AC Grounding &/or Bonding

[Brian Degulis]

OK once again for what it's worth, an isolation transformer is used to isolate ground. This being said. The primary side of the transformer is grounded to shore ground, that is the only thing on the boat that should be grounded to shore ground. The secondary actually has no earth ground, ground and neutral are tied together as a reference for the or to the hot side. Ground, and neutral are tied together on the secondary (output side) to eliminate potential (voltage) difference between them. You should read voltage between the white/green to the black. A test to see if you are truly isolated, connect one side of your ac meter to earth ground coming from the pedistal and the other lead to to the hot side of any outlet inside the boat, if you get any significant reading you are not isolated. You will find most so called isolated boats will fail this test, because some doo doo conected ground to the primary side of the transformer, usually a installed not familiar with isolated circuits.

Remember when you read hot to neutral you are not truly reading to ground you are reading the difference between the two. Yes you will read voltage hot to ground as utlimately they are tied together at the power source. On the same token you can read voltage neutral to ground, if fact that voltage should be under about 1.4 volts, if it is higher you are probably eating zincs. Most marinas do not pull in neutral on the 120 pedistals, they do usuaally pull it in on the 220 outlets. This is the reason the larger boats using 220 power use the isolation transformers and your plugs are 4 conductor (2 hots, 1 neutral, one ground) instead of 3 on the 120 (1 hot, 1 neutral, 1 ground.

Brian I'll have to look into the thing about being grounded on the hard, I am not sure about it. If no power is hooked to the boat, I doubt it is needed although lightening protection could be a factor.

Ted, your comments still confuse me. Put a clamp on meter on the cord turn off power from the pedistal and read something? Please explain to me how you can get any current flow with a clamp on meter with only one conductor connected to something (in this case ground), for many reasons this does not compute, at least to me.

I agree we are beating this subject to deaf, but it's not all bad, hopefully we are all learning something about an important issue.

On this subject I don't offer up opinions, I try very hard to stick to the techinical facts.

I sincerely hope this helps everyone

Grounding on the hard to prevent a shock hazard is only needed when the boat is plugged in on the hard.

Testing for isolation by reading voltage from a boats hot to earth ground won't work. You'll always get a reading because your boats Iso secondaries are conected to sea water ground which is in fact earth ground. You may not read full voltage but your going to get a reading for sure. It depends on the salinity of the water and the distance between the shore panel ground rod and the water.

Brian

 

[Trojan]

Ted I like the Rifkin's test you presented. I think it's a good easy test. I will be at the boat Sat and I'm going to check my panel. Being away from the boat it's sometimes hard to remember just how things are wired for sure. When you have been in many panels you tend to expect that all panels are wired correctly and it may not be the case. The electrical panels in some 30+ year old boats leave something to be desired.
I have never checked but are the battery chargers output isolated? I know the DC ground and the green ground are tied together.
This has been a good read. I sometimes don't give all the information I should and take things for granted. I sometimes think that we are all on the same technical plane.

BILL

 

[TedZ]

Brian said:

"Grounding on the hard to prevent a shock hazard is only needed when the boat is plugged in on the hard.

Testing for isolation by reading voltage from a boats hot to earth ground won't work. You'll always get a reading because your boats Iso secondaries are conected to sea water ground which is in fact earth ground. You may not read full voltage but your going to get a reading for sure. It depends on the salinity of the water and the distance between the shore panel ground rod and the water."


Naturally you have to be plugged in to have any issue with voltage on the ground plane. This grounding of boats on the hard is not my concept, i have seen folks like Roger of Jarrett Bay and others discuss it. It seems to me it is a safety precaution anticipating improper bonding boat-side like appliances etc.

I am not following your second paragraph. Where is the measurement you discuss taken? What i understand to be a test for isolation, if that is desirable, is to measure continuity from ground on the pedestal to ground on the boat. If it is open, the grounds are isolated.

Forget about ground rods at a marina. They are likely hundreds of feet away and ground loops almost certainly develop which will give a differing reading from hot to ground depending on how far away you are from the main panel. The NEC requires regrounding with a certain distance away from the main panel but i don't know if that applies to marinas. Probably does but i have never done it because it is hard to drive ground rods when you are over water!

The concept of iso secondaries, the hot(s) and neutral, being tied to sea water grounds implies that the grounds and bonds are tied together AND apparently to the neutral? I don't think that is what you want but it is likely quite common.

