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DC Earth Fault
junior
Posted: Thursday, October 22, 2009 9:44 AM
Joined: 14/01/2009
Posts: 1026


DC Earth fault.....Maybe one of you engineers have a suggestion. I'm an aluminum yacht moored for the winter, fender to fender, against a steel motor yacht. Yesterday the next door neighbor crew were draining the day tank and asked if I had a portable fuel transfer pump. I do, so I lent it to them and climbed into their machine room to lend a hand. We set up the pump, I alligator clip the pump into a handy 24v dc source and off we go. Pumping away and one of the crew accidentally knocked the pump causing a powered gator clip to touch the aluminum floor plates and ZAP !!!...that clip welded itself to the floor plate. This steel yacht has a serious DC earth fault. To keep my yacht from turning into an alka seltzer tablet I discreetly slipped ashore , disassembled my shorepower plug and disconnected earth. Is there any thing else I can do to protect myself against the yacht next door ?
kdhguard00
Posted: Thursday, October 22, 2009 10:18 AM
Joined: 16/09/2008
Posts: 31


I'm not your engineer and I'm curious to hear a solution, but when you say you "disassembled my shorepower plug and disconnected earth" are you saying that you propose to run off your generators all winter?  Of course, that's not unheard of or uncommon, I'm just not sure I'm understanding you correctly.

English Andy
Posted: Thursday, October 22, 2009 11:47 AM
Joined: 17/09/2008
Posts: 93


In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the earth.

Electrical circuits may be connected to ground (earth) for several reasons. In mains powered equipment, exposed metal parts are connected to ground to prevent contact with a dangerous voltage if electrical insulation fails. A connection to ground limits the voltage built up between power circuits and the earth, protecting circuit insulation from damage due to excessive voltage. Connections to ground limits the build-up of static electricity when handling flammable products or when repairing electronic devices. In some telegraph and power transmission circuits, the earth itself can be used as one conductor of the circuit, saving the cost of installing a separate return conductor.

For measurement purposes, the earth serves as a (reasonably) constant potential reference against which other potentials can be measured. An electrical ground system should have an appropriate current-carrying capability in order to serve as an adequate zero-voltage reference level. In electronic circuit theory, a "ground" is usually idealized as an infinite source or sink for charge, which can absorb an unlimited amount of current without changing its potential. Where a ground connection has a significant resistance, the approximation of zero potential is no longer valid. Stray voltages or earth potential rise effects will occur, which may create noise in signals or if large enough will produce an electric shock hazard.

The use of the term ground (or earth) is so common in electrical and electronics applications that circuits in vehicles such as ships, aircraft, and spacecraft may be spoken of as having a "ground" connection without any actual connection to the Earth.

So let's explore circuit ground versus earth ground...........................................

Voltage is a differential quantity i.e. the difference between two distinct and separate areas. To measure the voltage of a single point, a reference point must be selected to measure against. This common reference point is called ground and is considered to have zero voltage. This signal ground may or may not actually be connected to a power ground. A system where the system ground is not actually connected to another circuit or to earth (though there may still be AC coupling) is often referred to as a floating ground - which is quite common in modern day yachts; as the supply voltage from a marina can wander up and down in value (+ or minus 10% roughly) without causing too much damage to internal yacht electrical components.  If the supply voltage value drops to much (below 195v) or goes above 265v on a 230v ac circuit, then things tend to "get fried!".  8(

For Junior:  The best way to protect against the "battery effect" between hulls is to offer up additional lead anodes connected electrically to your internal earthing straps - don't forget to con check them!  The wire is then fed out through any suitable opening (think fire, think flood!) so the anode lead may be connected before it is suspend in the water between the two yachts.  Obviously the closer these anodes are to the other hull, will reduce the amount of electrons that are absorbed by your hull, so limiting the amount of corrosion.  I don't know the size of your yacht, but I would suspended these approximately every 5 - 10m (can be bonded to one earthing point!) and as close as possible to the other yacht for max effect ...... yes, it does work!  However, the thing I would really be worried about is their batteries discharging through this earth (shorting out) which would lead to a huge electrical fire onboard the other yacht.  If you moved berths, that would be a neater solution altogether, so long as you're upwind of the other vessel! 

