Cone of
Protection from Lightning - Faraday's Cage
This
spring seems to have brought the most extreme weather in history. With
heavy thunderstorms you will often find lightning. Lightning on the
water can bring life-threatening circumstances. For your safety and
the safety of others boating with you we have updated and are
republishing this article on Lightning Protection.
Capt. Matt
Even
though the odds are in your favor that your boat may never be hit by
lightning, if it happens it can have devastating effects. Don't
take a chance, protect yourself. If you are in a small boat and
close to shore when a thunderstorm approaches, get in and off the
water immediately. Better yet, don't go out if thunderstorms are
predicted. But what if you are miles offshore and a storm pops up?
Hopefully, you have prepared in advance.
The voltages involved in lightning are so high that even materials
that would normally be considered non-conductive become conductors,
including the human body. The voltages are so massive that if they
start to travel through a boat's structure - say through its mast -
then meet with high resistance (for instance, the hull skin) the
current discharge, in its attempt to reach ground, may simply blow a
hole in the non-conductive barrier. The safety conscious Captain
should make sure that his vessel is properly protected. Reference
should be made in detail to the standards for lightning protection as
set forth by the American Boat and Yacht Council (ABYC) and the job
should be performed by a licensed marine electrician.
In
theory, a lightning protection system is used to create what is know
as a "Faraday's cage," so called after the late
nineteenth-century scientist Michael Faraday. The principle of a
Faraday's cage is to provide a surrounding, well-grounded, metal
structure, in which all of parts are bonded together and carry the
same electrical potential. Such a "cage" attracts and
carries any lightning strike to ground much like lightning rods on
buildings. In other words, you need to provide an unobstructed way for
the lightning to dissipate its energy to ground (the water surrounding
you). Faraday himself risked his own life to prove this theory. The
additional benefit of a lightning protection system is that it tends
to bleed off any charge build-up in the general vicinity, possibly
averting a lightning strike in the first place.
So
how does a lightning protection system work? In a boat, the
"cage" is formed by bonding together, with heavy conductors,
the vessel's mast and all other major metal masses. A marine
electrician must tie in the engines, stoves, air conditioning
compressors, railings, arches etc. with a low resistance wire which
would ultimately provide a conductive path to ground (the water)
usually via the engine and propeller shaft, keel bolts, or better yet,
a separate external ground plate at least 1 square foot in dimension.
It is important that you ensure that your crew fall within the
protection of the "cage," something not always feasible when
the vessel is not built of steel or aluminum. On fiberglass or wooden
boats it is advantageous to have a mast or other conductive metal
protrusion extending well above the vessel, creating what is known as
a "cone" or zone of protection.
It
is generally accepted that this cone of protection extends 45 degrees,
all around, from the tip of the metal protrusion. This means that if
the aluminum mast of the average sailing vessel is properly bonded to
the vessel's other major metal masses and is given a direct,
low-resistance conductive path to ground, the entire boat should fall
within the protected zone. If the vessel has a wooden or composite
mast, a marine electrician can achieve the same effect by installing a
6 to 12 inch metal spike at the top and running a heavy conductor down
the mast and as directly as possible to ground, usually through the
engine and propeller shaft.
Again,
refer to the ABYC standards and have a professional marine electrician
install your lightning protection. This
is not a do-it-yourself project.
|
John Payne, on Lightning
protection...
Lightning has long been a problem for
mariners. As far back as the early 1800's boat builders were installing
lightning protection systems to minimize the catastrophic effects of strikes.
These methods were essentially the grounding of spars and rigging. More than one
vessel lost mizzens and masts along with compass problems as a result. Bonding
systems were also evolved as a response to dissipation of strike energy. In the
late 20th century, nearly 200 years on, the same measures are still valid. It is
a fact that virtually all classification societies and national marine
authorities lay down recommendations for protection of vessels from lightning
strikes, but very few cruisers bother to adhere to the them. From my own
experience the figure is around 5%. Of more importance is the startling
statistic uncovered during research that nearly 10% of fatalities on cruising
yachts are the result of lightning strikes, out of the 100 killed in the US
through lightning strikes yearly. Lightning causes along with death and injury
an enormous amount of damage in shore installations, in particular in the
telecommunications and electrical power distribution industries. For these
simple economic cost considerations many measures are taken to design systems
that minimize the effects of strikes. The main effects are high energy, high
voltage transients that result in flashover and damage. They can be either
resulting from direct strikes or induction into power or signal cables, or
induced, either through capacitive or inductive coupling.
