Guidelines for layout, components and installation of lightning protection system on Nordhavn 55
Ewen Thomson Ph.D. Consultant
Marine Lightning Protection Inc.
1. Overview
The lightning protection system comprises an interconnected network of air terminals, conductors, and grounding electrodes that form a conducting grid outside the interior space of the vessel. The protective zone provided by the air terminals gives coverage over all deck area and interior space. Since the VHF antennas mounted on the flybridge are not included in the protective zone, roof top connections are made between each antenna support and the closest air terminal and a transient voltage surge suppressor (TVSS) is connected in each antenna cable. Down conductors are preferentially routed externally to all wiring, plumbing and occupied regions, the exception being the down conductor leading from the air terminal on the centrally-located instrumentation mast. Three continuous loop conductors are incorporated into the network:- (i) the existing stainless steel in the flybridge superstructure; (ii) the stainless steel railing at main deck level, and: (iii) the existing copper bonding strap about 2' above the waterline. Grounding is provided via six Siedarc electrodes - one on each side at bow, amidships, and stern – and two 0.5 ft2 immersed grounding strips near the stern, one on each side outboard of the propellers. All grounding electrodes are preferentially installed above and near the waterline.
Major features are summarized in the diagrams in Appendices 1 to 6. Appendix 7 is a copy of Figure 18 from ABYC Standard E-11 showing a common connection for DC ground, AC ground, cathodic bonding ground, and lightning bonding ground. Appendix 8 gives instructions for lug terminations.
2. Air terminals
2.1. Materials
The preferred air terminal is constructed from ½" solid
copper rod with a rounded tip, tinned to a thickness of at least 0.3 mil. Alternative materials are ½" or thicker
stainless steel, aluminum or bronze rods or pipes with a wall thickness of at
least 120mil and an area of at least 0.3 in2.
2.2. Bow air terminal
It is recommended that the bow air terminal be attached
directly to the top of the bow pulpit, as shown in Appendix 5. The tip height should be 7' above the level
of the floor of the bow pulpit.
2.3. Amidships air terminals
2.3.1. Mounting
Presently the flybridge roof is supported on stainless columns that are secured by means of a stainless ring on top as shown in Figure 2‑1 below. If the stainless ring is replaced by a solid disk of 3/16" or more thickness, then the air terminal can be either welded to this disk or, preferably, screwed through both the disk and into the column flange below. In the latter case the air terminal could be a commercially available ½" tinned copper rod with a ½" -13 threaded base.
Figure 2‑1 Air terminal mounting on flybridge roof
2.3.2. Connection to antenna
The antennas mounted on the side of the flybridge are not
inside the zone of protection and are likely to struck by lightning. This would destroy the antenna and produce a
voltage surge on the antenna cable that could damage both the receiver and
other onboard electronics. While it is
impractical to ensure the survival of the antenna itself, protective measures
are highly recommended to reduce the likelihood of damage caused by the voltage
surge. As a first precaution, a
conducting connection at least equivalent to 2AWG copper (0.04 in2
for copper, 0.25 in2 for stainless steel) should be made between the
base of the air terminal and either the metal ferrule on the antenna or the
antenna support. Additionally, transient
voltage surge suppressors (TVSS) should be installed in the antenna coax as close
as possible to the air terminal. These
are preferentially mounted on common buses, one on port and one on starboard as
close as possible to the point where the cables pass through the coaming.
2.4. Main mast air terminal
Presently a lightning dissipator is mounted on a stainless
bracket at the top of the main instrumentation mast. Scientific opinion is solidly against the use
of these devices. However, its location
is an ideal place for a regular air terminal.
Appendix 5 shows the recommended height for this. A commercially available 48" or ½"
tinned copper rod with blunt tip is sufficiently tall if it includes all mast
head electronics within a 90-degree apex cone of protection. Such a rod can be attached by means of
½" -13 nuts above and below the present supporting bracket. In this case a ½" hole should be drilled
through the support.
3. Down conductors
3.1. Routing
Appendices 1, 3, and 4 show suitable paths for lightning down conductors. All down conductors should be made from insulated 2 AWG tinned copper battery cable terminated with the heavy duty lugs supplied by Marine Lightning Protection (see Section 3.3.1 and Appendix 8 for termination details). In general, the preferred route is the most direct path from air terminal to the closest grounding electrode with all direction changes by means of as smooth a curve (large radius of curvature) as possible.
