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Old 08-07-2022, 14:16   #1
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VHF ground connecte to 12V negative?

While debugging my electrical system I disconnected the battery negative from the house loads busbar. The boat's bonding system was still connected to the house battery negative. I expected that would disable everything on the boat by breaking the circuit back to the battery.



I was surprised that cabin lights and other stuff still worked. Disconnecting negative wires from individual circuits proved that the VHF and AIS negative wires were providing the return circuit to the battery.



It seems that the two radios (VHF and AIS) have an unexpected (to me) connection between their 12V negative and the boat's bonding system. So there was a return path from the negative bus to the radio, then through the bonding system back to the battery. Is that normal and correct?
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Old 08-07-2022, 14:46   #2
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Re: VHF ground connecte to 12V negative?

Sounds very odd to me
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Old 08-07-2022, 15:01   #3
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Re: VHF ground connecte to 12V negative?

Quote:
Originally Posted by maddmike View Post
It seems that the two radios (VHF and AIS) have an unexpected (to me) connection between their 12V negative and the boat's bonding system. So there was a return path from the negative bus to the radio, then through the bonding system back to the battery. Is that normal and correct?
Most 12v DC marine equipment have their chassis or case connected to the 12v negative, and marine VHF radios have the shell of their antenna jacks connected to the case. And finally, most VHF antennas (that I've seen) connect the shield of the antenna coax to their case.

So - if your VHF antennae are mounted to your mast, and your mast is bonded... there's your mystery return path.. Easy to check - just disconnect the antennae from the back of your radios.

Tip - when troubleshooting the DC side of a boat, it's better to disconnect positives, or just kill breakers or battery switches.
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Old 08-07-2022, 15:07   #4
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Re: VHF ground connecte to 12V negative?

It’s common but very unfortunate , no device should have a unseen connection. My vhf ( standard horizon ) has a specific and separate ground point.

If they are unseen , a risk exists that they become the unintended dc return paths for large currents , this over typically undersized and non protected dc returns.
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Old 08-07-2022, 17:21   #5
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Re: VHF ground connecte to 12V negative?

Quote:
Originally Posted by maddmike View Post
While debugging my electrical system I disconnected the battery negative from the house loads busbar. The boat's bonding system was still connected to the house battery negative. I expected that would disable everything on the boat by breaking the circuit back to the battery.



I was surprised that cabin lights and other stuff still worked. Disconnecting negative wires from individual circuits proved that the VHF and AIS negative wires were providing the return circuit to the battery.



It seems that the two radios (VHF and AIS) have an unexpected (to me) connection between their 12V negative and the boat's bonding system. So there was a return path from the negative bus to the radio, then through the bonding system back to the battery. Is that normal and correct?
So what is your bonding wire attached to?
If it's the battery negative, there is your answer.

If it isn't, is it connected to shore ground, on the AC leg?

Got to be connected to something grounded.
If not, you have voltage bleeding into the water.
Thru your underwater apparatus.
It's possible.
I'd check how much current is bleeding off, and isolate it.
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Old 08-07-2022, 17:38   #6
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Re: VHF ground connecte to 12V negative?

In a DC electrical circuit, ground is the current return path. For an antenna, ground is pretty much half the antenna. For most VHF radios the antennas pretty much take care of themselves. but good grounding is important for big whip antennas.
SSB radios are serious pieces of communication equipment and a metal hull makes an excellent ground plane. Even with VHF, Wood and plastic boats cannot always depend on just their negative DC paths to create an effective antenna. An SWR meter and experimentation to determine whether rigging, plumbing, railings help, or hurt, can pay big dividends.
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Old 08-07-2022, 19:45   #7
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VHF ground connecte to 12V negative?

He said he disconnected the battery ground so it has to be his charger no

Disconnect the charger ground from the busbar

Your charger is providing power and the return is via the ground busbar
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Old 08-07-2022, 19:56   #8
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Re: VHF ground connecte to 12V negative?

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Originally Posted by AKA-None View Post
He said he disconnected the battery ground so it has to be his charger no

Disconnect the charger ground from the busbar

Your charger is providing power and the return is via the ground busbar
Not necessarily, the OP also said "The boat's bonding system was still connected to the house battery negative."
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Old 08-07-2022, 20:11   #9
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Re: VHF ground connecte to 12V negative?