Ted

 

[TedZ]

Craig said:

"Tying the secondary of an isolated system to the ships bonding or grounding system, defeats the reason for the transformer. Now the question becomes and I can see a discussion comming, should the the secondary on an isolated boat be tied to ships ground? My training and experience tells me it should not."

I couldn't agree more, that is why i said:

"The concept of iso secondaries, the hot(s) and neutral, being tied to sea water grounds implies that the grounds and bonds are tied together AND apparently to the neutral? I don't think that is what you want but it is likely quite common."

To go further, i don't think the boat side grounds and bonds should be tied together except when USING a genny or inverter.

Ted

 

[Brian Degulis]

Brian Posted,

Testing for isolation by reading voltage from a boats hot to earth ground won't work. You'll always get a reading because your boats Iso secondaries are conected to sea water ground which is in fact earth ground. You may not read full voltage but your going to get a reading for sure. It depends on the salinity of the water and the distance between the shore panel ground rod and the water.

Brian,

I agree totally with what you, and given that, one has to ask, is the boat truly isolated?

Tying the secondary of an isolated system to the ships bonding or grounding system, defeats the reason for the transformer. Now the question becomes and I can see a discussion comming, should the the secondary on an isolated boat be tied to ships ground? My training and experience tells me it should not.

That would go against all common knowledge and I don't think any builder does it that way. That would be a completly floating system and would be almost impossible to acomplish. It would mean that the housings on your air conditioning compressors would be floating (no wired ground) yet seawater is flowing thru them creating a crude seawater ground. I can't see how it would work. Isolation on a boat means isolated from shore supply not isolated from sea water ground. The main purpose is to prevent electrical leakage from another boat or dock from finding ground through your boat which will happen if your hard wire conected to shore ground and shouldn't happen if you aren't.

Brian

Brian

 

[Traveler 45C]

I put the clamp on meter on my shore power cord and read .5a on the AC scale. Switching to the DC scale the reading was higher. Which is the correct scale to use for this test?

I shut down the breakers at the dock panel and the reading did not change.

I'm not exactly understanding this stray current ckt path and the effect of shutting off the breakers. How are you able to determine the problem is on the dock by shutting off the breakers? I would think that the stray current is on the grounding wire but could it also be flowing on the neutral?

 

[Traveler 45C]

Hmmm, I don't think this is all that complicated...

The reading should have been zero even with all the systems turned on as they were. I took readings on a few different power cords and they all were zero.

I'm in salt water.

There is definitely a problem somewhere. I already know that I've got some Hatteras induced wiring issures (AC grounding) and my bonding ckts are not in the best of shape. A galvanic isolator would block this current and I'll get one installed soon.

I'll try to do some further troubleshooting on Sunday.

 

[REBrueckner]

Traveler: You can go to YANDINA.com and check their electrolysis testing procedure....I've recommended their site several times...any questions to their website used to elicit a prompt reply...these folks make isolators (or used to) for West Marine and are real boaters. All you need is a volt ohm meter and maybe an hour....

http://www.yandina.com/electrolysis.htm


Brian's post #57 reply is correct....but I will mention again that floating secondaries, meaning onboard ungrounded two wire systems have been used in the past...my 1961 wood Matthews had such an isolation transformer based system.....Nigel Calder also mentions them......

 

[Brian Degulis]

I am not following your second paragraph. Where is the measurement you discuss taken? What i understand to be a test for isolation, if that is desirable, is to measure continuity from ground on the pedestal to ground on the boat. If it is open, the grounds are isolated.

Forget about ground rods at a marina. They are likely hundreds of feet away and ground loops almost certainly develop which will give a differing reading from hot to ground depending on how far away you are from the main panel. The NEC requires regrounding with a certain distance away from the main panel but i don't know if that applies to marinas. Probably does but i have never done it because it is hard to drive ground rods when you are over water!

The concept of iso secondaries, the hot(s) and neutral, being tied to sea water grounds implies that the grounds and bonds are tied together AND apparently to the neutral? I don't think that is what you want but it is likely quite common.

Ted


If I'm going to test for anything I need solid parameters. If I have to wonder wether contiuity I'm reading is a fault in my isolation or a ground rod that's close to seawater( Like at my home) then the test is worthless.

Brian

 

[Brian Degulis]

I think we need to understand the purpose of using a transformer in a boat. My understanding of it is this.

