Electrical issue

[Steve DAntonio]

I have an isolation transformer. Started with everything turned off and boat unplugged. Reading was -950 mv. I then slowly turned on DC devices then plugged into shore and slowly turned on AC devices and had little to no change in voltage throughout the process.

If you have a transformer, and it's wired for and achieving isolation, then you don't need a galvanic isolator, I should have said that.

-950 is acceptable for common underwater metals except aluminum, anything between -750 and -1100 in SW is acceptable. Based on your test is sounds as if you are OK and protected. It's possible the paint failure was a surface prep issue, and not related to corrosion.

 

[porman]

Steve, thank you for the explanation about salt water vs fresh.

 

[backinblue]

With the boat out of the water? Continuity tests can only be conducted while hauled out.

Not sure I agree with you about that. You can measure continuity to any metal that is part of the bonding system. It should be very close to zero ohms. Not sure why the water would matter, but please explain. In fact, the instructions I have for the electrode includes this test, but a little differently by moving the electrode to different fittings and noting that there is not a voltage change.

 

[Steve DAntonio]

"You can measure continuity to any metal that is part of the bonding system", from what?

While you can take several other types of corrosion analysis readings afloat, you can't check the continuity of a bonding system, i.e. between bonded, submerged components, with the vessel afloat, the water acts as a current path and will skew the results.

Having said that, the resistance between any bonded underwater metal and an anode should not exceed 1 ohm, again measured ashore.

Moving the positive lead of an ohm meter (with the reference cell in the negative lead and in the water) to different underwater metals, while afloat, is a valid test for determining protection levels and identifying components that may not be protected because of open or high resistance bonding wiring. While it may identify a bonding system continuity issue, it is different than measuring continuity and bonding system integrity. Ideally, all bonded metals should be at roughly the same voltage. If one isn't, and it is bonded, that is likely an indication of a bonding integrity issue, but it won't tell you the resistance between the anode and the protected metal (the cathode).

Typically, for corrosion analysis, continuity is tested while the vessel is out of the water, using an ohm meter and an extended test lead. One test lead is connected to a hull anode stud, and the other is touched to all the other bonded UW metals. Again, you should not see more than 1 ohm of resistance.

 

[backinblue]

I understand that the water can act as a conductor, but the resistance will be high compared to the bonding wires. So if I connect one lead to the main zinc block on the transom where all the bonding wires are joined together, and then take the other lead and measure resistance to any bonded component on the boat (thru hulls, trim tabs, engine block, etc) I should measure very near zero ohms (effectively verifying that there is not a bonding wire that is not connected or high resistance.) I don't see how water could affect this because the resistance through the water will be comparatively high.

 

[Steve DAntonio]

While a 1 ohm or lower test in the water probably indicates a sound bonding system, in the world of professional corrosion analysis, bonding system continuity is always carried out ashore.

In addition to the water having an effect on continuity readings, a bonded and anode-protected system is creating voltage on the bonding circuit, the anode is driving the other metals negative. This can effect continuity readings. And, if stray current is present, that too can have an effect on a resistance measurement. Ideally, for best accuracy, you would not make continuity measurements on a circuit that is active in this manner.

But, don't take my word for it, see this form, second section, "Bonding Continuity Test" http://westernmarine.com/acrobat/procorrosion.pdf

Once again, testing each U/W metal fitting with the ref cell is a good way to identify a bonding system resistance issue. With a sound, low resistance bonding system, every bonded metal should have roughly the same reading.

 

[backinblue]

Agreed, thanks for your help

 

[DrissZougari]

you will need to make an adapter to have access to the 3 wires inside your shore power and a clamp ammeters that can measures milliamps.
first put neutral wire and line wires (white+Black) inside the clam meter, the reading should be less than 30mA to consider your boat okay. if the reading is higher that 30mA, then your boat is leaking current into the surrounding water.
If you pass the above test, next move the clamp over the ground wire only. the meter will read the stray current in the water from your boat, surrounding boats and the Dock.
good luck

 

[Steve DAntonio]

you will need to make an adapter to have access to the 3 wires inside your shore power and a clamp ammeters that can measures milliamps.
first put neutral wire and line wires (white+Black) inside the clam meter, the reading should be less than 30mA to consider your boat okay. if the reading is higher that 30mA, then your boat is leaking current into the surrounding water.
If you pass the above test, next move the clamp over the ground wire only. the meter will read the stray current in the water from your boat, surrounding boats and the Dock.
good luck

Just to be clear, this test has nothing to do with corrosion. 99% of corrosion is DC in nature. The shore cord plays a role because the green grounding wire can connect your boat to other boats, but this is an issue even of the power is turned off. Vessels with isolation transformers are immune from this problem, and those with galvanic isolators are resistant to the problem, but not immune.

While a "break out" adapter can be handy for corrosion analysis, for this test it's unnecessary. Clamping the meter around the whole cord should show zero if the vessel has an isolation transformer. If it does not, anything over 30 mA would trip an ELCI, and could pose a risk to swimmers, and it means there could be a fault aboard, but that alone is not definitive, as that leakage could be coming from another boat. Turning the pedestal breaker off will then determine if the leakage is from this or another boat, if the reading drops considerably when the breaker is turned off, the leakage is from this boat, if there is little or no change, then most or all of the leakage is from another vessel, about which there is nothing you can do.

The term "stray current" is usually associated with DC and corrosion. When measuring an AC power fault this is usually called "leakage current".

Many clamp meters don't have the resolution to measure tens of miliamps of AC leakage, if you are doing this test make sure the meter you are using can do this accurately down to single milliamps.

30 mA is the ELCI trip threshold, so when I see it over that (and it’s from your boat) I know an ELCI can’t be used, which is why I emphasize that number, then 100 mA in fresh for ESD (electric shock drowning), and 500 mA in SW for heat generation and fire potential. Again all reading generated from the boat in question.