Gerry
Well-Known Member
Having raised a thread on the acceptable level of earth leakage in a marina a couple of weeks ago, I promised to share the marina's reply:
“I understand that there have been questions about the leakage current in the marina settings for the ground fault detector and possible damage from electrolysis.
I just talked to Jim Shaffer about all these issues. Jim is probably the industry expert on these subjects, has the best data base and understanding of the problems involved. He is currently working under US government grant to set the industry standards for these devices, ground fault monitors and ground fault detectors for the US coast guard.
The primary source of confusion that arises when reading the Marina Guard document “Electrical Safety in Marina” (16 pages) is between the table on page 4 thats states that 2 to 10 milliamperes will produce a “slight to strong muscular reaction” and 5 to 25 milliamperes will produce “strong shock, inability to let go”. This sounds pretty much like death for a swimmer. The seemingly conflicting statement on page 13 is “please note that in most applications, the Marina Guard is set to 10 amperes fault current”. This level of current (10 amperes) is 400 times what is stated above to cause “inability to let go”, or sure death.
Jim Shaffer was able to explain this difference or seeming inconsistency. He says that there can be 5 amps of current flowing from a boat into the water and a swimmer nearby will only feel a tingle, no danger. The conductivity of the sea water is absorbing a lot of current from the bonded electrical system on the boat and very little passes through the body of the swimmer.
In fact, in doing research he has recorded 44 cases of electrocution in a marina-type setting. Of these cases, 44 were in fresh water and none... none were in salt water. Fresh water, ironically, is more dangerous because then the person is then the conductor of the current!
What about the case where the swimmer is cleaning his prop and touches the prop shaft, for instance? In this case the majority of the current passes directly into the sea water even though the body fluids are similar to seawater THE INSULATING PROPERTIES of the skin isolate the swimmer from the harmful current. Jim refers to an experiment connecting 110 VAC to a glass of salt water and putting your finger in it and not feeling more than a tingle. If there is a cut the connection to the body fluids is more direct and the sensation is pain at the break in the skin.
This latter situation is a concern, Jim says that anyone who is working on a boat from in the water would be crazy to not disconnect the boat from shore power first.
I described the results from the recent installation of the Ground Fault Monitor on Dock 3. I recorded the readings as power was connected to the Power Pylons. The readings were about 1 to 2 milliamperes on three of the circuits and less on the rest. When all circuits were energized the reading was 3 milliamperes. Jim says that this is a very good level much than he has seen in other marinas where 600 milliamperes is not uncommon.
In light of the above it can be seen why, settings of 500 milliamperes to 5 amps are acceptable. I left the unit set at 40 milliamperes well above the 3 milliamperes to avoid false tripping and well below th 500 miliamps t o 5 to 10 amps Bender recommends.
About electrolysis. Jim Shaffer says that the only boats that he has seen have problems are aluminium boats or power boats aor sail boats with aluminum sail drives, our out drives or outboards. Then this only occurs with currents of several amps and over a period of weeks. The low level of current in our marina is well below this, more than 100 times less.”
“I understand that there have been questions about the leakage current in the marina settings for the ground fault detector and possible damage from electrolysis.
I just talked to Jim Shaffer about all these issues. Jim is probably the industry expert on these subjects, has the best data base and understanding of the problems involved. He is currently working under US government grant to set the industry standards for these devices, ground fault monitors and ground fault detectors for the US coast guard.
The primary source of confusion that arises when reading the Marina Guard document “Electrical Safety in Marina” (16 pages) is between the table on page 4 thats states that 2 to 10 milliamperes will produce a “slight to strong muscular reaction” and 5 to 25 milliamperes will produce “strong shock, inability to let go”. This sounds pretty much like death for a swimmer. The seemingly conflicting statement on page 13 is “please note that in most applications, the Marina Guard is set to 10 amperes fault current”. This level of current (10 amperes) is 400 times what is stated above to cause “inability to let go”, or sure death.
Jim Shaffer was able to explain this difference or seeming inconsistency. He says that there can be 5 amps of current flowing from a boat into the water and a swimmer nearby will only feel a tingle, no danger. The conductivity of the sea water is absorbing a lot of current from the bonded electrical system on the boat and very little passes through the body of the swimmer.
In fact, in doing research he has recorded 44 cases of electrocution in a marina-type setting. Of these cases, 44 were in fresh water and none... none were in salt water. Fresh water, ironically, is more dangerous because then the person is then the conductor of the current!
What about the case where the swimmer is cleaning his prop and touches the prop shaft, for instance? In this case the majority of the current passes directly into the sea water even though the body fluids are similar to seawater THE INSULATING PROPERTIES of the skin isolate the swimmer from the harmful current. Jim refers to an experiment connecting 110 VAC to a glass of salt water and putting your finger in it and not feeling more than a tingle. If there is a cut the connection to the body fluids is more direct and the sensation is pain at the break in the skin.
This latter situation is a concern, Jim says that anyone who is working on a boat from in the water would be crazy to not disconnect the boat from shore power first.
I described the results from the recent installation of the Ground Fault Monitor on Dock 3. I recorded the readings as power was connected to the Power Pylons. The readings were about 1 to 2 milliamperes on three of the circuits and less on the rest. When all circuits were energized the reading was 3 milliamperes. Jim says that this is a very good level much than he has seen in other marinas where 600 milliamperes is not uncommon.
In light of the above it can be seen why, settings of 500 milliamperes to 5 amps are acceptable. I left the unit set at 40 milliamperes well above the 3 milliamperes to avoid false tripping and well below th 500 miliamps t o 5 to 10 amps Bender recommends.
About electrolysis. Jim Shaffer says that the only boats that he has seen have problems are aluminium boats or power boats aor sail boats with aluminum sail drives, our out drives or outboards. Then this only occurs with currents of several amps and over a period of weeks. The low level of current in our marina is well below this, more than 100 times less.”
