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Old 25-03-2013, 06:55   #1
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Odd problem with DC bus in combination with AC

I'm facing a bit of an odd problem and I want to run it past CF before I start tearing my electrical panel apart.

The boat in question has AC outlets (three wire) scattered throughout the boat along with 2 wire DC leads available at each outlet as well.

The AC system is wired into a battery charger so that whenever shore power is connected, the battery charger provides an initial bulk charge followed by a maintenance float charge after a few hours.

The DC outlets are wired to a 12v outlet breaker.

When shore power is connected and the DC outlet breaker is off, the DC outlet leads show a -13.7vDC. When the DC outlet breaker is turned on (with shore power AC connected) the DC outlet leads go to a +14.48vDC (bulk charge).

I'm trying to figure out what in the boat's circuitry would cause that, but I think I'm going to have to track down the DC outlet wiring and just disconnect them from the panel and work backwards from there.

If it's any help, I don't believe that a battery isolator was installed and the boat has two battery banks.
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Old 25-03-2013, 07:40   #2
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Re: Odd problem with DC bus in combination with AC

Sounds like your black AC wiring might be connected to the DC ground buss. (Neg)

Yellow is recomended as a wire color for DC Neg because this is so often a problem.

When the charger is on it outputs the correct voltage.
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Old 25-03-2013, 12:13   #3
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Re: Odd problem with DC bus in combination with AC

Quote:
Originally Posted by keelsidedown View Post
When shore power is connected and the DC outlet breaker is off, the DC outlet leads show a -13.7vDC. When the DC outlet breaker is turned on (with shore power AC connected) the DC outlet leads go to a +14.48vDC (bulk charge).
Verbal description is woefully inadequate at presenting all the required info for this sort of issue.

At a minimum, please state exactly where your meter probes are connected when you have these readings.

A partial schematic drawing would help too, but if you had one, you'd probably already have the answer too
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Old 25-03-2013, 12:39   #4
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Re: Odd problem with DC bus in combination with AC

You could just be reading spurious noise on a high impedance multimeter input , Put a small DC load on and read the voltages with teh DC breaker off!

Dave
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Old 26-03-2013, 06:53   #5
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Re: Odd problem with DC bus in combination with AC

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Originally Posted by Lake-Effect View Post
Verbal description is woefully inadequate at presenting all the required info for this sort of issue.

At a minimum, please state exactly where your meter probes are connected when you have these readings.

A partial schematic drawing would help too, but if you had one, you'd probably already have the answer too
I tried to keep the verbal description as short as possible, but you're right, when it comes to electrical issues, there can really never be enough information.

Accordingly, I have uploaded a video to youtube. Sorry about the quality. Digital cameras have really lousy performance in low light conditions. Hopefully the video should add enough information to answer some of your questions.

GOPR0094 - YouTube

The meter was connected at the DC pigtail available at the outlet (I know, this isn't exactly up to ABYC standard, but it's a better than what was there before).

No schematic. I've been finding lots of wiring issues from previous work done by other owners (I had to get the compass LED unwired from the engine panel as it required the engine ignition key to be turned to the on position... along with the requisite engine warning buzzer blaring away in the background). My general impression is that they half-assed just about all of the non-factory wiring, turning a perfectly good, standards wired panel into something that probably verges on dangerous. It doesn't help that a genset was removed recently.

I probably need to look through my copy of Boatowner's Mechanical and Electrical Manual to understand how the AC system should be grounded. It had a genset, so the electrical system should have been designed to have an AC ground at sea and I suspect they took the short cut of connecting shore power to a common ground bus bar.
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Old 26-03-2013, 10:12   #6
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Re: Odd problem with DC bus in combination with AC

Quote:
Originally Posted by keelsidedown View Post
I tried to keep the verbal description as short as possible, but you're right, when it comes to electrical issues, there can really never be enough information.

Accordingly, I have uploaded a video to youtube. Sorry about the quality. Digital cameras have really lousy performance in low light conditions. Hopefully the video should add enough information to answer some of your questions.

GOPR0094 - YouTube

The meter was connected at the DC pigtail available at the outlet (I know, this isn't exactly up to ABYC standard, but it's a better than what was there before).

No schematic. I've been finding lots of wiring issues from previous work done by other owners (I had to get the compass LED unwired from the engine panel as it required the engine ignition key to be turned to the on position... along with the requisite engine warning buzzer blaring away in the background). My general impression is that they half-assed just about all of the non-factory wiring, turning a perfectly good, standards wired panel into something that probably verges on dangerous. It doesn't help that a genset was removed recently.

I probably need to look through my copy of Boatowner's Mechanical and Electrical Manual to understand how the AC system should be grounded. It had a genset, so the electrical system should have been designed to have an AC ground at sea and I suspect they took the short cut of connecting shore power to a common ground bus bar.


Wow! that's going to take some time for sure.. I used (with all battery's disconnected) a "ticker tracer" from (Low's or Home D.) to find and identify all my electrical wires. It was time well spent ("at the docks only") and found allot of Dead-End wires to no-where..lol This way you can quickly find where all the wires lead and re- document your vessel's drawings. However; some times you must open one end though if more the one wire share a common connections. Good luck! Linda P.S. If you need a new harness! I would recommend the harness 101 Spray by I-Wish Co.!!
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Old 28-03-2013, 13:16   #7
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Re: Odd problem with DC bus in combination with AC

Dave (goboatingnow) probably has it right.

You could well be seeing "ghost voltages", i.e., voltages induced by the nearby AC wiring. These have virtually no current, but you'd see them with a high-impedence multimeter.

Better to use an electrician's tool (like the Fluke T-Plus Pro tester) or put a small load on the DC side to verify.

Bill
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