AC ground current leakage

[smac999]

here was a weird one. simple boat wiring. standard AC and DC panel. no inverter.

had 130ma current on the AC ground wire with boat plugged in. even with shore main off. couldn't narrow down which AC circuit was causing the issue so I connected to dock with ground wire only and still had same current. so it was not current leaking from my own shore hot or netrual.

also meausred same current on the DC to AC ground jumper. so it must be coming in from the DC side. must be entering from water through shaft or through hulls. then going to dock?

another nearby boat measured 200ma when I clamped its' shore cable. but a few others measured 0. (but not with a low current meter) so they may have had some) I was hoping when I unplugged nearby boat, mine would go away so I could blame them. but mine increased and went from 130ma to 250ma. so appears the other boat is actually helping clear the fault in the water, not causing it....

see pic. I didn't have a clamp meter low enough to chase this. would have been neat to pinpoint which through hull it was entering. the 2 readings are with inline amp meter. I wasn't about to go around disconnecting every wire to follow it with inline meter.

is my assumption correct it must be external? i'll have to move the boat and measure again.

whats interesting is it likely would not trip a GFCI or ELCI since it's on the ground wire. the H and N Lilkly still in balance. wish I could have clamped those.

an iso transformer would prevent this though.

[mickt243]

Sounds like a fault on someone else’s boat, through the water, through your dc earthing system...

[kenbo]

Disconnect the 12vdc ground from the batteries and see what happens

[Andina Marie]

You can now get 50 amp high voltage galvanic isolators for Aluminum boats - - 2.4 volts instead of standard 1.2. Designed for Aluminum boats that already have up to 0.9 volts potential but also excellent for aggressive situations on all boats.

[CharlieJ]

#4
Galvanic isolators do nothing to protect against alternating current leakage. They only block galvanic current and pass alternating current that is present in the safety ground wire due to a line fault to ground.

[Andina Marie]

Originally Posted by CharlieJ

#4
Galvanic isolators do nothing to protect against alternating current leakage. They only block galvanic current and pass alternating current that is present in the safety ground wire due to a line fault to ground.

Isolators block ANY current that is due to a voltage less that the threshold.

DC current is not blocked if AC current is flowing because on every second half cycle the DC current is added on top of the AC current and flows up to 50% of the time.

If AC current is reducing the isolation you can bypass it with a Galvanic Capacitor. Yandina sells one rated at up to 5 amps however you should first make sure the cause is not on your own boat by checking if the AC current goes away when the main breaker is off.

[undercutter]

Originally Posted by CharlieJ

#4
Galvanic isolators do nothing to protect against alternating current leakage. They only block galvanic current and pass alternating current that is present in the safety ground wire due to a line fault to ground.

Are not the ground and neutrals always connected when on shore power?


When I went on the hard in Thailand, I grabbed the handrail and got quite a lift. Turns out the neutral wire was loose at the shore power plug on the boat so everything was traveling through the ground. Time for RCD's.

[CharlieJ]

#7
Yes, there is a N > G bond at every source. The common sources for boats are: shore side transformer (or possibly the main feeder panel for a marina), on board genset, inverter/charger (or inverter only) when inverting, and secondary of isolation transformer. No other N > G bonds are permitted onboard.

#6

Isolators block ANY current that is due to a voltage less that the threshold.

And with a threshold of approximately 1.2 volts, a GI will not block very much 120VAC leakage.

DC current is not blocked if AC current is flowing because on every second half cycle the DC current is added on top of the AC current and flows up to 50% of the time.

DC is not blocked when AC is flowing because the diodes have been biased into conduction by the >1.2 volt AC leakage.

If AC current is reducing the isolation you can bypass it with a Galvanic Capacitor.

Are you implying that the capacitor will bypass the diode pack in the presence of AC and that the GI will still provide galvanic isolation? If so, this defies physics.

[Andina Marie]

Originally Posted by CharlieJ

#7
<SNIP>

Are you implying that the capacitor will bypass the diode pack in the presence of AC and that the GI will still provide galvanic isolation? If so, this defies physics.

Yes, it is correct. If you put a Galvanic Capacitor (check Google and see detailed explanation.) across the isolator, the AC leakage current will flow through it and the voltage across the diodes will be kept below 0.3 volts for AC currents. The diodes are not switched into conduction and the DC current is isolated.

If the AC current is greater than the capacitor can bypass you have a serious leakage problem that should be diagnosed and corrected rather than covering up with a capacitor.

