Using Silver Cell to check for stray current

[Sea Lyon]

The last couple of years our Island Packet has been consuming zincs faster the it had been. Especially the zinc on the Maxi prop.

I purchased a Silver test cell from Zincs.Com.

Here are my results from first attempt at testing.
All thru hulls are bonded to the engine block.

All thru hulls measured -970 mV except engine intake at -258 mV
I assume must have bad connection on this bonding wire and will investigate.

The engine block shows -974mV

We tested with all DC off then turned on all equipment and got same results.

We then tested with all AC off. Readings dropped to -850mV when we turned main breaker on AC panel on. This is with all branch circuits off. Once i turn on branch circuits the readings return to -970mV.

The boat has 2 30 amp shore cords. Both with main breakers then going to rotary switch one feeding main AC panel the other goes to another panel suppling A air conditioner.

We also have a Whisper power generator feeding the rotary switch. It can power main panel and air conditioner.

Also a Magnum 2812 Inverter charger.

I tested with generator and inverter on and off with no change in the -970 mV . The only time it drops is when the AC panel main is on with no branch circuits on.

Any ideas on what's going on?

[smac999]

Sounds like you don’t have galvanic isolators. Or they are being bypassed.

Take a reading with boat not plugged in. This is your Baseline. Then plugged in main ac breaker off (double pole Breaking the neutral?) . Then ac breaker on. . Reading Should never change.

I would look at the magnum netreal wiring. The Input and output neutreals need to be separated. When the inverter does not have input power it will join the netreal to ground. Which will then bypass the galvanic isolator if there a neutreal connection to the dock and the output neutrals are not separated. Once you turn power (hot) to the magum it opens that bond and the galvanic isolator is no longer bypassed. Which is maybe the reason you see change with certain ac breakers.

Pretty common to see the inverter netreaul wiring wrong. (All common) That only causes the isolator to be byosssed for 30 secs each time the boat is plugged in. And Likly not causing zinc issues. That issue will blow gfci / elci. Breakers though.

[wrwakefield]

I can highly recommend this book regarding all aspects of corrosion we encounter- including anode considerations:

The Boatowner's Guide to Corrosion: A Complete Reference for Boatowners and Marine Professionals

It is my go-to volume on this topic- on par with Nigel Calder's works.

In case it is of interest.

Cheers! Bill

[Sea Lyon]

Originally Posted by smac999

Sounds like you don’t have galvanic isolators. Or they are being bypassed.

Take a reading with boat not plugged in. This is your Baseline. Then plugged in main ac breaker off (double pole Breaking the neutral?) . Then ac breaker on. . Reading Should never change.

I would look at the magnum netreal wiring. The Input and output neutreals need to be separated. When the inverter does not have input power it will join the netreal to ground. Which will then bypass the galvanic isolator if there a neutreal connection to the dock and the output neutrals are not separated. Once you turn power (hot) to the magum it opens that bond and the galvanic isolator is no longer bypassed. Which is maybe the reason you see change with certain ac breakers.

Pretty common to see the inverter netreaul wiring wrong. (All common) That only causes the isolator to be byosssed for 30 secs each time the boat is plugged in. And Likly not causing zinc issues. That issue will blow gfci / elci. Breakers though.

Thanks for responding, when I made post you are one of the people that I had would take a look.

We do have a Dairyland Galvanic Isolator, I checked it in December and it seemed to pass recommend test. It is a model with capacitors.

I did more readings with the Silver Cell the bonding system maintains a voltage of -.980 V with shore cords connected or disconnected seems to have no effect.

The voltage drops to about -.850 V for a few seconds when you cycle the main breaker. But then returns to -.980

It would appear that the boat is adequately protected???

I traced the power wires to the charger inverter. The neutrals both terminate on the neutral strip at the main panel. The power comes from the power bus of the main panel. the power feed from the inverter goes to main panel feeding the port and starboard outlets but not water heater.

Before leaving for the Caribbean we plugged into a marina in florida and tripped their new ground fault equipment. On investigation we found that as long as only one cord at time was plugged in we did not trip it.

I found that the wrong rotary switch had been installed separating the neutrals from each other. I replaced with proper switch now in generator setting the shore power hot and neutrals are all kept separate.

I have not been back to a marina with ground fault equipment since that change.

I also just put my DC clamp meeter on the ground wire at the Galvonic isolator. and am getting reading of .05 to .03 Amp . I assume that is too high????

