Neutral/ground bonding

[ol trunt]

I ran across this posting on another bus forum and received permission from the OP to post the discussion here. The OP is talking about upgrades in his own bus but includes a pdf which clearly explains bonding and grounding in 120v electrical systems. It is understandable to newbies and old guys like me with CRS.
Jack

"At least around here, bonding is supposed to be done at one point, and that point is the main power supply to the building. In other words, the primary source of the power. For a house, that is the main power center, aka the breaker panel. If you have a separate main disconnect before the breaker panel, then that's where the bonding should be done. Any subsequent sub-panels should not have a neutral/ground bond. If you have a separate main disconnect before the breaker panel, then your breaker panel is really a sub-panel.

Where it gets confusing for a bus is that people sometimes think of it wrong. When a bus (or any RV) is plugged into the pedestal it's basically a moving sub-panel. So, as a sub-panel a bus's breaker panel should not have a neutral/ground bond. That is done at the source feeding the pedestal.

However, when operating off generator the generator becomes the primary source, not the pedestal. Therefore, the generator should have a neutral/ground bond. The breaker panel in a bus should not have a neutral/ground bond - it creates a safety hazard in that a hot skin situation is possible.

Having the neutral/ground bond in the wrong place can create problems and keep breakers from tripping properly.

Here's a great PDF I found on the subject this morning as I looked to confirm what I thought:"

http://www.adamselectric.coop/wp-con...ng-Bonding.pdf

[TheHubbardBus]

Great thread, and great topic for discussion!


I look forward to reading @BlackJohn 's input. He has a valid and well-studied opinion on the subject which has caused me to rethink what I thought I knew.

[BlackJohn]

Originally Posted by TheHubbardBus

Great thread, and great topic for discussion!


I look forward to reading @BlackJohn 's input. He has a valid and well-studied opinion on the subject which has caused me to rethink what I thought I knew.

I just read what Jack posted and it seems to be well stated as to the subject matter.
Don't hold your breath for a detailed comment though TBH. This has been discussed many times on here and the more I read about it, the greater I see the confusion. Amateurs trying to make sense of it must get frustrated and rightly so. It is a difficult concept to get in one's head and thoroughly understand. Haven't looked at the pdf yet but I tend not to offer any advice on this. The reader has to understand what he has to do and with so much discussion around this, good and bad, I would rather not offer any advice. Bad advice will kill people.

Seasoned electricians and rv techs don't fully understand this so how can an amateur bus builder.

And e wonder why it is harder and harder to insure these buses. This is one huge reason, unqualified builders toying with something they shouldn't even consider. All to save a few bucks? How much are funeral costs for those people and their friends, families etc should a mishap occur?


John

[TheHubbardBus]

Understood, John. Just so you know, your words have been very helpful to me, and the process of considering the viewpoint you've touched on in other threads ultimately led me to a much better understanding of the nuances involved in a 'safe' (relatively speaking) rv electrical system... or any residential system for that matter. You didn't spell it all out - I had to figure it out myself. And that process helped immensely.

Ditto the sentiment on insurance. Insurance companies aren't running from skoolies without merit. It comes down to $$. If they're reluctant to insure, it's because an above-average percentage of them are going up in flames and/or costing lives. And that's all on 'us'.

[glenncooper]

Hello. I realize this post is over a month old, but thought I might try to shed some light on the subject. The referenced PDF had a lot of great information but might end up being a bit confusing. I'm an electrical engineer with thirty years of experience in industrial environments. I live full time in an RV that has a very sophisticated power system in it (that specifically deals with the bonding issue), and I'm building out a van that will specifically deal with the neutral / ground bonding issue.
Let me try to break this down simply.
In your home (or any other building), the ground wire, which is meant to keep you safe, is tied to the neutral wire at some point where the service enters the building.

In a mobile rig you have a challenge. There still needs to be a safety ground. If you're tied to shore power then that must come from the shore power feed, and as pointed out you should NOT also bond your ground to the neutral in your vehicle.