ABYC specifications suggest a capacitor to be included to achieve this function however Yandina is the ONLY company that provides a 4,000 microfarad capacitor.

[CharlieJ]

#9

Yes, it is correct. If you put a Galvanic Capacitor (check Google and see detailed explanation.) across the isolator, the AC leakage current will flow through it and the voltage across the diodes will be kept below 0.3 volts for AC currents. The diodes are not switched into conduction and the DC current is isolated.

You are absolutely correct...my fingers got way ahead of my brain. The capacitor will not block DC but will pass AC thus preventing the diodes from being forced into conduction.

My basic premise remains: galvanic isolators do nothing to prevent/mitigate AC leakage.

And, by the way, the parallel capacitor in ABYC Standard A-28 is not a "suggestion":

28.5.4 Galvanic isolators using diodes shall use a non-polarized capacitor(s), or equivalent method of meeting the requirements of A-28.12.4.

[EngNate]

Very likely two things at work causing the OP's observations. One, there is (excessive) voltage loss on the Neutral conductor serving multiple receptacles on the dock, and there are Ground-to-Neutral connections on some boats, including possibly the OP's, and these connections pass that voltage to the Ground conductor. Loss elevates the voltage on the Neutral, and when this voltage is impressed on the Ground currents circulate between boats. Ground and Neutral are to be connected only at the prime source of power. Any other connection puts the Ground in parallel and sharing the load current with the Neutral. Further detail would digress a bit.

Probability is any marina will have some boats with the improper G-N connection, and there will be loss in wiring, so small amounts of these currents will be present. When the loss is excessive the effect is more pronounced, and definitely has an adverse influence on corrosion.

So, to minimize the effects on your boat and others, insure that there is no continuity, measured at the shore inlet on your boat, between the Ground and Neutral conductors. Check this with the ship/shore switch in all positions just to be sure. This will be the same for boats equipped with isolation transformers if the ground connections are correct. Note that if an incandescent indicator is used for Reverse Polarity warning it will read continuity. LED or neon indicators are preferable. One can go around and check neighbors' boats from the dock too. Likely is, once you've disconnected all with a GN connection that are on your main circuit a fault current such as this will disappear. I have removed jumpers between G and N busses on a number of boats, also found improperly wired inverters and isolation transformers the cause of fault currents.

"this post is not intended to contain the complete solution to any particular issue"...

[Andina Marie]

Agree with EngNate 100%. At a yacht club in Cape Canaveral we had 6.3 volts between neutral and ground. Ground wire in the docks was only 10 gauge compared to much heavier main conductors so cross feeds from neutral to ground at the end of the dock had a big effect.

The problem arises with boats that have a generator or inverter. While the neutral should not be connected to ground when on shore power, it SHOULD be connected to ground when generating power on board. But unfortunately a majority of transfer switches don't have enough poles to switch the ground. In recent years quality inverter manufacturers have added a relay the switches the ground connection when the inverter is on but gensets frequently need to be switched manually.

[transmitterdan]

A lot of marinas connect “earth ground” to neutral at the box. They don’t bring a separate ground circuit down from the transformer feed. So there will always be some voltage between the “ground” terminals of every pedestal. The voltage between grounds depends on the current drawn by the entire feeder on the dock. Even worse is when there is a high resistance connection in the neutral path at a splice on the dock.

So when boats connect AC ground to battery ground on board (which they should not do when on shore power) that completes a circuit between neighboring boats and then current flows. A galvanic isolator will not impede the current.

If current (AC or DC) is flowing in your boat’s AC ground wire there is something you can do about it on your boat. If your AC ground is fully insulated from hot, neutral, DC power and the water there can be no current.

[smac999]

Originally Posted by transmitterdan

So when boats connect AC ground to battery ground on board (which they should not do when on shore power) that completes a circuit between neighboring boats and then current flows. A galvanic isolator will not impede the current.

If current (AC or DC) is flowing in your boat’s AC ground wire there is something you can do about it on your boat. If your AC ground is fully insulated from hot, neutral, DC power and the water there can be no current.

The ac to dc bond is required by abyc. It can’t be removed. An isolation transformer would fix this issue though. Of course fixing the boat or dock causing it is best choice.

I don’t recall there being a dig diff in ac voltage from g-h and n-h. But I did not write it down so I don’t recall. If it was big I would have made a note.

[transmitterdan]

The ABYC “rule” makes no sense. It’s better to “break” the stupid ABYC rule than kill people and boats in poorly designed marinas. Just my opinion.

When on internal ship’s power it makes sense.