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Lightning Ground Question

Tom Metzger

Tom Metzger

Sustaining Partner
As usual, Maine Sail is right. I have used this method of attaching grounding to the keel on boats I have owned. It's the simplest way to do it.
Mike Jacker
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Not so fast, Mike. MS's method is good, but the 2/0 cu wire is way over sized. According to my guru, Ewen Thomson, AND ABYC main conductors (ie: mast to keel connection) must be a minimum of #4 AWG stranded copper. Secondary connections should be a minimum of #6 AWG.

2/0 will work, but it is hard to deal with and is expensive. I used #2 primarily because it's what I had since I use it for all of my high current needs on my boat. The voltage drop with #4 cu is 5 volts with 10,000 amps over 2 feet. Completely insignificant when dealing with lightning. Heating is not a problem and #4 is strong enough to withstand the mechanical stress.
 
Joliba

Joliba

1988 E38-200 Contributing Member
Yes, I did use smaller than 2/0, also. I was referring to his method of attaching the ring terminal of a ground wire or cable to a plate attached between 2 nuts on a keel bolt.
 
V

Vagabond39

Member III
An interesting idea using keel bolts for grounding. But, isn't yory keel covered in fiberglass, gel coat, sealer, and ablative paint?That seem to insulate it from the water, not ground it.
 
M

Maine Sail

Member III
Tom Metzger said:
Not so fast, Mike. MS's method is good, but the 2/0 cu wire is way over sized. According to my guru, Ewen Thomson, AND ABYC main conductors (ie: mast to keel connection) must be a minimum of #4 AWG stranded copper. Secondary connections should be a minimum of #6 AWG.

2/0 will work, but it is hard to deal with and is expensive. I used #2 primarily because it's what I had since I use it for all of my high current needs on my boat. The voltage drop with #4 cu is 5 volts with 10,000 amps over 2 feet. Completely insignificant when dealing with lightning. Heating is not a problem and #4 is strong enough to withstand the mechanical stress.
Click to expand...


Oversized and lightning.... ? I'll keep my 2/0, which has already done its job once. Hard to work with? Not at all but I can assure you that 4/0 at 10F gets a bit tedious when stripping it.. Expensive? For 2' 2/0 it is $6.60 more expensive than 2GA. My jumper is about 7"... The guidelines are just that, guidelines. They are also bare minimum suggestions.

I get to do lightning damage assessments as part of my living. How our boat is wired for lightning damage mitigation is from my experience in assessing this damage on other boats.. I have seen some pretty mind blowing stuff like a VHF whip shot through solid fiberglass like an arrow and a 15 pound bilge cover/cabin sole blown 6' across a cabin and impalled into the nav station. I also saw an 85k carbon spar, which had a 4GA primary down conductor totally destroyed.. There was no fixing the spar and due to the age of the boat, and cost to repair it, the insurance company totaled it..

You can't prevent a strike but there is potential to minimize hull damage.. If you feel comfortable with 4GA or 2GA that is great, and all that matters...
 
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Tom Metzger

Tom Metzger

Sustaining Partner
Maine Sail said:
Oversized and lightning.... ? I'll keep my 2/0, which has already done its job once. Hard to work with? Not at all but I can assure you that 4/0 at 10F gets a bit tedious when stripping it.. Expensive? For 2' 2/0 it is $6.60 more expensive than 2GA. My jumper is about 7"... The guidelines are just that, guidelines. They are also bare minimum suggestions.
Click to expand...

When you had read Ewen Thomson's papers from the University of Florida you see that his recommendations are based on research, not anecdotal postings.

Yes, 2/0 cable is better than #4 for lightning protection, 4/0 is better than 2/0, and a Hummer is safer than the cars we drive, but we don't all rush out to buy Hummers.

When one is installing this connection, using 2/0 is fine if one has the wire and terminals, but if he is going to buy for the job I would go with ABYC rules or a size larger. The rules are the minimum, Not the "bare" minimum. There is a big difference between selling to a customer and helping a friend.
 
M

Maine Sail

Member III
Tom Metzger said:
When you had read Ewen Thomson's papers from the University of Florida you see that his recommendations are based on research, not anecdotal postings.

Yes, 2/0 cable is better than #4 for lightning protection, 4/0 is better than 2/0, and a Hummer is safer than the cars we drive, but we don't all rush out to buy Hummers.
Click to expand...