Lightning Physics
What causes lightning? Within the cloud
formation, strong updrafts and downdrafts generate high electrical charges. When
the voltage reaches a sufficiently high level both cloud to cloud and ground
discharges occur. Strikes occur when the ground is at positive polarity and the
cloud negative region attempts to equalize with ground. Alternatively strikes
can occur when the positively charged cloud top equalizes with the negative
ground, or when the positive charged ground equalizes with the negative charge
cloud or the negatively charged ground equalizes with the positive charged cloud
top.
Lightning consists of a number of components,
which form a multidirectional flow of charges. Peak currents can exceed 200,000
amperes (200kA) at over 30,000°C for a matter of milliseconds (25-100 ms). The
positively charged ions rise to the cloud top, and the negative ions migrate to
the cloud base. Regions of positive charged ions also form at the cloud base.
Eventually the cloud charge levels have sufficient potential difference between
ground and another cloud to discharge. The processes are as follows:
* Leader. The leader consists is a negative
stream of electrons consisting of many small forks or fingers that follow and
break down the air paths offering the least resistance. The charge follows the
fork finding the easiest path as each successive layer is broken down and
charged to the same polarity as the cloud charge.
* Upward Positive Leader. This is a positive
charge that rises some 50 meters above the ground. (Sometimes from your mast
head)
* Channel. When the two meet a channel is
formed.
* Return Stroke. This path is generally much
brighter and more powerful than the leader and travels upwards to the cloud
partially equalizing the potential difference between ground and cloud. (Often
directly through your vessel)
* Dart Leader. In a matter of milliseconds
after the return stroke, another downwards charge takes place following the same
path as the stepped leader and return stroke, sometimes followed by multiple
return strokes (multi-pulse surges).
· Multi-pulse Surges. These occur in over
70% of strikes. This phenomenon is where up to 20 re-strikes follow the initial
strike at intervals of around 10-200 milliseconds apart. In addition continuing
currents of 200-500 amps with durations of up to 1-2 seconds may also occur. The
movements happen so fast that it appears to be a single event. This sequence can
continue until the differential between cloud and ground has been equalized.
Protection Systems
Protection can never be achieved using a
single method, and a number of measures should be used to minimize the risks.
The main objectives of any systems are:
· Capture of the strike at a nominated
point, ie mast head
· Conduction of the strike current to ground
safely using a well installed down conductor that reduces side flashes
· Dissipation of the strike energy to
ground, through a low impedance ground system so that rises in ground potential
are minimized
· Equipotential ground bonding of all
relevant systems and components
· Protection of power supplies from high
voltage transients and surges that may damage equipment
· Protection of conductors both power and
signal from induced surges that may damage equipment
As a reference to official guidelines the
measures are in accordance with NFPA 302 Fire Protection Standard for Pleasure
and Commercial Motor Craft
Lightning Protection Zone. The most reliable
protection system is one that grounds any strike directly and the principles are
as follows:
* Grounding. The primary purpose of a
grounding system is to divert the lightning strike discharge directly to ground
through a low resistance circuit suitably rated to carry the momentary current
values. This has the effect of reducing the strike period to a minimum, and
reducing or eliminating the problem of side strikes as the charge attempts to go
to ground. As electricity follows the path of least resistance to ground, little
goes down the stays.
* Cone of Protection. The tip of the mast, or
more correctly a turned spike clear of all masthead equipment gives a cone of
protection below it. The cone base is the same as the mast height. This
protective cone prevents strikes to adjacent areas and metalwork, which in a
yacht can mean stays, rails or other items lower than the masthead.
