|
HStripTM
A 0.5 ft2 immersed
grounding strip fabricated from
tinned copper
Overview
·
Dimensions of
36" x 2" x 3/16" provide 0.5 ft2 of contact
surface when immersed.
·
Ground strip is
fabricated from solid copper and tin electroplated.
·
Connector is
3/8" diameter tinned brass.
·
Detailed installation
directions included regarding placement, precautions & installation
The Problems
The
accepted method for a lightning protection system for a boat is to use a 1
ft2 ground plate and this is required in most lightning
protection standards for watercraft.
While a copper ground strip offers the best conductivity, immersed
copper fittings tend to turn green and corrode. On the other hand, if bronze or stainless
steel is used in the ground plate there are problems with the resistance of
the connection. A preferable option is tin-plated copper.
Any
immersed ground plate or strip can increase its effectiveness by means of
the dynamic ground concept. This is
explained in Thomson
(1991) and has been adopted by the major standards agencies such as
ABYC and NFPA. Basically, since
sparks form preferentially from sharp edges, thereby increasing the
effective area of the grounding conductor, the edges of a ground plate or
strip should be sharp and exposed. Also, for the same contact area, a long
thin ground strip is more effective than a square ground plate. So the
simplest geometry – a rectangular strip with sharp edges – is
the best.
The
connection to the ground strip needs to be at least equivalent to a down
conductor. ABYC and NFPA recommend a
down conductor area of at least 21 mm2. In a typical lightning protection system the
connectors are the weakest links since it is difficult to make a perfect
contact, so that a connection that is much larger than a down conductor is
preferable. Another difficulty is
that readily available fasteners made of stainless or bronze have much
lower conductivities than copper, and brass in not recommended for constant
immersion. So copper is best, but
this metal is soft and tinned copper fasteners are not readily available.
A
significant problem with the installation of any lightning conductor below
the waterline is the risk of the through-hull connection causing an
explosion during a lightning strike.
There are many instances of this happening and the result is a large
leaking hole below the waterline.
.
The Solutions
Our
HStripTM has a contact surface area of
0.5 ft2 and provides four sharp edges to enhance the dynamic
ground effect of the strip. Two of these
provide more than 1 ft2 of immersed area. Each is made of
solid 3/16" copper and tin plated to 0.3 mil. As such, two HStrips
connected together with our HStripConTM
internal connector can be used in any application requiring a one square
foot ground plate or ground strip.
Or the two strips can be split up and placed one on each side of the
boat just below the waterline to give a total immersed area of 1 ft2 that
is closer to the waterline and can be positioned to shield other immersed
fittings such as propellers, though-hull sensors and thrusters.
Both
electrical connections and securing is done through identical 3/8"
tinned brass bolts. Electrical
connection is made through a heavy duty lug for the down conductor
connection, or a GapConTM spark gap
connection if electrical isolation is desired. If the spark-gap option is chosen, the HStripTM is electrically isolated from the
lightning protection system for voltages less than 600V. Isolation of immersed metals fittings
reduces galvanic corrosion and eliminates electrolysis problems in marinas
with ground current leakage. It also
dramatically decreases the possibility of introducing fault current into
the water from inadvertent reversal of live and ground wires, which is a
major cause of electrocution deaths from boat wiring problems.
Included
with the HStripTM are detailed
installation instructions, explanations of the hazards involved, and
recommendations for precautions.
However, the main protective procedure we recommend is the addition
of SiedarcTM electrodes to either
terminate perimeter down conductors, or, with the daisy-chain connection
option, to provide additional above-water grounding terminals and sideflash protection.
While our 1991 paper concluded that a one
square foot immersed area was theoretically adequate in salt water, the
statistics also showed that the electronics damage in a grounded boat in
salt water was effectively the same as that in an ungrounded boat. We feel that an external network of
lightning conductors and bonded fittings is the key to effective
protection. Hence the HStripTM alone should not be relied upon,
even in salt water. However, it is
the obvious first step.
Applications
HStrips in sailboat
In
this application, the HStrips are installed on
each side just below the heeled waterline approximately in line with the
mast. Hence we establish an equipotential region around the main saloon, as explained
in Section 4.3 in our Grounding
Concepts page. Fig. 4-6 from
this page is shown below, where the dashed lines represent surfaces of
constant voltage. No sparks are
possible in an equipotential region where there
is a constant voltage everywhere.
The
two SiedarcTM electrodes above the
waterline and outboard of the HStrips provide
additional grounding terminals via spark promotion and lower the sideflash risk from the down conductor in the hazardous
region near the waterline. In conjunction with additional down conductors and SiedarcTM electrodes at the bow and stern,
and the loop conductor at deck level, a protective conducting shield is
formed around the whole interior of the boat.
HStripTM on Mirage Great Harbor
N37
The
optimal location for grounding terminals is as close to the waterline as
possible so that the current has the shortest distance to travel to the
water surface.. By placing the HStrips at the stern of Young America, a Mirage Great Harbor N37, we ensured a location
that was continually immersed but at the periphery of the hull. In addition, two SiedarcTM
electrodes were installed abeam of the HStrips to
provide additional down conductors and grounding terminals at the aft end
of this passagemaker. Another major advantage to an aft location
is that the HStrips effectively shield the
propellers. Since the engine block
was connected to the DC ground and hence to the lightning protection
system, the propellers otherwise were attractive exit points. To further lower the risk of lightning
current damaging engine and gearbox bearings on its path to ground, Young America was fitted with an
insulating coupling between each engine and its propeller shaft. Also, to completely isolate all underwater
fittings from each other and hence avoid galvanic currents in the water, we
added a GapConTM spark gap at the
connection point of each HStripTM. The complete lightning protection
system comprised two HStrips, six SiedarcTM electrodes, four air terminals,
and a network of interconnecting conductors that formed a shielding grid
around the boat.
More information
Brief technical
details and photos of the HStripTM, GapConTM and HStripConTM
Prices & availability
|
