Electrical question - inverter 'chassis ground'?

[BrentwoodBayliner]

Hi everyone - I'm installing a new Xantrex Freedom X 3000 watt inverter on my Bayliner 3270.

When visiting my local (trusted) marine supplier picking up installation wiring, I was advised to ignore the manufacturer's advice about installing a 'chassis ground'.

The unit is already grounded on the AC side through shore power, and on the DC side through the negative connections to engine blocks etc.

The store tech said the addition of another ground wire from the inverter chassis to 'boat ground' (ie: engine blocks) could set up a 'loop' which could be detrimental. Better off without it he said..

So.. question for the electrical gurus.. how do you feel about these comments? Are they accurate? Why would the manufacturer not mention this?

I'm not trying to save money (even though it's about 8 feet of 2/0 cable) I just want to be safe. I'm also installing all the suggested fuses and breakers at both ends of the AC and DC wires.

Thanks in advance for all your help and advice!

 

[O C Diver]

Follow the directions explicitly or seek the help of an accredited MARINE electrician!!!

It is incredibly important to not create a pathway to put stray current into the water through the bonding system of the boat!!!!! People die while boating in freshwater because of stray current introduced into the water through the bonding system.

Ted

 

[BrentwoodBayliner]

Thanks Ted. I think I understand all that, but if I DO attach the 'chassis ground' wire, it will go to the engine blocks, and therefore be connected to bonding as well. So won't I have the same problem?

 

[O C Diver]

If you have a chassis ground, follow the manufacturer's directions on whether it goes to the bonding system or the shore power ground. This isn’t a universal question like should the DC negative be connected to the bonding system. READ the directions and if in doubt, call the manufacturer.

Ted

 

[O C Diver]

The issue here is whether the chassis is tied to the shore power ground or not.

Ted

 

[SteveK]

Xantrex. My previous inverter instructions in the box were IIRC connect chassis to AC ground OR was it battery negative. In any event those instructions were updated/corrected years later for the same Xantrex freedom 20 opposite of what the instructions in the box came with.

Perhaps the guy selling the wire had a reason to make the suggestion. Why not use a multi meter and see it the chassis is bonded to AC ground or DC negative already before hooking it to either.
On my todo list. I always wondered why it would not already be bonded at the factory.

 

[BrentwoodBayliner]

Thanks Ted - it's not supposed to be connected to AC shore power ground. The install manual says "the chassis ground lug should be connected to the boat's DC grounding bus or engine negative bus".

And therein lies the crux of my question - two (big) wires are now connected between the inverter and DC ground, which creates a 'loop'. That's what I was being warned about.

From several forums I've now heard strong opinions on both sides about whether it should be connected or not. If it was being installed in a dirt house, absolutely it would go the 'earth' ground. But we don't have that when the boat is away from the dock.

 

[BrentwoodBayliner]

Thanks Steve - good suggestions. I'll follow those up. The store guy's only comment was: "I've installed hundreds of these and I've always done it this way.." It actually COST him a sale of 8 ft of 2/0 cable. They are acknowledged as the most 'pro' store in town.

BTW - you are famous in my house. I OFTEN quote your signature line about two engines..

 

[twistedtree]

And therein lies the crux of my question - two (big) wires are now connected between the inverter and DC ground, which creates a 'loop'. That's what I was being warned about.

It only creates an alternate current path if the chassis and DC negative are common inside the Xantrex. A meter will tell you if they are, but I expect they are not.

 

[CharlieJ]

The engineers at Xantrex and the folks writing the ABYC Standards know a whole lot more about this subject than the counter guy at your local chandlery.

This has nothing to do with ground loops, electric shock drowning, EMP, the phase of the moon or any other subject. The case ground is a redundant path back to the negative bus so that, in the very unlikely event of a catastrophic failure inside the inverter/charger, the only path back to the negative bus will not be via the AWG 10 serving the AC in/out. AWG 10 when challenged by a short internal to the inverter/charger will become a strip heater because the B+ is protected by a 300A fuse.

RTFM.

 

[BrentwoodBayliner]

Thanks Charlie - excellent advice. Best answer so far!

 

[High Wire]

Agree. It’s all about a DC fault.
There is a provision for the chassis ground to be one size smaller than the DC feeder leads. So that provides a little cost break.

 

[SteveK]

Charlie. Why a separate line back to the ground buss. A rather large negative wire is right there beside the chassis. why is that wire not attached to the chassis?

 

[fgarriso]

I am sure the store tech knows so much more about the system than the manufacturer does. NOT!

Just pick up the phone and call the manufacturer's support line and ask them the question. It is better than getting shocked or someone else getting hurt.

 

[CharlieJ]

@Soo-Valley #13:

Charlie. Why a separate line back to the ground buss. A rather large negative wire is right there beside the chassis. why is that wire not attached to the chassis?

The chassis would then be part of the normal DC circuit and all operating current would be passed thru the chassis. Doable but a bad idea.

 

[High Wire]

Charlie. Why a separate line back to the ground buss. A rather large negative wire is right there beside the chassis. why is that wire not attached to the chassis?

If a positive lead going to the inverter internal guts falls off and contacts the chassis, there is no pathway back to blow the fuse except the chassis ground. That ground has to be able to handle the fault current until the fuse opens.

