AC Ground Leak - Measuring

Paul Mermelstein · Jun 9, 2015

I am not an expert, but I have a couple of thoughts. What happens if you disconnect the boat from shore power, and run a short jumper from the green on the power cord to the green on the pedestal? Any current there would indicate that you could indeed be the ground path for some external problem (either another boat or the pedestal itself).

It might be a good idea to see if the current is DC. That could indicate a galvanic current path between boats.

coolista · Jun 9, 2015

Important! That you are another's ground might not be too off target, but the condition suggests the entire dock system might have a 'ground plane' that floats. If true, this is a serious condition that can injure or create fires.
Please ask your electrical service provider to verify his grounding rods are making sound connection to the true earth - usually by wiggling them a bit or driving them farther into the ground with one or two strokes of a hammer. If his system ground is to pipes inside his building, make sure the clamping yoke is making pure contact with the pipe.
Then, make sure there is no corrosion between those ground rods/pipe and your boat by opening each intermediary electrical panel (presume one-to-three, depending on dock layout). Inspect grounding circuit for visible corrosion, then tighten each panel's set of ground circuit screws - just a bit of pressure to the screwdriver to make sure no corrosion is hiding in the screw yokes. It may seem trivial but this check takes little time/effort, and a floating ground can be seriously dangerous to you and is far more readily a risk to your electronics. Resume your on-boat testing.

Bill Roosa · Jun 9, 2015

Since you can't measure DC with a clamp on type (least not an inexpensive one) I'd rule out DC leakage from your boat. The simple test to see "which way" the 15 mA is going is to disconnect the shore power at the dock and run a wire to the sea and measure the voltage between the plug and the water. then do the same thing with the boat side of the cable. One should show a small voltage and that would be the source of the current.
AND BE SURE YOU ARE ONLY TESTING THE GROUND when you test the dock side!!!!

walt · Jun 9, 2015

- Clamp on Black/White (Hot/Neutral) = 0mA
- Clamp on Green only = 15mA

This was done with ALL branch circuits turned off.

To me this tell me that whatever current is flowing on the Hot/Neutral, it is equal and cancelling out (or there is none). The 15mA flowing on the ground must be sourced from outside the boat?

On your boat at least in the state you measured, all the current on the hot = all the current on the nuetral (black /white shows no current). Therefore, your AC system is NOT leaking to the green ground.

However, you still have an active DC (ie, 12 volt) system on your boat that is also likely connected to the green AC wire (DC ground likley connected to green). It is possible that the DC system using its energy storage could be injecting current into the water and using the green earth wire as the return path.

What you would also need to do is to completely disconnect your batteries on the 12 volt side (you can leave the grounds all connected) so that you are sure there is no active 12 volt anywhere on the boat that could lead to the water. If you remove the possibility of your boats 12 volt creating the green wire current and you still can measure the green wire current, then this means that the marina green wire has a voltage on it when it should not.

Note.. hall effect sensors often used in current clamp meters do measure DC current.

The green wire is safety only so no where in the marina should the green wire have any current. But... if someone wired an AC system incorrectly and put current on the green wire, a voltage develops simply because of ohms law (the wire itself is the R). If you are on this green wire net and it has a potential, it would show up like you measured - and of course be a source of corrosion unless you have some sort of isolation (like a diode or transformer isolation).

Note that if someone is wrongly running current on the green wire (like using the green instead of the white), if none of that current is going into the water, a clamp on amp meter around the extension cord will NOT find this. The reason is that the sum of the current into the cord still matches the current out. It is only when you leak current into the water that the clamp on meter around the whole cable finds a problem. If the dock has an ELCI/GFCI plug, it wont allow you to put current on the green wire in the first place.

uncledom · Jun 10, 2015

walt said:
On your boat at least in the state you measured, all the current on the hot = all the current on the nuetral (black /white shows no current). Therefore, your AC system is NOT leaking to the green ground. However, you still have an active DC (ie, 12 volt) system on your boat that is also likely connected to the green AC wire (DC ground likley connected to green). It is possible that the DC system using its energy storage could be injecting current into the water and using the green earth wire as the return path. What you would also need to do is to completely disconnect your batteries on the 12 volt side (you can leave the grounds all connected) so that you are sure there is no active 12 volt anywhere on the boat that could lead to the water. If you remove the possibility of your boats 12 volt creating the green wire current and you still can measure the green wire current, then this means that the marina green wire has a voltage on it when it should not. Note.. hall effect sensors often used in current clamp meters do measure DC current. The green wire is safety only so no where in the marina should the green wire have any current. But... if someone wired an AC system incorrectly and put current on the green wire, a voltage develops simply because of ohms law (the wire itself is the R). If you are on this green wire net and it has a potential, it would show up like you measured - and of course be a source of corrosion unless you have some sort of isolation (like a diode or transformer isolation). Note that if someone is wrongly running current on the green wire (like using the green instead of the white), if none of that current is going into the water, a clamp on amp meter around the extension cord will NOT find this. The reason is that the sum of the current into the cord still matches the current out. It is only when you leak current into the water that the clamp on meter around the whole cable finds a problem. If the dock has an ELCI/GFCI plug, it wont allow you to put current on the green wire in the first place.

If there is a DC leak you won't read it on an A/C clamp meter.
If there is an ELCI/GFCI device and an isolation transform, all load on the secondary side will look like a balanced load on the primary side.

Maine Sail · Jun 10, 2015

walmsleyc said:
Our Club has started testing members shore power cords, and have also done in water testing for AC leakage problems recently. They have been using a clamp on Ammeter for basic testing.

My boat showed a reading of 15mA at the pedestal, and at the boat, and at the AC panel. I investigated a little further, and found the following when using the clamp at my AC panel on the main AC supply to the panel:

- Clamp on Black/White (Hot/Neutral) = 0mA
- Clamp on Green only = 15mA

This was done with ALL branch circuits turned off.

To me this tell me that whatever current is flowing on the Hot/Neutral, it is equal and cancelling out (or there is none). The 15mA flowing on the ground must be sourced from outside the boat?

Is my boat a ground path for someone else's leak? What other explanation could there be?

I need to do more detailed testing, but I also tested while turning each branch circuit on. None of them made a difference to the readings except the hot water heater, which caused a jump in the reading (Hot/Neutral/Ground) to ~50mA (don't remember if I tested ground current separately, but will do that).

It is noted on the internet that water heaters can be a common problem with AC leakage, but there isn't much info on exactly why. Would it be the heating element leaking to the fresh water, and then to the case/ground?

I would appreciate any thoughts/advice.

Chris

Chris,

Unless you were using a Fluke 360, a Yokogawa 30031 an Extech 380942 or similar to these three, a 15mA reading means little due to the poor accuracy of most clamp meters in this range. Best to disconnect and insert a DVM in series for any sort of accuracy because I am pretty sure you don't want to spend $350.00 - $700.00 on an AC leakage clamp. If your marina is using typical clamp meters they too are getting misleading results...

Also any measurement of current imbalance really needs to have current flowing. Testing your hot & neutral with no loads will not tell you much without current flowing, unless of course you have a "leak". Course you still need an accurate leakage grade clamp meter to do this, if trying to use a clamp..