Now I've finished offering engineers advice...............I'll return to studying for my MCA Master 3000 ticket! LOL 


Chief
Posted: Thursday, October 22, 2009 11:53 AM
Joined: 02/06/2008
Posts: 341


Which clip welded itself to the deck? If it was the + then that boat has a problem. If it was the - then the boat has a negative grounded DC system rather than an isolated system and there is nothing wrong with that as long as the hull is not used as a return path for DC circuits. When your clip was welded and the pump was running that was not good because it used the hull as a return, but it did not indicate any problem with the system itself. On the other hand, removing the ground connection from your boat to the shore power source may have serious and potentially fatal results if you have an AC ground fault and some poor crewmember, diver, or dockwalker touches the hull or swims near the hull. If you are concerned about impressed current corrosion, hang a bunch of zincs over the side by a wire bonded to your hull. But in the meantime, replace that shorepower ground before something really bad happens and I don't mean corrosion. Just for grins, go look at your own generators or main engine starting system. Do you have isolated starters with both battery leads on the starter or is the negative lead bolted to the crankcase? Take a meter and read the voltage between the battery positive and the hull, got anything? You may or many not have the same DC grounding system as your neighbor. Check your electronics as well, particularly those with antennas.
junior
Posted: Thursday, October 22, 2009 1:45 PM
Joined: 14/01/2009
Posts: 1026


Sorry, by earth fault I was not indicating its polarity, simply electricity on the hull. The neighbor is DC negative on hull. The yacht I am on is 100 percent isolated, all equipment, engines , dc motors, sensors, floats free of the hull, no DC current touches earth. NONE. At present I have a small positive DC leakage on my hull that I suspect is coming off the carbon brush dust in a well used DC motor. When I connect shore power to earth, I am connecting my boat to the next door neighbors with a copper cable...I inherit his fault ? Do I ??? I'm not a electrician, but my understanding is that all ac equipment on the yacht, by building code, is grounded to earth so that when I'm running on generator AC, any short is transmitted to earth " sea water" via this ground wire and not my body. The earth connection on the dock box, as I understand it, is simply to protect the dock box, plug, and short link before the induction transformer. When I remove earth, I do indeed create a danger to anyone acting as a ground when handling this cable and its associated components. Of course, when the yacht is standing free, out of sea water, on the shipyard hardtop, she has no natural sea water earth, so the earth connection at shore power box is critical and must be utilized to prevent a workman from acting as the discharge conduit. . As to hanging extra Zinc, I'm also wary of this. Anodes are placed on a yacht to create a current. Adding to this current with additional zinc will unbalance the calculated loading and is to be avoided without profession advice. From what I understand the yacht with the under zinced electrical field must add anodes, not it neighbor. Again, I'm an amateur at this. Perhaps I should give McDuff a call for clarification .
Chief
Posted: Thursday, October 22, 2009 2:06 PM
Joined: 02/06/2008
Posts: 341


Yes, if your hull is connected to the shore power safety ground as it should be you may inherit your neighbor’s problem. That is if he has a problem. Why do you believe he does?

 

You can install a galvanic isolator on the shore power ground to prevent DC current from coming onboard uninvited. I wouldn’t waste my time or hull material debating whose responsibility it is to hang more zincs. You perceive a problem that may not even exist and have created a lethal hazard in response, who buys the zincs is the least  problem here.

 

It is a pity the previous contributor posted all that stuff about earths and grounds, it just made a bigger hash of the confusion many people have about grounds and faults and earth grounds and ground faults and now even short circuits. Please contact McDuff, this is not the place for a basic electricity class. And before you do anything else, reconnect that safety ground connection! You are exposing anyone near the yacht to a lethal hazard and yourself to a lawsuit at best and a possible jail sentence. And that my friend is professional advice.


junior
Posted: Thursday, October 22, 2009 4:28 PM
Joined: 14/01/2009
Posts: 1026


A Galvanic isolator, on earth, sounds logical cheap and easy so Ill inquire in the morning. I assume an islolator works like a Diode. As to why I think the neighbor is HOT, well Ive never seen a yacht whose DC system resembles an automobile, with the chassis used As a conductor common ground.. That My neighbor had Enough DC negative amperage on the floor plates to weld the DC positive gator clip down indicates to me that he has a major fault or this motoryacht is, just as I suspected , simply a Winnebego with three decks . I have no problem with anodes, again cheap and easy if they work. If they upset the designed electric field of the anode system, MORE anodes can be just as dangerous as LESS. No technical adviser was available when I contacted McDuff...Ill try again. Oh and presently I'm trying to get to the bottom of this...HOW DO I ISOLATE my boat from the neighbors , with phone calls to electrical contractors in three different languages...head is spinning.....last guy sounded like he spoke Italian with an Indian accent........Call center ????
Chief
Posted: Thursday, October 22, 2009 6:10 PM
Joined: 02/06/2008
Posts: 341


A galvanic islolator uses several diodes. They “disconnect” the safety ground until just over a volt is present on the safety ground then they conduct and you have a ground connection.

 

Just because the DC negative is grounded to the hull does not mean it is a “common ground.” Unlike a car, the hull is not used as the current path in any circuit. Like I wrote earlier, this is not a good place to hold a basic marine electricity class so I won’t go any deeper. Just realize that since the negative on that boat is connected to the hull, the hull did conduct enough to power your DC pump, or at least create a shower of sparks. I must assume that it was the negative clip that fell off. The positive was still connected and the circuit was made through the deck plates and hull. Using the hull as a return is not an acceptable way to power a device on a boat but it doesn’t mean there is anything wrong. That boat doesn’t have a major fault and it is not a bad design. You are getting all concerned for nothing.