Lightning Protection Systems. Most
classification societies, American Boat and Yacht Council and other advisory
bodies generally recommend lightning protection in the form of a directly
grounded mast and spike. Other devices have come onto the market, and
effectiveness of most has yet to be conclusively confirmed.
The Great Myth. Lightning rods and grounding
actually attract lightning strikes! The presence of a properly installed
lightning protection system will give a number of advantages:
* It will safely ground the strike energy
* It will in most cases limit the damaging
effects of heat and reduce the current levels flowing, as well possibly the
length of time the strike takes (this is in milliseconds). This reduces damage
to equipment and crew
* In GRP vessels, in particular multihulls,
under certain weather conditions, a static charge builds up on the deck. It is
my contention that ungrounded vessels actually promote strikes to the vessel due
to this condition. Additionally grounding the mast dissipates this charge, and
in the process removes a common cause of radio (RFI) noise that occurs as small
arc occurs as the static charge goes to ground.
Mast. Lightning will generally strike the
highest point, and take the path offering the lowest resistance to ground. The
mast is usually the strike point. Note that a stainless steel VHF whip does not
constitute any protection.
Mast Spike. The mast spike ideally should be
a copper rod with pointed end. To avoid metal interaction, stainless rods are
commonly used but should be of a thicker section than the more conductive and
lower resistance copper. The spike should be at least six inches higher than any
other masthead equipment, including VHF aerials. Many commercial units (Dynarod
and Seaground) have an offset in the rod, which although not being the required
straight section would be satisfactory. The purpose of the point being sharp is
that it facilitates what is called point discharge. Ions dissipate from the
ground and effectively cause a reduction in potential between the cloud and the
sea. In many cases the strike may be of lower intensity or not occur at all.
Dissipation Systems
Lightning Protection Device (LPD). This was
an Italian development of some 10 years ago, and consists of a high performance
varistor. The device is designed to interact with the electrical charges of the
initial stepped leader where current values are relatively low and avoid the
return strokes. Charges accumulate on the atmospheric electrode and varistor
poles. The varistor conducts and the charge condition on the electrode alters.
These charges leave when some streamers form to meet the leader.
No Strike Charge Dissipater. This is based
somewhat on the pinted spike only that there are many points in a brush
arrangement. This is to stop development of a stepped leader forming and
minimize the strength of any that do occur,
Mast Cable. Much of the damage in a strike
results from heat, as the large current flow into a resistive cable acts as a
heater. It is essential that cable cross sectional area is sufficient, typically
35mm² or greater. Under no circumstances use soldered joints alone, as they
will melt during a strike causing further havoc. Always crimp connections and
ensure that all bonded connections are clean and tight. All connections must be
bolted.
Grounding. A good ground requires direct and
permanent immersion in seawater. It must also have sufficient area to adequately
dissipate the strike energy. Through hull fittings must never be used as a
primary ground point unless you want to sink the vessel. The bonding cable from
the mast base to the ground plate should be as straight as practicable without
sharp corners as side discharges occur and this is called corona discharge.
Similar side discharges can occur from boat to boat in crowded marinas. Normally
I enclose the cable in high quality electrical conduit to reduce the possibility
of side strikes on the cable, as electrical insulation will frequently break
down under high voltage conditions.
Steel/Alloy Vessels. Connection of the mast
base with a large, low resistance bonding strap to the hull or as more practical
the mast step is sufficient.
GRP Vessels. A keel acts as a good ground and
is sufficient. Bridge out with a stainless link at least two keel bolts to
spread the contact area. On multihulls you have to install a large separate
ground plate, such as a radio ground (Dynaplate, Wonderbar or Seaground). This
will ensure that there is a large and efficient ground area. Do not use the
radio RF ground plate as the lightning ground. Never bond the lightning system
to the corrosion system bonding, machinery or electrical system negatives or
grounds. Never bond the lightning system to bronze through hull fittings (unless
you want to sink the vessel!).
Wooden Vessels. Wooden vessels normally have
a metal mast track. The track should be properly grounded. If possible a copper
strap can also be run, although this is not always practical. Direct bonding to
a ground plate or the keel should use the same grounding method as GRP.
