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This article was originally published in the
October 15, 1996 issue of Practical Sailor. The author, Stan Honey,
is a renowned sailor, navigator and electrical engineer.
Marine Grounding Systems
ground n. 12. Electricity A. A large
conducting body, such as the earth or an electric circuit connected to the
earth, used as an arbitrary zero of potential.
In a normal house on land, the problem of grounding is simple.
It consists of the green grounding wire in the AC wiring system and serves
the purpose of preventing shocks or electrocution. The ground
connection is usually made by clamping to a metal water pipe or by driving
a long copper stake into the ground.
On a boat, things are considerably more complicated. In addition to
the AC ground, we need a DC ground or return line, a lightning ground, and
a RF ground plane for the radio systems. Our first thought might be
to simply make the ground connection to a metal thru-hull, propeller shaft
or other underwater metal. This underwater metal will be grounded by
connection to the seawater will serve as our “water pipe”.
Unfortunately, a connection between any of these systems and underwater
metal can, and probably will, give rise to serious electrolytic corrosion
problems. This article will discuss the particular requirements of
each system, resolve the contradictions between the systems and present a
consistent and correct solution for a complete, integrated, marine
grounding system.
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Figure 1. The boats electrical
system should be connected to seawater at one point only, via the engine
negative terminal or its bus.
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DC Ground
Every light or appliance should be wired with its
own DC return wire. Never use the mast, engine, or other metal object
as part of the return circuit. The DC load returns of all branch
circuits should be tied to the negative bus of the DC distribution
panel. In turn, the negative bus of the DC distribution panel should
be connected to the engine negative terminal or its bus. The battery
negative is also connected to the engine negative terminal or its
bus. The key factor here is that the yacht's electrical system is
connected to seawater ground at one point only, via the engine negative
terminal or its bus. See figure one.
AC Ground
See Practical Sailor August 15, 1995 for a
detailed treatment of the green wire. The best solution is a heavy
and expensive isolation transformer. The acceptable solution (for the
rest of us) is to install a light and inexpensive Galvanic Isolator in the green wire, between the shorepower
cord socket on your boat, and the connection to the boat's AC panel.
Then, connect the grounding conductor (green) of the AC panel directly to
the engine negative terminal or its bus. Note that this meets ABYC's recommendations. In choosing Galvanic
Isolators, make sure that you select one that has a continuous current
rating that is at least 135% the current rating on the circuit breaker on
your dock box. Certain Galvanic Isolators (e.g. Quicksilver)
include large capacitors in parallel with the isolation diodes, which in
certain situations theoretically provide better galvanic protection.
Unfortunately, these units cost substantially more than conventional
Galvanic Isolators. If you feel like spending real money on galvanic
isolation, you might as well do it right and buy an isolation transformer.
It is also a good idea to use a Ground Fault Interrupter (GFI) in
your AC wiring. GFI's will occasionally
"nuisance trip" due to the humidity surrounding the wiring on
boats, but the additional safety that they offer (particularly to nearby
swimmers) in disconnecting power in the presence of ground currents is
worth the nuisance. If your GFI starts to nuisance trip, it is
probably a very good idea to track down and clean up your damp wiring in
any event.
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Figure 2. Ground fault circuit
interrupters (GFCI) should be installed in each AC circuit. A GFCI will
disconnect power in the presence of ground currents, helping prevent an
electrocution.
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Lightning Grounds
Connect a 4 AWG battery cable from the base of your aluminum mast to the nearest keel bolt from
external ballast. If you have internal ballast, you should install a
lightning ground plate. One square foot is recommended for use in
salt water; fresh water requires much more. Do not rely on a
thru-hull or a sintered bronze radio ground (e.g. Dynaplate)
for use as a lightning ground.
For additional comfort, also run a 6 AWG wire from your keel bolt or
ground plate to the upper shroud chainplates, and
to your headstay chainplate.
Don't bother with the backstay if it is interrupted with antenna
insulators. Have each of the cables that are used for lightning
ground wires lead as directly as possible to the same keel bolt, with any
necessary bends being smooth and gradual.
Given that you have grounded your mast solidly to the ocean, your mast will be at exactly the same electric
potential as the ocean. There is no chance that you can dissipate the
charge between the ocean and the atmosphere, so don't bother with a static
dissipater at the masthead. Wire "bottle brush" static
dissipaters may be useful to dissipate
seagulls, however, but that is beyond the scope of this article.
RF Ground
Your VHF doesn't need to use the ocean as a
counterpoise, so here we are dealing only with the ground needed for your
HF/SSB radio.