3.2.
Loop conductors
3.2.1. Flybridge
The existing stainless steel superstructure in the flybridge forms a continuous loop around the flybridge, although it is tortuous in parts.
3.2.2. Main deck
The handrails at main deck level also form a continuous loop
if all lifelines are attached. Note
however that the lifelines do not meet the specifications of a main lightning
conductor.
3.2.3. Galvanic bonding strap
The existing ~1"x3/16" copper bonding strap for
cathodic protection is continuous from port stern around the bow and back to
starboard stern. A connecting conductor
(as described in Section 3.1) should be connected between the ends of this bonding
strap to ensure a continuous loop that satisfies the specifications of a main
lightning conductor. ABYC Standard E-11
stipulates a common connection for DC ground, AC ground, cathodic bonding
ground, and lightning bonding ground, so that this bonding loop can also be
used in the lightning protection system to connect all down conductors and
grounding electrodes.
3.3. Connections
3.3.1. General
All lugs are for 2AWG cable and have a 3/8"
diameter hole. The manufacturer's
directions for connecting cable to lug are included in Appendix 8. For better electrical continuity, 0.1-0.3 ml
of PenetroxE joint compound should be injected into each lug before inserting
the stripped cable. Also, application of
heat shrink (with a 1.5" cable overlap) is highly recommended to improve
resilience and further impede moisture intrusion. PenetroxE should also be applied between
surfaces that are to be connected before attachment.
3.3.2. Stainless stanchions & handrails
Use the 90 degree lugs for any connection that involves a straight through connection, as illustrated in Appendix 2. If a sufficiently large hole can be made through the roof or coaming, the lug can be bolted in direct contact with the bottom of the stanchion base. Attach with either a 5/16 " or 3/8" machine screw (preferably tinned brass, stainless, or bronze) after making a corresponding thread through the center of the stanchion base. Otherwise, use a ¾" expansion stud (preferably tinned copper, bronze or stainless) with a 3/8" hole through the center.
3.3.3. Splices
Splices between cables can be made by terminating each cable
in a lug at the point of the splice, stacking the lugs together and securing
with a through bolt and nut. When
splicing into a main down conductor, assure that the down conductor lugs are at
180 degrees and the lug on the spliced cable is angled down as much as possible
to avoid U-shaped loops.
4. Grounding electrodes
4.1. Siedarc electrodes in through-hull fittings
Two flush-head electrodes with parallel connectors are
installed near the bow, two mushroom-head electrodes with parallel connectors are
installed amidships, and two mushroom-head electrodes with daisy-chain connections
are near the stern, as illustrated in Appendices 1,3,4 and 6. Generally, each electrode should be installed
above the stationary waterline and below the lowest flange of any nearby
plumbing through hulls. In addition, the
two stern electrodes should be positioned so that the cable from the electrode
to the HStrip is perpendicular to the strip.
Each electrode cable is attached
to the bonding loop by the shortest path possible, with the lug at 90 degrees
to the bonding strap, and using the same 3/8" machine bolt to attach the
electrode cable,and corresponding down conductor to the bonding strap. Detailed installation instructions are
included with the electrodes.
4.2.
HStrip immersed grounding strips
Place each HStrip between the propeller and the waterline at a depth where it will be immersed during all normal modes of operation. The connection from the bonding strap to each HStrip should be made by as direct a path as possible from the strap, through the dasiy-chain Siedarc electrode, to one of the HStrip bolts..
5. Notice
The lightning protection system described in this document
is fundamentally consistent with the revised standard presently being
considered by the National Fire Protection Association to replace the
watercraft standard (Chapter 8) in NFPA 780.
Any departures are considered by the author to be improvements over this
revised standard. For example, instead
of a single one square foot grounding strip we use two 0.5 ft2
strips that are placed between the fitting to be shielded – the propeller – and
the likely current exit path, the waterline.
In general, lightning protection systems are limited by the current
state of knowledge and no implication is intended that this, or any, protection
system will prevent damage or injury.
6. Appendices
Appendix 1
Appendix 2
Appendix 3
Appendix 4
Appendix 5
Appendix 6
Appendix 7 Figure 18 from ABYC E-11 2003
Appendix 8 Termination directions for heavy duty lugs