Shut all AC circuits off and still get the same reading.

Thanks for taking time to read!

Bob

[Sea Lyon]

Thanks Bill , I just looked at that book online. I think it would held me. Will order as soon as we are someplace long enough to get it!

[smac999]

Originally Posted by Sea Lyon

The voltage drops to about -.850 V for a few seconds when you cycle the main breaker. But then returns to -.980
It would appear that the boat is adequately protected???

I traced the power wires to the charger inverter. The neutrals both terminate on the neutral strip at the main panel. The power comes from the power bus of the main panel. the power feed from the inverter goes to main panel feeding the port and starboard outlets but not water heater.

the neutral on the output should be going to it's own bus with the neutrals of the inverter loads. otherwise the galvanic isolator is bypassed during the first 15 secs of turning on the shore power. which it looks like you are seeing. that bypass will also cause issues with gfi / elci breakers.

see the 2 attached pictures I made a while ago. they are geared at tripling gfci's. but if you follow you'll see that during the inverter startup the boat has a path from dc ground to the dock ground on the white wire which bypasses the galvanic isloator on the green wire.

if it's done right the voltage will not change when cycling the breaker

as a mentioned before. ~15 secs (varies a bit by inverter, they will release the n-g bond only after checking the incoming power and the charging starts) of bypassing the galvanic isolator each time you plug in likely isn't causing big issues in zincs since it would not be doing much for long. so you may still have other issues.

but if it's like this you may want to fix for future gfic / elci breakers


you'd need a pretty high end special clamp meter to read 50ma dc. I would probably call that an error on a standard meter. at that low you are better off with a regular inline amp meter but you'd have to disconnect the ground wire to run through the meter to measure it.

if the boat voltage with the silver cell is not changing pluged in vrs not pluged in then no dc current is flowing. I would take that as more accurate then clamp meter reading

[Sea Lyon]

Originally Posted by smac999

the neutral on the output should be going to it's own bus with the neutrals of the inverter loads. otherwise the galvanic isolator is bypassed during the first 15 secs of turning on the shore power. which it looks like you are seeing. that bypass will also cause issues with gfi / elci breakers.

see the 2 attached pictures I made a while ago. they are geared at tripling gfci's. but if you follow you'll see that during the inverter startup the boat has a path from dc ground to the dock ground on the white wire which bypasses the galvanic isloator on the green wire.

if it's done right the voltage will not change when cycling the breaker

as a mentioned before. ~15 secs (varies a bit by inverter, they will release the n-g bond only after checking the incoming power and the charging starts) of bypassing the galvanic isolator each time you plug in likely isn't causing big issues in zincs since it would not be doing much for long. so you may still have other issues.

but if it's like this you may want to fix for future gfic / elci breakers


you'd need a pretty high end special clamp meter to read 50ma dc. I would probably call that an error on a standard meter. at that low you are better off with a regular inline amp meter but you'd have to disconnect the ground wire to run through the meter to measure it.

if the boat voltage with the silver cell is not changing pluged in vrs not pluged in then no dc current is flowing. I would take that as more accurate then clamp meter reading

Thank you for explaining, I am starting to see what's going on! When you say the neutral mis wiring can cause problems with gift/eici breakers are you referring to the marina installed equipment or on the vessel?

I wonderded about that small current being a meter error. My clamp on is a Fluke but its the most basic one that reads DC. The one they recommend for this kind of work is pretty pricy.

Thanks again we are making a series of passages then I will dig deeper.

[GordMay]

The range of protection, measured in negative millivolts is -550 to -1100mV.
The exact value will vary, depending upon the boat configuration. Wooden boats for example have a protection range of -550 to -600mV. An aluminum boat would have a range of -900 to -1100mV.

See ABYC E-2 Cathodic Protection, TABLE II - RECOMMENDED RANGE OF CATHODIC PROTECTION BASED ON AG/AGCL REFERENCE CELL [Page 17 of 20].
The imperative thing to remember is that these numbers from the ABYC standard E-2, are based on measurement in SALT WATER, at a specific speed, and temperature range.

There is one key statement, found in the ABYC E-2 standard that applies:
“A minimum negative shift of 200mV, relative to the corrosion potential of the least noble metal being protected, must be maintained.”

ABYC E-2 [2013 Old]* ➥ http://www.pcmarinesurveys.com/E-2%2...protection.pdf

* Thanks to CF member, 'boatpoker'