The ability to use 120V shore cord and have limited 240V on the boat

The ability to boost dock voltage which is often low

To isolate and create a power source that is unique to your boat one that does not share neutral or hot with anything on shore and shares ground only to the extent that seawater and earth ground are common. The benifit to isolation is that an electrical leak from another boat or shore supply does not use your boat to find ground because your boat is not conected to shore ground. Your boat is working on a seawater ground only. This is exactly what your gen set does it has no conection to earth ground or any other power source.

If you agree with the above then what would be the advantage to isolating secondary neutral and seawater ground from each other. It's my understanding that the two are conected on land based supplies for safty purposes. With the neutral and ground conected at a single point neutral is always at ground potential.

Basicly on your boat your transfomer is your power company and your boat is the grid and end user. So IMHO follow the same guidlines as your power company does.

Brian

 

[Traveler 45C]

Trav,

I hope I am understanding you correctly. with power connected to the boat and something running you should read the current that is being used. What do you mean you should get nothing? What am I missing here.

Think about a 2 conductor power cord. If you were to clamp only one of the wires you would read the current through that wire. If you clamped both of them, your reading the positive and negative current at the same time, which is effectively zero (+1a + -1a = 0, for example).

If you've got one, clamp a 2 conductor power cord and see. If you have a 3 conductor cord, we know that the 3rd wire, the grounding wire is non-current carrying. Since we know the reading should be zero, any reading must be coming from the grounding wire and shouldn't be there. If it is, something is wrong.

Rob, I'm familiar with that website.

 

[TedZ]

I think we need to understand the purpose of using a transformer in a boat. My understanding of it is this.

The ability to use 120V shore cord and have limited 240V on the boat

The ability to boost dock voltage which is often low

To isolate and create a power source that is unique to your boat one that does not share neutral or hot with anything on shore and shares ground only to the extent that seawater and earth ground are common. The benifit to isolation is that an electrical leak from another boat or shore supply does not use your boat to find ground because your boat is not conected to shore ground. Your boat is working on a seawater ground only. This is exactly what your gen set does it has no conection to earth ground or any other power source.

If you agree with the above then what would be the advantage to isolating secondary neutral and seawater ground from each other. It's my understanding that the two are conected on land based supplies for safty purposes. With the neutral and ground conected at a single point neutral is always at ground potential.

Basicly on your boat your transfomer is your power company and your boat is the grid and end user. So IMHO follow the same guidlines as your power company does.

Brian

That is absolutely the way it is done now EXCEPT the power company doesn't put 60 or 70 VAC on a manufactured ground above the real ground potential of zero. That is what we are doing in most instances with an iso. Which is back to the beginning. As long as the ship's ground plane doesn't come in contact with the dock-side ground plane all is fine for 30 years! Get a swimmer in the water or a metal dock or hand rail in the rain and watch out.

Making 240 VAC out of 120 VAC great...half the ampacity but fine. Buck-boosting with xformers fine if you have the switches and many do. You just need to understand this is not without some risk - what isn't - i know. Many will say, "that's life with a xformer" and that is ok if that is what you accept. There are transformers that are wound in such a way as to not have neutral/ground secondaries that are above the real earth ground by more than a fraction of a volt or a few volts tops. If with a meter you read a voltage between the boat's bonded items and shore ground that is say 30 volts or more - meter on a boat's handrail if bonded and the green conductor in the power pole RECEPTICAL - then some thought has to be given to safety. That is all this is about.

Ted

 

[Brian Degulis]

Then you must see the same safty problems when running on the gen set Right?

I think you grossly underestimate the seawater ground and it's relationship to earth ground. They are the same most of the time or within a volt or 2. I read 0 volts between my bonding system and shore ground. The problems you describe simply aren't happening because the seawater ground is better than you think it is.


Brian

 

[TedZ]

Well make the measurement.

Don't tell me the secondary neutral is within a volt or two of the primary's neutral. It isn't right and you won't find one EE who will agree with you. If you are bonded on the ship's side correctly i.e. no neutral ground connectiuons, there will be no voltage difference ground to ground...couldn't agree more.

BUT as we have said ad nauseum, if you have ship side neutral-ground connections such as appliances present, then what you say is not correct and is a dangerous assumption.

If your boat is perfect in that regard, wonderful. Congratulations. Most are not.

