Electricity: electrical systems and devices – Safety and protection of systems and devices – Ground fault protection
Reexamination Certificate
1999-09-13
2004-04-20
Jackson, Stephen W. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Ground fault protection
C361S045000, C361S079000
Reexamination Certificate
active
06724589
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a system for protecting boat's electrical systems and personnel, and prevents degradation to both noble and sacrificial elements on the boat due to both Alternating Current (AC) and external Direct Current (DC) electrical effects. It also protects other boats and personnel at the marina from electrical problems arising within the protected boat. In all of these cases, protection is provided against potentially lethal electric shock to personnel, as well as uncontrolled currents in the marina, boat, and surrounding water that contribute to severe electrolytic corrosion, or externally induced galvanic action.
BACKGROUND OF THE INVENTION
It is normal practice to provide power sources for boats at marinas, which commonly is a 3 or 4 wire AC system for the electrical devices on the boat, including battery charger, lights and other auxiliary devices.
It is also common practice to provide protection for the hull, conductive fittings, paint, and other conductors in and on the boat against corrosion that is commonly called DC galvanic action, which arises, for example, when a metal boat and a dissimilar metal such as the propeller are both in the aqueous, particularly salt seawater solution.
However, problems have arisen concerning these AC and DC boat systems. In many cases the AC power source at the marina is not properly wired and may have power, neutral, and safety ground lines interchanged (commonly called reversed polarity). In many cases the AC system on the boat is not properly wired, or has AC leakage to the boat ground. These problems cause leakage or fault AC currents, which can be life threatening to boat and marina personnel, and cause serious harm or failure to underwater parts and electrical devices on the boat.
The DC systems provided to overcome the galvanic action provided on the boat are frequently interconnected with the AC safety ground. Leakage currents through this path result in accelerated corrosion. AC voltage on the AC safety ground represents a serious electrolysis and safety problem for all boats, marina apparatus and personnel.
These effects cause four types of damage:
First, this interconnect causes all underwater protective active elements (typically zinc's) and the more noble underwater metallic parts of all the boats with a DC ground connected to the AC safety ground wiring in the entire harbor to be hooked together electrically. The result is a very large area electrolytic corrosion network, making the entire harbor into a much larger wide-spread area battery which may rapidly deplete the more active elements. This is caused by the combined effect of galvanic corrosion from all boats.
A second form of damage is electrolytic corrosion caused by introducing harbor AC electrical power commonly connected into the DC ground systems of boats. This arises due to improper wiring, leakage, and AC return line voltage drop that is inducing low levels of AC voltage into the common AC shore power safety ground. This can result in depletion of both the active elements, and the more noble underwater elements of a boat. This type of damage is particularly insidious, because it attacks all through hull fittings, corroding and eventually destroying them, so the boat can easily be subjected to flooding due to failure of the through hull fittings. This phenomenon has reportedly caused unprotected or unmanned boats to sink at the dock. Even wet, normally non-conductive underwater material, like wooden hulls, have been reported to be damaged by this action. It is also a common problem in destroying conductive surfaces including copper based bottom paint. This effect can be visually identified after it has occurred by large numbers of small “pock marks” in the paint when the boat is hauled out of the water.
A third failure mechanism is electrical damage that develops as a result of leakage current unbalance in shore electric power delivered to the boat. This happens when current is being supplied to the boat, but not returning through the intended return line. The unbalanced current can be leaking off through the water into the marina, or through the AC safety ground. This type of fault has reportedly killed persons by electrocution that were swimming, were in or on, or went into the water near a boat with this type of failure. As little as 15 Milliamperes (ma) is considered the safe level of current that a human body can withstand. This type of failure in an otherwise properly wired boat may be caused by movement of electrical contacts touching each other in an electrical distribution panel, or by sea water invading a circuit that is intended to be insulated, but leaks off to ground through the wet area. Some electrical systems, like refrigerators, heaters, air-conditioners, or timer run devices may have faults that do not activate until switched on unexpectedly, long after the operators have departed the boat.
A fourth common damage prevention deficiency is the ability to test, isolate both AC and DC faults and leakage, and document deterioration trends. This can provide identification of a potential problem's existence and point of origin prior to extensive or irrecoverable damage, particularly desirable with the boat in continued operational use.
U.S. Pat. No. 3,362,900 discloses a cathodic protection system in which a transformer
20
together with transistors
48
,
50
provide cathodic protection from the boat
110
vac. This is an active DC protection system for balancing out any galvanic current damage to the hull of the protected boat. It is intended to solve the boat's self generated electrolysis effect. However in this system there is no protection for the boat or personnel from AC currents, which may be faulty from the marina power source. The Electrical Test/Isolator (T/I) described herein makes no claim for protection against this type of problem, only against externally induced, or electric fault destructive effects.
U.S. Pat. No. 3,636,409 also utilizes a transformer
2
and a filter system
11
is connected to the boat ground
9
. As shown in
FIG. 2
the protective system includes a first pair of rectifiers
12
and
13
in series with each other and the second pair of rectifiers
14
and
15
in series with each other and of opposite polarity to rectifiers
14
and
15
. Capacitors
16
,
17
are in series with each and in parallel with the rectifiers. However this system does not provide protection against faulty AC marina supply systems.
U.S. Pat. No. 3,769,926 is somewhat similar. In
FIG. 2
this patent discloses a protective system including opposite polarity diodes
36
a,
36
b,
and
38
a,
38
b
connected in parallel with each other. It is noted that this system does not include the capacitors as were provided in the previous reference system. However this patent provides no protection against faulty AC marina supply sources.
U.S. Pat. No. 4,117,345 includes a ground isolator circuit which monitors the DC provided for galvanic protection, and also monitors the AC circuit between the boat ground connection and the ground connection at the dock. The reference points out that any voltage above about 2.5 volts RMS could conceivably be dangerous when contacted by a person in the water. This system provides protection against DC voltage and thereby prevents externally induced corrosion of the hull of vessels due to electrolysis. It also limits the AC potential between the hull and the shore ground to a level safe for humans, at the stated 2.5 volts RMS potential value. However this reference does not provide corrosion protection below the stated level, or against the AC marina supply system that is faulty for reasons other than externally induced stated levels of the boat to dock ground connection. Further, this is a relatively complicated system with many circuits, parts, and expensive electrical components, resulting in decreased reliability and increased cost. Furthermore the system requires continuous AC or DC power to function properly. Furthermore if the system detects an unsaf
Cummings Henry W.
Jackson Stephen W.
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