. Single point ground, at the source. Neutral is grounded at the source.
GFI circuits detect an imbalance between line and neutral usually indicating a ground fault. You are the ground fault when you get a shock which is why you need to be very aware of your surroundings when working with live circuits. Don't become a ground fault. :-}

Rwnielsen 10-13-2021 06:04 PM

This is the wiring, from Century Exhibits (coach builder) that was in my bus when I got it. This is never correct.
It's a Blue Bird built in 1998 with 2, onboard generators. I have removed this panel (for a smaller one) and will bond the generators before I use them again. I'm using a portable with a short shore power cord and a bonding plug.

https://www.skoolie.net/forums/membe...cture27052.jpg

Rucker 10-13-2021 06:36 PM

Quote:

Originally Posted by Rwnielsen (Post 455935)
Sorry, I wasn't more clear. The ground buss in the panel is bonded to the enclosure. The green screw is used to bond the neutral to ground and is only used at service entry. If your panel is mounted to the framing of your bus then it (the frame) becomes incidentally bonded (grounded) as well.

The rest of your post, I agree with. Single point ground, at the source. Neutral is grounded at the source.
GFI circuits detect an imbalance between line and neutral usually indicating a ground fault. You are the ground fault when you get a shock which is why you need to be very aware of your surroundings when working with live circuits. Don't become a ground fault. :-}

Yes-I think it's safe to say given the above challenges that every AC outlet on the bus should be GFCI; and further if a GFCI recep is tripping, don't just replace it with a regular recep. Something is wrong.

Tip: checking voltage differential between the neutral wire and the ground may help with troubleshooting in those cases. The multimeter should read zero volts.

Rucker 10-13-2021 06:45 PM

Quote:

Originally Posted by Rwnielsen (Post 455940)
This is the wiring, from Century Exhibits (coach builder) that was in my bus when I got it. This is never correct.
It's a Blue Bird built in 1998 with 2, onboard generators. I have removed this panel (for a smaller one) and will bond the generators before I use them again. I'm using a portable with a short shore power cord and a bonding plug.

https://www.skoolie.net/forums/membe...cture27052.jpg

Hard to believe they put that in a bus. They shouldn't have put that in a woodshed.

But then again, pages and pages are written on proper grounding, and it's challenging to sort through all the noise.

Clearly, even for a manufacturer.


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