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Marine Lightning Protection

Grounding strips     

Prices & ordering

See our Prices page for prices, part numbers, and ordering information.

General considerations

Function

Immersed grounding conductors conduct the lightning current directly into the water. However, since the charge neutralized by lightning resides on the water surface each HStripTM should be installed as close to the water surface as possible. Another function of an immersed strip is to shield or bypass other immersed conducting fittings such as propellers,bow and stern thrusters, and transducers.

Area

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.

Edges

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.

Connections

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 HStrip TM

Dimensions

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.

Connections

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.  Use the GapConDaisyTM if you would like to daisy-chain the spark gap connection into a loop from deck to mast base. 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. 

Installation

In all of our systems we recommend that at least one HStrip be placed on either side of the hull. This provides a protective equipotential region inside the boat, 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. In conjunction with additional down conductors and SiedarcTM electrodes at forward, amidships and aft, and the loop conductor at deck level, a protective conducting shield is formed around the whole interior of the boat.

Use with SiedarcTM electrodes

The addition of SiedarcTM electrodes with the daisy-chain connection option provides 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 described above. The diagram on the left shows a typical installation where a connection is made to the mast at both deck level and floor level. This forms a continuous shielding loop around the saloon and provide redundant paths for current flow. The photo on the right shows the part of the lighting system on an Outbound 46 just outboard of the mast. In this case the toe rail was integrated into a continuous loop at deck level and interconnected through a daisy-chain SiedarcTM electrode to an HStripTM on each side.

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 (left photo above), a Mirage Great Harbor N37, we ensured a location that was continually immersed but at the periphery of the hull.  In the next N37, we placed them even closer to the waterline by installing them above the chine. 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.

Products

Overview

In its typical application, the HStripTM is mounted below the waterline and directly below a SiedarcTM electrode, and connected to a loop conductor surrounding the boat at about deck level, as shown above. The GapConTM air gap can be added to isolate the grounding strip to minimize galvanic currents, and two HStrips can be installed end-to-end connected by the HStripConTM to provide one square foot contact surface at one location.

HStripTM

This is a 0.5 ft2 tinned copper grounding strip for use as an immersed water terminal. Heavy duty tinned copper lugs and attachment hardware are included. Dimensions are 3/16" x 2" x 36" with attachment and connection holes on 24.0" centers. Estimated DC resistance in salt water is 0.29 ohms, compared with 0.36 ohms for a square 1 ft2 plate. (See however the important **Note below concerning ground resistance estimations.) The HStripTM is supplied with two round-head tinned brass screw through bolts for attachment and electrical connection. The round head on each bolt increases attachment security.   Electrical connection point is to either or both of the two bolts.    Two HStrips are recommended to give a total immersed area of one square foot.  Use the HStripTM connector below for connecting end-to-end.  A spark gap connector can be added for galvanic isolation, as described below.

HStripTM connector

The HStripConTM is a tinned copper coupling connector for installing two HStrips end-to-end. The resulting 3/16" x 2 "x 72" strip has an estimated DC resistance in salt water of 0.2 ohms. It is installed inside the hull to bridge adjacent connecting bolts of HStrips when placed end-to-end. 

Gap connector

The GapConTM is a tinned copper connector with integral spark gap for (1-10kV) isolation of an HStripTM immersed grounding strip, or two HStrips interconnected via a HStripConTM. It is designed to be crimped directly to a 2AWG cable and comes in both terminal and daisy-chain options.

**Note on ground resistance estimations

The various assumptions made in estimating ground resistance ignore some important factors such as time variations in the lightning current waveshape, removal of charge from the surface of the water, and sparking effects. The errors introduced are significant. Hence all the above estimates of ground resistance should be used only for comparison, not absolute values. See our ProBoat 2004 letter for a discussion of dynamic effects. Add to this the observations of situations where no apparent current flowed from an immersed keel or grounding plate, and the role of immersed grounding becomes even more obscure.