 

[Circumnavigator]

As noted above, you ground every metal chassis on the boat -- inverters, chargers, pumps, whatever -- for safety in the event that something fails inside and the chassis becomes hot. You do this even if the unit is mounted on a metal wall in a metal boat.


The DC negative is not connected to the chassis because you don't want the inverter splitting the negative DC current between the wire directly to the battery bank and the grounding system. If it were connected and the direct wire failed, you'd be drawing the inverter load through the grounding system.


Jim

 

[SteveK]

@Soo-Valley #13:

The chassis would then be part of the normal DC circuit and all operating current would be passed thru the chassis. Doable but a bad idea.

If a positive lead going to the inverter internal guts falls off and contacts the chassis, there is no pathway back to blow the fuse except the chassis ground. That ground has to be able to handle the fault current until the fuse opens.

I am failing to understand. With a bonded boat to ABYC standard the DC negative is bonded to the AC ground.
A separate line from chassis would also be bonded but now at a point many feet away travelling parallels to the existing DC -. Why is a separate line better going to the same point? As it is from the DC - and AC ground is it not the same difference.

 

[CharlieO.]

I need a book with pictures and diagrams, any recommendations? For complete boat electrical systems.

 

[Circumnavigator]

I am failing to understand. With a bonded boat to ABYC standard the DC negative is bonded to the AC ground.
A separate line from chassis would also be bonded but now at a point many feet away travelling parallels to the existing DC -. Why is a separate line better going to the same point? As it is from the DC - and AC ground is it not the same difference.

The AC ground and DC negative are bonded at only one point -- not in the inverter. As I tried to say above, any connection between them in the inverter will split the DC current between the wire going from the inverter DC negative to the battery and the grounding system. Not a good idea.


Jim

 

[SteveK]

The AC ground and DC negative are bonded at only one point -- not in the inverter. As I tried to say above, any connection between them in the inverter will split the DC current between the wire going from the inverter DC negative to the battery and the grounding system. Not a good idea.


Jim

Jim, I never said they were bonded inside the inverter. i am asking why a separate wire is needed to go to the ground bonding from the chassis when both the AC ground and DC - already also go to the same connection point.
Why not just take a 6 inch wire from DC - and call it a day

 

[diver dave]

Answer: Potential fault occurs from DC+ to inverter chassis. Dc+ supplies hundreds of amps into ac green wire run. Wire run catches fire before fusing. This is the logic of very large inverter bonding conductor. .

 

[ofer]

i am confused too.

1. should DC from batteries be connected to boat grounding?

2. if you weren't sure how your grounding system is connected. is it better just to leave chassis ground not connected to anything. i don't mind getting shocked and die, i just don't want my boat catching on fire.

 

[diver dave]

How does this sound? All big metal objects on the boat should be connected together.
Saildrives appear to be a popular exception.

 

[Circumnavigator]

Jim, I never said they were bonded inside the inverter. i am asking why a separate wire is needed to go to the ground bonding from the chassis when both the AC ground and DC - already also go to the same connection point.
Why not just take a 6 inch wire from DC - and call it a day

For the same reason that the AC circuits in your house have a separate green bonding conductor even though it is connected on the panel to the AC neutral (white) wire. If there is a fault inside the connected unit, you want a completely separate system there to carry the load and blow the fuse or breaker. The DC negative and the grounding system are connected at only one place, just as your household white and green wires are connected only at the entrance panel.


Jim

 

[SteveK]

As long as the chassis is only connected to either AC ground OR DC - they are still only connected at one point.

 

[twistedtree]

As long as the chassis is only connected to either AC ground OR DC - they are still only connected at one point.

If the AC ground terminal, chassis frame, and DC ground (not negative, but ground) are connected together in the inverter, then I think it would be fine to use just one grounding wire, but it would have to be big enough to carry the DC fault current which is much greater than the AC fault current. It also would assume that the boat's AC ground, DC negative, and ships ground are all tied together, which they aren't always.


Simplest and hardest to screw up and/or misinterpret down the road is to just run both wires. You need to run the fat one regardless, and that's the hard part.

 

[SteveK]

If the AC ground terminal, chassis frame, and DC ground (not negative, but ground) are connected together in the inverter, then I think it would be fine to use just one grounding wire, but it would have to be big enough to carry the DC fault current which is much greater than the AC fault current. It also would assume that the boat's AC ground, DC negative, and ships ground are all tied together, which they aren't always.


Simplest and hardest to screw up and/or misinterpret down the road is to just run both wires. You need to run the fat one regardless, and that's the hard part.

why did you reply to my post with this. The whole thread is about grounding the chassis, NOTHING about internal connections.

 

[twistedtree]

why did you reply to my post with this. The whole thread is about grounding the chassis, NOTHING about internal connections.

You were asking about using a single grounding wire rather than two, or maybe it was someone else?

 

[alormaria]

I need a book with pictures and diagrams, any recommendations? For complete boat electrical systems.

Two books come to mind:


Boatowner's Illustrated Electrical Handbook by Charlie Wing


Boatowner's Mechanical and Electrical Manual by Nigel Calder