Mount your automatic tuner as close to the backstay as possible, preferably just under the
after deck. Run copper ground tape from the tuner to the stern
pulpit/lifelines, to the engine, and to a keel bolt. It is good
practice to include the HF/SSB radio itself in this network of ground
tapes. If the builder of your yacht had the foresight to bond into
the hull a length of copper tape or an area of copper mesh, be sure to run
a copper ground tape to this as well, and say a blessing for builders such
as these. Sintered bronze ground plates (e.g. Dynaplates) can be used as radio grounds in situations where the ballast or
engine is unavailable or awkward to connect. If the ballast, engine,
and lifelines are available, however, they generally make a high
performance ground.
Bonding and Electrolytic Corrosion Due to Hot Marinas
Do not bond any thru-hulls or other immersed metal
that can be electrically isolated. Specifically, keep your metal
keel/ballast, your metal rudder shaft, your engine/prop, and all thru-hulls
electrically isolated, from each other, and from the engine.
It's worth understanding the reason. In an increasing number
of marinas, there are substantial DC electric currents running through the
water. If your bits of immersed metal are bonded, the electric current
will take the lower resistance path offered by your boat in preference to
the water near your boat, and the current will flow into one of your bits
of metal, through your bonding wires, and then out another bit of
metal. The anodic bit of metal or thru-hull that has the misfortune
to be on the "out current" side of the current running through
your bonding system will also become "out metal" and will
disappear, sometimes rapidly.
Your zinc is only intended to protect against the modest galvanic
potentials and therefore currents that are caused by the dissimilar metals
that are immersed and electrically connected together on your own
boat. Your zinc is incapable of supplying enough galvanic potential
to protect against substantial DC currents that may be flowing in the
water. These DC currents in the water will cause electrolytic
corrosion to your bonded thru-hulls or metal parts.
Zincs and Protection from Galvanic Corrosion
Use zincs to protect against the galvanic currents
that are set up by dissimilar metals on your boat that are immersed and
that are in electric contact with one another. The best example is
your bronze propeller on a stainless shaft. The best protection is to
put a zinc right on the shaft next to the propeller, or a zinc on the propeller nut. An isolated bronze thru-hull doesn't need protection because
it is not in electrical contact with another immersed dissimilar
metal. If electrically isolated, high quality marine bronze, is
electrochemically stable in seawater; nothing good can come from connecting
wires to it.
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Figure 1. Conductors
running from the external keel or ground plate to the mast, stays
and to the metal fuel tank will protect against a lighting strike, and
there will be no DC connections to the engine or to the electrical
system.
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Stainless steel is a special case. Generally, it is a bad idea
to use stainless steel underwater, because it can pit. When it pits
the "nobility" of the metal changes locally, and you end up with
tiny galvanic couples that are made up of different parts of the same piece
of metal and the pits grow deeper. One school of thought suggests
that if you must use stainless steel underwater (e.g. you need its
strength), then you should connect a nearby, immersed zinc to it; this
protects the stainless steel from itself, reducing the rate of
pitting. The electrochemistry of this assertion is compelling enough
to recommend that you protect a stainless steel rudder shaft with a zinc. This may be done by mounting a zinc on the hull near the rudder shaft, and
electrically connect it (inside the hull) to the stainless rudder
shaft. For the reasons described above, ensure that your metal rudder
shaft is not electrically connected to anything else. Your stainless
steel propeller shaft will be protected from itself, by the same shaft zinc
that protects the propeller from the stainless steel shaft. In both
cases the pits, if they appear, will appear where the stainless steel is
not exposed to the water. Trouble areas are in the cutlass bearing,
inside the rudder bearing, and just inside the top of the rudder.
Keep your metal keel/ballast electrically isolated from all other
bits of metal. If you have the misfortune to have an external iron or
steel keel, however, mount a zinc directly on it
to reduce the rate of corrosion. Leave lead keels/ballast isolated.
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Figure 1. To avoid making
another DC ground to the engine via the HF/SSB radio copper ground strip,
fasten the copper tape securely to an insulating piece of phenolic or to a terminal strip, cut a 1/10" gap
across the tape, and solder several 0.15 uF
ceramic capacitors across the gap.
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Inconsistencies in the Ground Rules
So now, you are annoyed with the
inconsistencies. We said to leave all bits of immersed metal
electrically isolated when we described electrolytic corrosion and hot
marinas, but then we said to connect wires and copper tape to your keel and
engine for lightning and RF grounds. So what to do?
