Advanced Marine Electrics and Electronics Troubleshooting A Manual for Boatowners and Marine Technicians

121

• The exposed conductor ground plate, or
  the cumulative total area of multiple
  through-hull fittings and other exposed
  underwater metal objects tied into the
  grounding system, needs to be of suffi-
  cient size to ensure a low-resistance elec-
  trical connection to earth ground. The
  ABYC recommends at least 1 square foot
  of surface area in salt water, which will
  generally work for everything but SSB.
  SSB manufacturers generally recommend
  at least 1 square meter or yard of exposed
  surface area. Since fresh water is less elec-
  trically conductive than salt water, several
  square feet of surface area should be used
  for lightning protection.


• To reduce the risk of the lightning “side
  flashing” as it seeks a path to ground,
  heavy metal objects in proximity to a
  lightning conductor should be electrically
  bonded to the lightning conductor. Also,
  lightning tends to jump out of “corners”
  created in the lightning ground wiring
  system, so minimize sharp bends.

The Elements of a Lightning
Protection System

The basic elements of an LPS are shown in the
illustration. The primary conductor (also
called a down conductor) from the masthead
must have the equivalent conductivity of a
piece of 4 AWG (21 mm^2 ) wire. You can
achieve this by using an aluminum spar as the
conductor. Carbon fiber spars are considered
nonconductive for this purpose, so 4 AWG
wire needs to be added. At the upper end, the
conductor must terminate at what the ABYC
technical report calls an air terminal—in other
words, a lightning rod. The air terminal
should be the highest point on the mast by at
least 6 inches (150 mm).
The down conductor must be tied directly
to a grounding plate (or its equivalent) of at

grounding systems

least 1 square foot in surface area. You can

achieve equivalency in exposed surface area by

tying in other underwater metal objects such

as rudders, metal keel ballasts, through-hull

fittings, and the like to the system, but you

should never connect the down conductor

directly to a through-hull fitting. If you do,

and lightning strikes your boat, the strike may

blow the fitting out through the bottom of the

boat. A tinned copper distribution bus bar

makes a good link for the system if multiple

underwater metal objects are connected

Air terminal

Parallel path Parallel path

Bonding

conductors

(6 AWG)

Engine

Metal tank

Immersed

ground plate

or strip (1 sq. ft.)

4 AWG conductor

short as possible with

8" min. radius bends

Primary

lightning

conductor

(4 AWG)

Secondary

lightning

conductor

(6 AWG)

The essential elements of a lightning protection system. (Reprinted with

permission from Boatowner’s Illustrated Electrical Handbook,second

edition, by Charlie Wing)