I have read every ounce of Thompson's research I can get my paws on. I've also collaborated with Boat US and multiple insurance companies to get even more actual data on strikes. You would be SHOCKED at the sheer lack of data collected. I am currently working with Boat US to get better damage tracking reports by surveyors so we can get a much larger n=X pool of data to draw from. Boat US is working on a data analysis & compilation of lightning claims but the person working on it is still at least a year out. Every survey has to be gone through and that takes lots of time..

As of now there is a good pool of evidence, including the Florida Sea Grant data by Thompson, that suggests bonded boats suffer less hull damage, but more data would always be better. Thompson generally recommends 2GA over the ABYC TE-4 4GA... His take on this subject has also evolved a lot over the years. He's a good guy and a pleasure to talk to.

Tom Metzger said:
When one is installing this connection, using 2/0 is fine if one has the wire and terminals, but if he is going to buy for the job I would go with ABYC rules or a size larger. The rules are the minimum, Not the "bare" minimum. There is a big difference between selling to a customer and helping a friend.
Click to expand...

There are no "rules" for lightning protection because we simply don't fully understand. ABYC TE-4 is a technical report but not a requirement under the ABYC standards for builders. And yes those suggestions are the minimum suggested sizes, but they are suggestions not rules.

If I was helping a friend I would give him the 2/0 or 4/0 and not even charge him....:) As for a customer I always give them the choice and go over what the "suggested sizes" are. I make no more or less selling 2GA vs. 2/0 and in the whole scheme we are talking floor mat change differences anyway...... I have seen lightning blow a 4" hole clean through a ground based solid metal transformer a few miles from my house. If it can do that I think even my 2/0 is subject to being too small in the right strike...

Each boat owner has to do what makes them comfortable in regards to lightning. I would be more comfortable with 4/0 but not enough more to crawl down in there and swap it out, heck if I had 2GA I would probably leave it too......:)
 
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Tom Metzger

Tom Metzger

Sustaining Partner
.... I have seen lightning blow a 4" hole clean through a ground based solid metal transformer a few miles from my house. If it can do that I think even my 2/0 is subject to being too small in the right strike...
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This is not comparable. With utility power equipment you have a different situation. The lightning causes an insulation failure which brings to bare continuous (compared to lightning) high current AC faults. I've seen substation transformers weighing tens of tons move a foot on their pads from faults close in on the secondaries that produced shorts in the windings - no lightning involved in the station. Relatively common with slow oil filled circuit breakers in old stations.
 
jreddington

jreddington

Member III
The ABYC standards address this. To meet this standard (in quick summary), you need an air terminal at the top of the mast and a primary lightning conductor of 4 AWG. This should run as straight down as possible to an immersed ground plate, 1 ft square with an electrical conducting bolt protruding through the hull to connect the lightning cable to.

The chain plates of every shroud should be connected to the main line with a secondary minimum of 6 AWG wire. Large metal objects within six feet of the main conductor such as engines, tanks, winches, etc. should be bonded to the primary conductor with 6 AWG cable.

Seacocks and thru-hulls should be bonded to the plate with 6AWG directly, not through the primary conductor.

There are a number of additional details which make this a complex project, and still does not result in 100% protection. Improperly done, you increase the chance of side flashes.
 
Tom Metzger

Tom Metzger

Sustaining Partner
The ABYC standards address this. To meet this standard (in quick summary), you need an air terminal at the top of the mast and a primary lightning conductor of 4 AWG. This should run as straight down as possible to an immersed ground plate, 1 ft square with an electrical conducting bolt protruding through the hull to connect the lightning cable to.
Click to expand...

Jim - In spite of a little mast bend, I think the keel stepped aluminum mast can be considered "as straight as possible" so no need for the copper down lead from the masthead. :) Retrofitting the recommended system to an Ericson is VERY difficult. Also, 1 square foot of ground plate is way too small for fresh water.
 
R

Ryan L

s/v Naoma
Maine Sail said:
This is how I isolate my VHF whip from the spar...

I found another possible solution but I'm interested in other perspectives regarding this idea...

