Grounding lightning to water:

A guide for using Siedarc^TM electrodes in a marine lightning grounding system

Ewen M. Thomson Ph.D.

Copyright 2004 Marine Lightning Protection Inc. All rights reserved

Concepts and devices described in this document are the property of Marine Lightning Protection and covered by existing and pending patents.

Please enquire about licensing procedures for any intended use.

1. Summary

Siedarc^TM electrodes are effectively lightning rods near the water and have the same specifications as an air terminal. The electrodes supplement the grounding requirements of the typical watercraft standard of a single 1 ft² ground plate. They provide multiple discharge paths into the water to overcome the fundamental problems inherent in a single, centrally-located grounding terminal. With electrode terminations near the waterline, lightning conductors can be routed outside sensitive areas rather than through the middle of the vessel. In this way a marine lightning protection system can be designed with similar conductor geometry to a land-based system. By minimizing the risk of sideflashes to the water, an extensive bonding system for conducting fittings is possible. The recommended layout for lightning conductors in the grounding system comprises:

· a loop conductor around the perimeter of the boat at deck level

· a network of bonding conductors at deck level connected to the loop conductor

· multiple external main conductors

· internal main conductors only where necessary

· a grounding terminal at the end of each main conductor.

2. Introduction

2.1. Components of a lightning protection system

A typical lightning protection system in a watercraft has four major components.

1. Strike termination devices, or air terminals, provide terminals for the lightning to attach to the watercraft.

2. Main conductors conduct the bulk of the lightning current down towards the water.

3. Grounding terminals allow the current to exit into the water.

4. A network of bonding conductors equalizes voltages between the lightning protection system and conducting fittings.

2.2. Function of grounding system

The grounding system provides the connecting paths between the lightning protection system and the water. It has the following functions:

· to initiate current flow into the water;

· to establish a low impedance network for the peak lightning current;

· to prevent sideflashes from conducting fittings; and

· to minimize shock hazards for the crew.

2.3. Benefits of Siedarc electrodes

Present standards for lightning protection systems specify an immersed grounding terminal with a 1 ft² grounding area. However, grounding is more effective if multiple grounding terminals are distributed over the hull. See http://www.thomson.ece.ufl.edu/lightning/IEEE.html Also, since current tends to flow into the water in spark channels, contact area can be augmented through spark initiation. See pages 4 - 6 in the April/May 2004 issue of Professional Boatbuilder (http://www.marinelightning.com/ProBoat.pdf )

Using spark-promoting Siedarc^TM electrodes for these extra terminals, rather than immersed ground plates, has several advantages:

· Electrodes are designed to meet NFPA standards for an air terminal

· Each electrode requires only one mounting hole.

· Electrodes can be faired into the hull, thereby reducing drag and avoiding galvanic corrosion.

· Electrodes are mounted preferentially above the waterline.

· Routing lightning conductors around sensitive areas reduces emi and shock hazard.

· Electrodes are shaped to promote initial current flow.

2.4. Scope

This Guide discusses the role and implementation of grounding electrodes in a marine grounding system. Its scope is to:

· illustrate limitations in existing standards for lightning protection;

· identify particular sideflash risks;

· suggest layout for lightning conductors and grounding electrodes

Yacht configurations that are appropriate for the use of this type of grounding are those with:

· fiberglass hulls, and

· aluminum masts

Yacht configurations that require special consideration are those with:

· carbon fiber hull, or

· carbon fiber reinforcing structures in hull, or

· SSB ground strips embedded into hull, or

· carbon fiber masts.

2.5. Summary objectives

The main objectives for incorporating electrodes in the design of the grounding system are:

· to place electrodes near fittings that are sideflash risks;

· to distribute electrodes widely, preferentially along the waterline;

· to route connecting conductors externally to simulate a Faraday cage.

3. Improving on traditional single-plate grounding

3.1. Existing standards

Recommendations for lightning protection systems are published by several authorities, including ABYC, NFPA, ISO ABS, and Lloyds. Generally, these specify:

· an immersed ground plate or strip with an area of at least 1ft²

· a copper down conductor with a cross sectional area in the range size 21 – 58 mm²

· bonding of all metallic fittings close to the lightning conductor.

3.2. Problems

There are several problems with this scheme:

· The 1ft² area for a ground plate is completely inadequate in fresh water, and even in salt water sideflashes can occur from fittings close to the water.

· A single centrally-located down conductor maximizes the risk of generating sideflashes to other fittings and shock hazard

· Bonding conducting fittings to the lightning protection system increases the risk of a sideflash to the water.

These problems are illustrated in the two case studies discussed in Appendix A.

3.3. Solutions

These problems can be addressed by:

· supplementing the 1 ft² ground with multiple extra grounding terminals,

· adding additional lightning conductors outside sensitive areas,

· placing bonding conductors as far as possible from the water.

4. Designing the grounding system

4.1. Concepts

The main features of the grounding system are as follows:

· One or more main conductors are split into several grounding branches.

· Main conductors are routed preferentially external to interior spaces.

· Each grounding branch is terminated in a grounding terminal.

· Multiple grounding paths maximize the discharge area around the hull.

· Sideflash risk from a fitting to the water is reduced in the vicinity of any grounding terminal.

· Sideflash risk between fittings is reduced through bonding.

Steps in the procedure for designing the grounding system are to:

· identify regions of high sideflash risk'

· place grounding terminals,

· route lightning conductors.

4.2. Identifying sideflash hazards

4.2.1. Types of sideflashes

On the basis of the cases discussed in Appendix A, we can distinguish two types of sideflashes:

· Internal sideflashes connect from a fitting to another fitting inside the boat.

· External sideflashes connect to the water.

4.2.2. Fittings & risk

The risk of a sideflash depends on:

· the shape of the conducting fitting;

· for a fitting amidships, how close it is to the water;

· for a fitting near the beam, how close it is to the waterline.

Relative risks are summarized in Table 4-1.

Fitting	Sideflash risk		Comment
	Internal	External
CFC hull reinforcement	Moderate	Very high	Can destroy hull
Gel coat blister	Low	Very high	Also applies to water-soaked laminate
Immersed transducer	Low	Very high	Can blow out
SSB foil ground on hull	Low	Very high	High risk to hull integrity
Chainplate	Low	High	Avoid current flow through stay
Mast base	High if elevated	High	Aluminum mast is excellent conductor
Tank	High	Moderate	Same for metal and water tanks
Prop shaft	High	Low	Immersed in water
Keel bolt	High	Low	External sideflash through ballast
Plumbing	Moderate	Moderate	Depends on location
Batteries	Low	Moderate	Ground connected to battery negative
Bilge water	Low	Moderate	Especially if in contact with lightning protection system
Encapsulated ballast	NA	Moderate	Current flow through ballast highly desirable

Table 4‑1 Relative sideflash risk for fittings

Regions where the sideflash risk is high are illustrated in Figure 4‑1. This is a simplified yacht with mast, forestay, single set of sidestays, keel bolts connecting to ballast, and a conducting tank forward. In a more typical yacht the region of high sideflash risk would be more likely to encompass the complete volume of the hull below the waterline.

Figure 4‑1 Regions of high sideflash risk

4.2.3. Carbon fiber hulls and fittings

Carbon fiber composites (CFC) present several design problems for lightning protection. Specifically,

·