The ground in your NMEA 180 could be carrying your DC negative load. That very thin wire could easily turn red hot if you key the VHF. Also the antenna mounting bracket on your aluminum mast could be the DC negative return. There are many unexpected paths to DC negative. Battery charger was already mentioned. Inverter is another one. As already posted you are better off turning off the circuit breaker.
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Old 08-07-2022, 20:54   #10
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VHF ground connecte to 12V negative?

He needs to find out where that circuit path is before something draws serious power and melts wires

The only item that should ever be directly connected to the battery is the sump pump every thing else should be off the bus bar

Edit

I believe we have multiple wiring diagrams showing that
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Old 08-07-2022, 22:12   #11
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Re: VHF ground connecte to 12V negative?

If you mounted the antenna to a metal boat with a metal mount. Then yes. You have connected the boat itself to the dc neg through the vhf antenna and radio.

Switch to plastic mount or make a plastic spacer

If it’s mounted to fibreglass then you have other issue.
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Old 09-07-2022, 02:53   #12
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Re: VHF ground connecte to 12V negative?

Quote:
Originally Posted by smac999 View Post
If you mounted the antenna to a metal boat with a metal mount. Then yes. You have connected the boat itself to the dc neg through the vhf antenna and radio.

Switch to plastic mount or make a plastic spacer

If it’s mounted to fibreglass then you have other issue.


I was not aware the mounting structure of a vhf antenna was connected to ground.
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Old 09-07-2022, 06:10   #13
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Re: VHF ground connecte to 12V negative?

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Originally Posted by goboatingnow View Post
I was not aware the mounting structure of a vhf antenna was connected to ground.
Many do. On this common type, the shell of the connector is part of the mounting.

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Old 09-07-2022, 08:03   #14
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Re: VHF ground connecte to 12V negative?

Hmm my banten antennas ( which is the only brand I use) does not have the mounting bracket grounded. The shield in the coax is ground afaik
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Old 09-07-2022, 09:52   #15
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Re: VHF ground connecte to 12V negative?

Thanks for all the comments.
Quote:
marine VHF radios have the shell of their antenna jacks connected to the case. And finally, most VHF antennas (that I've seen) connect the shield of the antenna coax to their case.
So - if your VHF antennae are mounted to your mast, and your mast is bonded... there's your mystery return path.. Easy to check - just disconnect the antennae from the back of your radios.
Quote:
The only item that should ever be directly connected to the battery is the sump pump every thing else should be off the bus bar
All of the above is what I expected. To clarify: everything on the boat has its DC negative connected to one of two bus bars: one for the start battery and one for the house battery. The two busbars are connected together at one place (the current-measuring shunt). The bonding system (keel, mast, pushpit, pulpit, engine, and chainplates) is connected with a copper braid to the start battery negative bus.

This configuration makes a 12V DC circuit for the house stuff, a separate 12V circuit for the start battery (engine and windlass), and the bonding system. Those three systems are connected at one place to keep everything grounded, without creating ground loops.

Quote:
no device should have a unseen connection. My vhf ( standard horizon ) has a specific and separate ground point.
My Icom radio also has a designated frame ground, although I don't think the AIS does. There's also a frame ground connection on the Furuno chartplotter and autopilot, neither of which is connected to the device 12V negative. My understanding was that the frame grounds should be connected to the bonding system because their purpose is to ground all exposed metal for safety.

Quote:
Tip - when troubleshooting the DC side of a boat, it's better to disconnect positives, or just kill breakers or battery switches.
Yes. In this case, however, I was trying to trace all the negative wires and make sure I knew where everything was connected. For example, I found that the windmill had its positive connected to the house battery and negative connected to the start battery's busbar. That's functional, but confusing.


Quote:
Most 12v DC marine equipment have their chassis or case connected to the 12v negative
This seems to be what's happening in my VHF and AIS. It also seems wrong because it creates a ground loop and, as others mention above, means DC negative current can flow through unexpected paths. My chartplotter and autopilot have their case grounds isolated from the 12V circuit, which is what seems the right way to build it.
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