Page History: Ground Leak
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Page Revision: 2008/11/12 18:35
To my horror, the port called the other day to inform me that they put a clamp on Aeolus' power cord and detected a 90 milliamp current leak. They've just finished rebuilding all of the docks and have a newfound commitment to compliance with electrical codes. I'm all for this as who wants electrolysis eating their propeller away? Especially not me, considering I just dropped over $3,000 on a new MaxProp folding propeller a few months ago!
Emerald City Diving had just done a flyby a few weeks ago and found my zincs down 50% after only 3 months, so I guess I do have a problem in my own house to resolve.
I promptly cracked open my copies of Nigel Calder's Boatowner's Mechanical and Electrical Manual and Don Casey's Sailboat Electrics Simplified to figure out how to put my trusty multimeter on the job to track down the offending circuit. And so the odyssey began...
After visits from two different electricians and several posts on internet discussion forums, here is some of what I learned that might be helpful to you, whoever you may be who googled up this page:
First as a disclaimer I am not an electrician! I am sure some of my explanations are at a minimum oversimplified, and I have no doubt that some of them may have some bad info or reveal a lack of complete knowledge on the subject. I have found that not very many people really understand ground or current leaks or galvanic corrosion. While I certainly learned a lot about the subject, I am not an expert!
I spent a lot of time on the DC side of things. Way too much time. The general troubleshooting steps on the DC side are:
Shut everything off an measure volts between the positive cable and positive battery post. The problem here is that there are way too many "vampire" loads that can't truly be shut off. The classic example is the memory on the stereo to keep your station presets. I think my Promariner battery charger, Xantrex SD400 inverter, and Xantrex XBM battery monitor all draw a handful of milliamps even when "off" just to keep an eye on the batteries.
Measure amps between the positive cable and positive battery post. See the same problem as above...there are a lot of legitimate sources of milliamp-level loads
Measure resistance (Ohms) between the positive cable and the negative battery post. I think this is the best measurement because it is irrespective of the small but legitimate loads that many devices draw, and it measures the real problem: somewhere you have a leak to ground. The books say if it is below 10,000 Ohms (10kOhms) it is a problem. If it is above 10,000 Ohms don't worry about it.
After a lot of investigation and general cleanup (I couldn't believe how many "bridge to nowhere" circuits the boat had...wires going from the DC panel off through the cabin and not connected to any device, or how about the random wires that went from the DC panel back to connect to the DC + and - wires for the alternator?? I yanked a lot of superfluous wire, and my general trust of the previous owners' wiring jobs went down several notches), I narrowed the problem down to two circuits:
The inverter (more on this below)
The pump for the Newport diesel cabin heater. This is a diesel fuel pump in the engine compartment. It has a path to ground via the copper fuel line which is connected to the steel fuel tank, which is connected to the ground bonding circuit back to the DC negative bus bar. The resistance is pretty high though...about 40,000 Ohms. I may go back someday and insert some non-metallic hose in the fuel line to break this circuit...but probably won't do anything.
Since the port was measuring the leak through my shore power cable, they were looking at an AC current leak. AC is supposed to return to ground on the neutral wire, with the green ground wire being a safety alternate path. So all of your amps coming in on the hot wire should go back on the neutral wire. Well, not all of mine were coming back...some of them were leaking out into the water through my boat's grounding system and thru-hulls or prop or other metal underwater that is all tied together with big #6 AWG wires.