Ted

 

[Brian Degulis]

On our boats the secondary neutral and seawater ground are conected at a single point in the panel. Seawater ground is at or very close to earth ground eliminating the problems you describe.

I notice you keep avoiding any comment at all as to why running an iso transformer is any different than running your gen set as far as no earth ground is concerned. I'd like to hear your thoughts on that?


Brian

 

[TedZ]

Sorry. Don't mean to avoid it i thought i was very clear that the genny and inverters bond neutral to ground and to not run them when plugged in at the dock.

If you are asking do gennys manufacture a neutral, they do not nor do inverters. These devices are primary supply units and the neutral and ground are supposed to be bonded together. Accordingly, there is the correct voltage between the legs and neutral/ground. I install BIG generators and i always ground them to mother earth. If it were a problem i would have pyrotechnics!

Unless you get a transformer that has for example, a 120 VAC primary and two 120 VAC secondaries to form the 240 VAC secondary, then the neutral is going to be arbitrary with respect to the primary's neutral. It depends on how it is wound and likely no two are exactly the same. Ask a transformer manufacturer if there is any question on this issue.

Hope this answers the question.

Ted

 

[Brian Degulis]

You seem to think that neutral from the Iso secondary is not bonded to ground. On our boats Neutral from the Iso secondary is bonded to seawater ground the exact same way our generators are. That's why there is very little or no diffrence in potential between shore neutral and ground and the boats neutral and ground. That's why no one is experiencing the problems you speak of. There is no avoiding being in contact with earth ground and our boats ground. Our railings are bonded only to the boats seawater ground and of course anything on the dock like the power pedastal is bonded to earth ground. If things actually were as you suggest we would being getting shocks all the time in fact most of us would probably be dead by now.


Brian

 

[TedZ]

Well it seems to me that Brian and Craig need to get their thoughts resolved.

Brian says his neutral is bonded to the "sea water" ground, i.e the ships bonding system.

Craig says bonding the iso secondary in any way defeats the isolation. He feels that isos are 1:1 devices.

I guess those two smart and respected boaters' posts tell the story of isos and how little they are understood.

First, isos are not necessarily 1:1 devices. They have taps like most transformers for varying voltages.

Second, the assumption that a sea water ground is the same potential as a dock ground is not true nor is there any guarantee that it is ever true. Sea water is resistive just like fresh water but less so.

This is crazy. Measure it and report back here. So simple. Open your iso and check the neutral to neutral voltage, i.e primary's neutral to secondary's neutral. To be totally accurate, you should disconnect the secondary neutral as it could be being sunk by various grounding philosophies but try it the simple way first.

If there is a voltage difference then decide where it is being dissipated if it is, or where it exists ship side and what is energized with this voltage.

If you have an iso it is the only responsible thing to do.

Good luck.

Ted

 

[Brian Degulis]

I'm not talking theory here or hypothetical's our secondary neutrals are bonded to seawater ground. I don't have to check the Iso neutral against shore neutral I'm not disputing that they may be different. I have checked my boats seawater ground against shore ground and they're at the same potential. As I said before If they weren't we would all be getting shocks on a regular basis simply by grabbing a rail on the boat and a shore ground. I think this shows how a well respected EE gets so immersed in theory that simple truths and realities are ignored.

Brian

 

[Traveler 45C]

I think you guys all have your wires crossed

I wasn’t able to do any testing yesterday as I planned; other repairs had to get done. But I did find this pic.

It pretty much says it all. It clearly shows the isolation xfmr secondary’s neutral tied to ground (the ship’s ground plate, in the water) as it should be. The generator should be wired the same as the xfmr. Notice the inverters output neutral is also tied to ground. That connection is made when the inverter is the primary power source. When not, that connection is switched out*. The bonding ckt should be tied to ground also. Note if no iso xfrm is used and operating from shore power, neutral and ground should not be tied together on board.

The article from which I pulled the pic can be found here:
Grounding and Circuit Protection for Inverters and Battery Chargers
http://www.jamestowndistributors.co...Protection+for+Inverters+and+Battery+Chargers

Here are some other good reads:

Preventing Hazardous Ground Faults on Boats
http://www.jamestowndistributors.co...e=Preventing+Hazardous+Ground+Faults+on+Boats

*Neutral (White) to Ground (Green) Bond Switching
http://www.jamestowndistributors.co...tral+(White)+to+Ground+(Green)+Bond+Switching