But when you're not on shore power you need to have these bonded together.

The solution I've seen (and the only one that makes sense) is some type of relay that provides the neutral <> ground bond when you're not connected to shore power.

My RV does this with an external relay where the coil is powered by the shore power. When the shore power is off it's assuming we're running on the inverter or generator, and the relay attaches the neutral and ground.

In my van build I won't use a generator, but I will have solar and I'm using a Renogy inverter / charger. This inverter/charger handles this internally. I've tried to attach an image from the Renogy manual where it shows this.

I would suspect that most inverter/charger units do this automatically. This doesn't help you if you're running a generator - in that case you'd have to provide the relay externally.

If you don't ever have a shore power option then you can just bond these together.

Hopefully this helps some.

[john61ct]

The only connection my mobile rigs have to any AC, or specifically shore power, is through the AC-to-DC rectifier / charger.

If I had an aircon unit I'd run that as its own dedicated circuit with a plug that could be transferred from a genset to the shore pedestal, I would never own an inverter big enough to feed aircon.

There are a couple small inverters dedicated to specific loads, their chassis "grounded" to the overall chassis common, which is also connected to DC negative return buses at various points along with the engine block.

In effect the only time anything has a true Earth Ground is while the charger is plugged into shore power.

Does that sound safe?

[glenncooper]

John,
The way you've described that sounds basically correct. The really key detail is that you unplug the air conditioner from shore power and plug it into the generator. When plugged into the generator there should be one place that the neutral from the generator is tied to the earth ground. This might already happen in the generator - not sure about that.


Basically sounds like you're on the right track.

[TheHubbardBus]

Originally Posted by glenncooper

In a mobile rig you have a challenge. There still needs to be a safety ground. If you're tied to shore power then that must come from the shore power feed, and as pointed out you should NOT also bond your ground to the neutral in your vehicle.

But when you're not on shore power you need to have these bonded together.

The solution I've seen (and the only one that makes sense) is some type of relay that provides the neutral <> ground bond when you're not connected to shore power.

My RV does this with an external relay where the coil is powered by the shore power. When the shore power is off it's assuming we're running on the inverter or generator, and the relay attaches the neutral and ground.

You're describing a transfer switch.

[glenncooper]

Not exactly a transfer switch - I obviously have one of those too. My transfer switch operates automatically when the generator is started. My grounding relay operates when the shore power is gone. In most cases those are the same thing, but not necessarily. My transfer switch is also "full", which I think is also why the grounding relay is there (bought it that way - not my design - but still a correct design).

[TheHubbardBus]

I see what you mean. But I still don't understand the reason the NG-switching functionality isn't incorporated in the transfer switch itself. Could you elaborate?

[glenncooper]

I believe the reason that the ground-neutral link is not part of this transfer switch (at least in my case) is this: First - it's important that earth ground be tied to the neutral at some point, and only one point. If you're connected to shore power then that tie comes from the shore power. In my situation the rig is wired so that the transfer switch switches when the generator starts, not when the shore power goes away. I'm guessing that the thinking on this was that this gives you the ability to run and test the generator under load while the shore power is still connected (again - not my design - I bought it that way). So in this case you could have the generator running but still have a valid (but unused) shore power connection. So the ground-neutral relay does NOT switch, and the bond still happens via the shore power, but the transfer switch DOES switch, meaning you're running on generator power. I could certainly see a situation where you have both work together, i.e. the transfer switch and ground-neutral bond (same relay..) switch when the shore power is off. That would make sense. For my new van build (still really hoping to get a bus, but not there yet...) I won't have a generator and so the inverter charger takes care of the ground-neutral switching for me. Much cleaner. Not sure if this helped or muddied the waters.....

[TheHubbardBus]

Thanks so much for the explanation, Glenn. I understand completely now. And learned more in the process. Much appreciate your input!

[glenncooper]

Glad I could help....