Emergency Ground. A heavy gauge copper cable
can be clamped to a stay over a half-meter section. The other end should be
clamped to a ground plate, and hung over the side. Do not use chains and anchors
(another great myth), as they are ineffective as a ground.
Electromagnetic Pulse. A vessel can have
damaged equipment from a strike within a few hundred meters. Insurance companies
don't like to accept claims on damage unless you can show total damage to
masthead systems. A strike sends out a very large electromagnetic pulse, which
is a strong magnetic field. This field is induced into wiring and systems as a
high voltage spike, doing just as much damage. If you suspect damage from an
induced electromagnetic pulse from a localized lightning strike, check with all
vessels adjacent to yours, and get statements to support the contention.
Generally all the electronics will be out if this is the case as the mast and
any wiring acts as a large aerial.
Sidestrikes. It is common in very closely
moored vessels and crowded marinas to have lightning strikes literally jump from
vessel to vessel as it attempts to find ground on ungrounded vessels. Usually
the strike exits from stays, chainplates and spreaders. In many cases the strike
will go to water from the chainplates causing serious damage to hull and
fittings. In many cases the rig may topple.
St.Elmo's Fire (Brush Discharge). This
phenomenon is more common on steel vessels and when it occurs usually precedes a
strike, although the effect does not occur all the time. The vessel in effect
becomes a large ground mass. Ionized clouds and balls of white or green flashing
light that polarizes at vessel extremities characterize the discharge. The
discharge of negative ions reduces the potential intensity of a strike. Damage
to electrical systems is usually induced into mast wiring, as the steel hull
itself acts as a large Faraday cage.
Corrosion Factors. Considerable care must be
taken when bonding various items of equipment into a lightning protection
bonding system. On steel and alloy vessels the hulls are the one ground plane
for all equipment and all grounds are held at the same potential. In GRP and
timber vessels it can be more complicated, but problems may arise where
indiscriminate bonding of through hull fittings and other items is carried out.
It is easy to create differences of potential between various items creating a
corrosion nightmare. After connecting up a lightning system, it is prudent to
monitor the corrosion rate of anodes, and observe any underwater bonded items.
Bonding. Most authorities recommend that all
stanchions, chainplates, and large metallic equipment such as stainless water
tanks should be bonded to the lightning ground. Failure to bond can result in
side flashes as these can offer an alternative path. The bonding should be made
at the point closest to the main conductor. I prefer not to bond the stays and
chainplates as often recommended. My reasoning behind this is that if a good low
resistance path is made from mast to keel or groundplate the strike energy will
be directed that way. Grounding stays offers alternative high resistance paths,
encouraging side strike activity. Current flows can also cause crystallization
and permanent damage to stainless stays and fittings in a severe strike (try and
get that past the insurance company!). Bonding must be undertaken with care.
Dissimilar metals such as the aluminum mast copper strap, and steel must be
interconnected to ensure no galvanic corrosion can occur. More importantly
interconnection of various grounding systems must be undertaken with great care.
It is only necessary to bond internal metallic equipment within six feet of the
mast. In practice this is rarely water tanks under bunks etc, but should include
tankage under the cabin sole.
Lightning Safety. In an electrical storm, the
following precautions should be taken to avoid any shock or something more
serious.
* Stay below decks at all times.
* Stay well away from mast, boom shrouds,
chainplates and the mast compression post or mast if below deck.
* Take a position and plot it prior to
shutting down, or in case of all electronics equipment being blown.
* Turn off all electronic gear and isolate
circuit breakers if at all practical. Disconnect aerials also if practicable.
* Do not operate radios until after the storm
unless in an extreme emergency.
* After a lightning strike, be aware that the
compass may be incorrect.
* Check all running rigging and fittings
after a strike as damage can occur that may seriously affect vessel capacity to
sail.
* Check all through hull fittings for damage,
if you have decided to risk bonding them. Usually if they are damaged or gone,
you will see water ov
John Payne
|