RF ground. The RF ground needs to be a ground for RF signals
only. It does not need to conduct DC, and as described in
"Bonding and Electrolytic Corrosion..." above, you do not want to
connect another DC ground to your engine and to your keel etc.
The solution is to find a dry secure place along each of the copper
RF ground tapes that are running to your engine and keel. Fasten the
tape securely to an insulating piece of phenolic
or to a terminal strip, cut a 1/10-inch gap across the tape, and solder
several 0.15uF ceramic capacitors across the gap. These capacitors
will be transparent to the RF, which will be happily grounded by the ground
tape system, but they will block any DC currents from running through the
RF ground system, and will avoid any resulting susceptibility to hot marina
electrolytic corrosion. It is worth selecting the capacitors
carefully, because they may carry a significant amount of RF current.
An acceptable choice of capacitors and vendor are listed at the end of this
article.
Lightning Ground. The lightning ground needs to be a direct DC
connection to the keel or to a ground plate to handle currents due to
lightning strikes. So how do we keep the keel or ground plate
electrically isolated as required in "Bonding and Electrolytic
Corrosion..." above?
The solution is to connect the keel or ground plate directly to the
mast, but make sure the mast is not electrically connected to the boats DC
ground system. If your steaming light, masthead light, tricolor, Windex light etc. are wired carefully and
correctly, they each will have their own DC return wire; there should be no
ground connection between their wiring and the mast itself. Make sure
that this is the case. This should also be true of your masthead
instruments. The unintended DC connection between mast and DC ground
is typically made by the masthead VHF whip, which connects the shield of
the coax to the bracket connected to the mast. That shield also
connects to the VHF radio which is DC grounded by its power
connection. The easiest solution is to insert what is called a "inner-outer DC block" into the coax.
This RF device puts a capacitor in series with the center conductor, and
another capacitor in series with the shield. This device is
transparent to the VHF RF signals in the center conductor and shield, but
blocks any DC current in either the center conductor or shield. This
device can be made by a good radio technician, or purchased from radio
supply houses, pre-fitted with any kind of coax connection on both
ends. The commercial units look like a coax "barrel"
connector. A vendor is listed at the end of the article.
Once the DC connection from the mast to the VHF is broken, check for
any other connections with an ohmmeter, and straighten out any other wiring
errors or unintended connections. If your metal fuel tank is also
bonded to the lightning ground system (per ABYC) then make sure that it
does not have DC connections either to the engine via the fuel line or to
the electrical system via the fuel level sensor. A piece of approved
rubber fuel hose in the fuel lines to the engine solves that connection,
and a well designed fuel level sensor will not make electrical contact with
the tank.
When you're done, there will be heavy conductors running from the
external keel or lightning ground plate to the mast, stays, and to the
metal fuel tank, but there will be no DC connections to the engine or to
the yacht's electrical system. See figure 3.
Summary
By using capacitors to block DC connections in a
few key areas, it is possible to have perfect ground systems for AC, DC,
RF, lightning, and corrosion, and have a boat that is immune to stray DC
currents that are traveling through the water in "hot marinas."
In the old days, the technique of bonding everything together worked
okay. In its defense, the "bond
everything together" approach makes your boat less sensitive to
electrolytic corrosion that can result from faulty wiring on your own
boat. The problem is, the "bond everything" approach leaves
your boat totally defenseless to wiring errors in
nearby boats and nearby industry, that cause stray DC currents to run
through the water.
Today the technique of bonding everything together would still work
fine if your boat spent all of its time on the high seas, in remote
anchorages, or in marinas that were wired perfectly and in which all of the
nearby yachts were wired perfectly. Having underwater metal bonded
together in crowded marina's today, however, is
asking for expensive trouble. As outlined above, it is avoidable
trouble. It is possible, with careful wiring and a few capacitors, to
have the best of all worlds, good RF and lightning grounds, ABYC approved
DC and AC grounds, and security against electrolytic corrosion caused by hot
marinas.
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Sources:
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Inner-Outer DC
Blocks: PolyPhaser, Model IS-IE50LN-C1, This Inner-Outer DC block also
contains a lightning arrestor. It costs about $120 from www.aesham.com 800 558 0411 It uses type N connectors.
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Capacitors for use to block DC
in SSB grounding tape: Digi-Key,
(800) 344 4539. Type X7R Monolithic Ceramic capacitor, 0.15uF,
$0.91 each, Digi-Key part number P4911-ND.
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