 

 If it was the opposite, and it was the positive clip that fell off and hit the deck, check to see if your pump is one of those portable reversible things that you change pump direction with a switch or by simply changing the polarity of the power connection. By your description I believe it is. Was it really the positive connection or was it just a red colored clip?

 

Unless that deck plate was electrically isolated from the hull and had a  +DC connection to it and nothing else, a fault of the severity required to weld the clip would have already started a fire somewhere else or blown a fuse. I really do suspect that when the clip hit the deck it sparked and scared the onlookers who then got all upset and imagined the worst.

 

 

You are overly analyzing the zinc thing as well. If you have a DC leak from another boat via the water and your hull is properly grounded via the shorepower ground, the extra zincs will go away, not your hull. There is not as much science in zincing a boat as you might wish to believe. It is more art and experience on the part of the designer and builder than careful evaluation of the incalculable. The last thing it is is dangerous.  Nobody will get hurt and your hull won’t dissolve around you. If you have an impressed current cathodic protection system there may be issues with getting out of range readings, so check with the maker. But disconnecting the shorepower safety ground may well kill someone if you have an AC ground fault.


junior
Posted: Thursday, October 22, 2009 6:34 PM
Joined: 14/01/2009
Posts: 1026


Ok, perhaps Im just thinking of the thousands of little bits of dissimilar metal hovering in the kilometers of electically charged electrolye ,sea water, plumbing running around the yacht. Gee and another thing Id like to understand is ... the standard wiring diagram of an Induction transfomer has no physical electrical connection to the yacht ...only induction, yet I can read conductivety from the earth pin to the hull. a simple drawing would be.... http://www.smartgauge.co.uk/galv_tran.html Am I missing something. I always understood the shorepower earth conductor as a protection against inbound faults or a fault in the induction transformer itself .
Chief
Posted: Thursday, October 22, 2009 7:13 PM
Joined: 02/06/2008
Posts: 341


 There isn't much more I can tell you except to take a course or hire a good electrician to come onboard and tell you what you have.

I don't know if you have a shorepower isolation transformer or not. I do know that all the transformers on your boat are induction transformers ... that is how they work, induction. They may or may not be isolation transformers.

As far as galvanic corrosion goes, you don't have to worry about the neighbor's DC crawling up your heat exchanger.  Like most of the internal bits you're worried about, they are quite happy to dissolve themselves. Those little cylindrical zinc thingies on the end of the little brass plugs that never get checked are there to slow down that process.


junior
Posted: Thursday, October 22, 2009 7:29 PM
Joined: 14/01/2009
Posts: 1026


Yes indeed..the shore power transformer is an isolation transformer...and still crawling around the lazarette with a voltmeter and flashlight....have to ponder it again in the daylight. Good news is that I Dont see any bubbles forming around the neighbors waterline...I look safe for another day
benjaminfisher
Posted: Thursday, October 29, 2009 3:25 PM
Joined: 10/05/2008
Posts: 21


You should have a Capac monitor on board an aluminium vessel to measure the potential difference between the hull and the water.

I would recomend that you never dock an aluminium vessel next to a steel vessel. I have moved slips in the past to avoid the potential disaster of having the bottom fall off.

Also, a good marine electrician can take measurments at your slip to make sure that the dock or anther vessle is not going to create problems for you. I would recomend doing this in any slip you will be staying long term.

Extra zinc is always a good idea, but will get expensive if you stay where you are.

Good luck!


junior
Posted: Thursday, October 29, 2009 6:40 PM
Joined: 14/01/2009
Posts: 1026


Hi Ben. I never heard of CAPAC ? . What does it do ? I do have a very sensitive DC ground fault detection circiut monitoring both the dc plus and dc negative poles. This yacht is aluminum and 100 percent isolated at the moment. As far as lying alongside a steel boat. Steel and aluminum like each other...they are closer together on the scale of dissimilar metals. You would not be happy alongside a copper bottomed yacht. Steel is friendly. On an aluminum yacht, whenever you have a critical mechanically fastened joint or flange like a tank top that may be wet, submerged in bilge water, use steel fasteners, not stainless steel. . Besides, Very very little metal is exposed underwater to electrolyte on a yacht... Yachts are heavily insulated from sea water with epoxy filler and epoxy barrier coat systems. there is virtually no 'wet ' aluminum or any metal on this yacht. I have no exposed propeller shaft because the yacht runs a fully enclosed oil bath system. Thru hull stand pipes and flanges are plastic lined. Even the Bronze propeller is fully encased in epoxy primer. A critical task for an anode system is to protect the this underwater paint system What always worries me about an electrically leaking yacht next door plastic, steel whatever, is what it does to all the wet systems inside the yacht. Some systems I can preserve by flushing out the sea water electrolyte and preserving with anti freeze. Some systems like complex stainless steel bilge plumbing manifolds I cant isolate because I need them. Crevise corrosion in stainsteel welds is a bummer to fix.
 
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