A company called Poly Phaser makes a unit called S-B50LU-C0. It's a surge protector with UHF connectors that works for VHF frequencies. It blocks DC on the center pin unless there is a current overload in which case it shorts the current to ground. I spoke with their excellent customer support and was given this suggestion- Connect the coax from the radio to the B50 as normal. Ensure the B50 is grounded to boat's DC ground w/ at least AWG 6 wire. Prepare a length of coax with a normal UHF connector at one end and a UHF connector NOT CONNECTED TO SHIELD on the other end. Use that to connect the VHF to the B50.

This would eliminate the problem and offers additional protection to the radio (although I imagine it could work without the B50 as well?)

At first glance it seems like a simple but effective solution. Thoughts?
Click to expand...
 
Tom Metzger

Tom Metzger

Sustaining Partner
Ryan L said:
Maine Sail said:
This is how I isolate my VHF whip from the spar...

I found another possible solution but I'm interested in other perspectives regarding this idea... [SNIP] At first glance it seems like a simple but effective solution. Thoughts?
Click to expand...

Ryan - It's nifty, but doesn't isolate the mast from the the dc ground which is what I think you want to do. It isolates the center conductor, not the shield.

I get the impression that you are trying to replace a simple idea for an insignificant problem with a complex solution. That much more hair in the scupper.
Click to expand...
 
R

Ryan L

s/v Naoma
Tom Metzger said:
Ryan L said:
Ryan - It's nifty, but doesn't isolate the mast from the the dc ground which is what I think you want to do. It isolates the center conductor, not the shield.

I get the impression that you are trying to replace a simple idea for an insignificant problem with a complex solution. That much more hair in the scupper.
Click to expand...


Tom unless I'm missing something this way will isolate both center pin AND shield? Center pin via the surge protector, shield via simply not connecting the shield (as described in my previous post). I hear what you're saying regarding hair in scupper (neat expression, hadn't heard that before) but if this works it's just a simple in line installation and solves the ground problem as well as protect the radio. Right?
Click to expand...
 
Tom Metzger

Tom Metzger

Sustaining Partner
Ryan L said:
Tom unless I'm missing something this way will isolate both center pin AND shield? Center pin via the surge protector, shield via simply not connecting the shield (as described in my previous post). I hear what you're saying regarding hair in scupper (neat expression, hadn't heard that before) but if this works it's just a simple in line installation and solves the ground problem as well as protect the radio. Right?
Click to expand...

Ryan - My bad, I missed the ungrounded shield. The coax with a disconnected shield is not coax. You will lose radio signal, particularly when transmitting.

Check with the manufacturer. I'm not a qualified antenna engineer, but it sounds bad to me.

An unrelated thought: I wonder how effective Maine Sail's solution is in a salt water environment. Does the salt on the surface of the insulator block provide a path to ground for the corrosion current? I don't know... just asking.
 
R

Ryan L

s/v Naoma
Tom Metzger said:
Ryan - My bad, I missed the ungrounded shield. The coax with a disconnected shield is not coax. You will lose radio signal, particularly when transmitting.

Check with the manufacturer. I'm not a qualified antenna engineer, but it sounds bad to me.

An unrelated thought: I wonder how effective Maine Sail's solution is in a salt water environment. Does the salt on the surface of the insulator block provide a path to ground for the corrosion current? I don't know... just asking.
Click to expand...

I did check with the manufacturer, did you read my post? :) I spent almost an hour on the phone with two of their experts. One was an engineer specializing in RF stuff and the other was a career signals guy from the Coast Guard. Not connecting the shield was their idea.

I tested transmitting with only the center pin connected and anecdotally do not seem to have any loss of signal strength (there is an automated radio check system in our region).

I think Maine's solution is excellent as well.
 
R

Ryan L

s/v Naoma
Finally a simple effective solution!

I ended up solving this problem by grinding the opening on the mounting bracket so that there was slight play when the antenna was installed. Then I placed a layer of 20mil PVC tape on the top and bottom of the bracket. I used a razor blade to cut out the mounting "hole" on the bottom and top but I left a 1/16th or so or tape sticking "proud" into the mounting circle on the bracket. When I insert the antenna (pressing it down into the mounting circle) the tape stretches into the tiny gap between the antenna and the mount, thus insulating the threads and providing a tight mount. The antenna ground is now isolated from the bracket and thus from the mast (confirmed with a meter). Super simple, inexpensive, secure, and effective so far.
 
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