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 Post subject: RF Ground?
PostPosted: 30 Dec 2020 21:36 
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Able Bodied Seaman

Joined: 06 Aug 2020 09:02
Posts: 42
I'm installing a new Raymarine AIS700. The manual says to run an RF ground to protect against near lightning strikes but NOT connected to any point connected to vessel's 0V negative terminal.

I traced out the bonding cable in the boat. Turns out the grounding system includes the 0V negative terminal mounted to a bus bar below the engine. All other ground points lead there then to the engine block.

Anyone deal with this? I can't see installing a dedicated RF ground. Not even sure how I would do it.


 
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 Post subject: Re: RF Ground?
PostPosted: 31 Dec 2020 09:28 
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Skipper

Joined: 09 Aug 2017 15:35
Posts: 501
Location: Maine/USVI
Dedicated Dynaplate? A single bronze thru-hull disconnected from the bonding cable?


 
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 Post subject: Re: RF Ground?
PostPosted: 01 Jan 2021 15:01 
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Able Bodied Seaman

Joined: 06 Aug 2020 09:02
Posts: 42
My sense is that the ABYC grounding and bonding code has changed substantially in the past 40 years since #240 was built. A few interesting things about the grounding system: 1) it uses rather thin unsheathed brass wire, perhaps 10 gauge. This seems very thin to me for this purpose. 2) Forestay, backstay, and sidestays all have wire going to them. The starboard upper is grounded through the stainless strut that runs up behind the settee cabinets from the transverse stiffener in the bilge to the chainplate. 3) The starboard forward upper is tied into the ground. 4) The forestay and sidestay wires connect with the mast step wire then run aft to two bolts through the bilge. I assume these are long bolts that thread into the lead keel ballast. 5) The wire bends into and out of the bilge sump. Seems strange to do this since there will always be some water in the sump. 6) The wire is affixed to a bus bar connected to the engine tray. 7) A separate wire connected to the bus runs to the fuel tank and a different wire runs to the backstay chainplate. Cool I have a yellow wire, maybe 10 or 12 gauge, that runs to the AC panel. 9) My battery negative runs to the bus.

The fact that the battery negative terminated there as a ground and is fully connected with the lightning grounds is troubling to me. Does this not represent a potential electrical hazard to the electrical system? In fact, reading Stan Honey's article on grounding and bonding (http://honeynav.com/wp-content/uploads/ ... unding.pdf), he shows that lightning ground terminating to a single keel bolt and separate from the DC ground and the RF ground.

I don't think I need a dedicated RF ground since I don't use a SSB/HF radio. Therefore, I think I only need a battery negative terminated to the engine block and the lightning ground.

So, I'm wondering if anyone has modified the old grounding system to bring it up to new code and what it was that you have done. Right now I'm trying to rationalize why I shouldn't just upgrade my bilge wire and leave it terminated on the two bilge bolts and taking it off the engine block, then leaving my DC negative and AC negative leads on the engine block.

Also, if you haven't unscrewed and cleaned your mast step wire you might want to make that a priority. Mine was heavily oxidized. See pics.


Attachments:
File comment: Bus for all grounding wires. DC negative wire is thick black one. Yellow is AC.
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File comment: Forward bilge bolt and junction for all rig grounding wires.
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File comment: Oxidation on mast step where grounding wire attaches
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 Post subject: Re: RF Ground?
PostPosted: 03 Jan 2021 11:25 
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Able Bodied Seaman

Joined: 06 Aug 2020 09:02
Posts: 42
I hope I'm not beating on an issue that folks have no interest in, but now that I'm installing a very expensive new electronics suite I am paranoid about a lightning strike (I live in the Chesapeake).

Inconsistencies exist in descriptions and ABYC code of how to properly wire a lightning ground. ABYC TE-4 "Lightning Protection" goes into some detail. I will paraphrase what I *think* the important points are:

1) An Air Terminal is recommended. This is simply a copper or aluminum rod that protrudes vertically from the mast and is higher than your VHF antenna. Anecdotally, driving around my very crowded marina (many hundred of boats) I haven't yet seen one boat fitted with an obvious air terminal. We get a lot of lightning here in the summer. An air terminal seems super easy to fit - thread an aluminum rod into the top plate. Maybe this would prevent VHF antennas from getting vaporized?
2) Primary conductors should be at least #4 AWG wire. For a T37, this would connect the mast step with the forward grounding bolt in the bilge for the most direct path.
3) Secondary conductors should be at least #6 wire and connect all standing rigging and the fuel tank and be routed as directly as possible to the grounding bolt (we have two in the bilge, one in back of the mast step and one approx. 7 ft aft in front of the engine).
4) Rules exist for interconnecting electrical equipment. Is this really a "drain wire"? I'm not sure. But I have drain wires on my Axiom MFD and my AIS. It seems to me these need to go to a secondary connector. TE-4 says the following:
If the case of electrical equipment is internally connected to the DC negative and the negative cable is #6 AWG or larger this cable may also serve as the secondary lightning conductor.
If the case of the electrical equipment is not internally connected to the DC negative or the connection is less than #6 AWG then a secondary lightning conductor shall be installed.


It turns out that my DC negative cable is larger than #6 and goes to the grounding bus that connects to the engine. So, the way I read this is that I can connect my electrical drain wires here. Right?

5) Other large metal objects include bow and stern pulpits. Are these connected to the secondary connectors? I have no idea but maybe there's a branch that stems off from the copper wire going to the forestay and backstay. BTW, the T37 copper wire can't possibly meet the new ABYC secondary conductor standard. It seems too thin to me and also it has crazy bends all over the place.
Attachment:
ABYC Grounding Schematic.jpg

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6) It seems that for a T37, the lightning grounding terminal is the keel (I assume we don't have a lightning grounding plate because those bolts are bedded into the lead keel). Is this right? If so, then it seems superfluous to have a conductor connecting the two keel bolts. Stan Honey's article seems to confirm this.
Attachment:
Stan Honey Grounding Figure.jpg

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Honey also says of the grounding system that there should be "no DC connections to the engine or the yacht's electrical system.

So, here are my thoughts on what to do from here to more adequately protect my boat:
1) Keep my lightning protection system separate from my RF drain and DC neg by removing the conductor running between the keel bolts. I will be wiring new mast lights so these will need to have their own DC return line (i.e. not be grounded to the mast).
2) Attach a proper #6G secondary conductor to the fuel tank and run it to the forward keel bolt. Remove original copper wire from the DC neg bus.
3) Connect my electronics drain wires to my DC neg bus, which is connected to the engine and the aft keel bolt as per the Honey diagram.
4) Remove the backstay conductor
5) Attach an air terminal to the mast

Perhaps I'm making this more difficult than it should be but it just seems there is a lot of conflicting and confusing information out there.


 
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 Post subject: Re: RF Ground?
PostPosted: 04 Jan 2021 12:07 
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Rail Meat

Joined: 06 Sep 2020 10:08
Posts: 14
I'm totally interested in this topic, but I don't have a solution to the conflicting and confusing information. I've puzzled over the same issues.

When we were boat shopping earlier this year I noticed that bonding methods were one of the most inconsistent things from boat to boat. So we aren't the only people who find it confusing.

FWIW, I was on a brand new 75 foot catamaran in Bermuda (watching the 2017 America's Cup challenger finals) that got hit by lightning in the middle of the night. The VHF antenna appeared to be the entry point (the phot shows what was left of the antenna). There was evidence there that the path down to the water was somewhat erratic: it looked like some of it came most of the way down the headstay before jumping out to someplace else (I don't remember what it was). That leaves me believing that whatever we do, the lightning might not follow our plan; there's no foolproof solution.


Attachments:
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 Post subject: Re: RF Ground?
PostPosted: 04 Jan 2021 14:02 
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Skipper

Joined: 29 Dec 2010 10:24
Posts: 179
Interesting topic,I have never been able to understand the reason for the two ‘keel bolts’ as the ballast,into which I assume they thread ,is encapsulated with fiberglass and so is not connected to water. Maybe I am missing something!
I got a 3ft x4””x1/4” piece of copper,bolted it on (outside)just foreword of the mast with two bronze bolts and connected it to the mast base with battery cable. But I think,if lightening hits you direct ,everything is toast ! On our fire dept & I have seen several results.

_________________
Hull #6


 
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 Post subject: Re: RF Ground?
PostPosted: 04 Jan 2021 14:29 
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Able Bodied Seaman

Joined: 06 Aug 2020 09:02
Posts: 42
I think that all we can do is provide the best path to ground as possible to avoid serious damage during a near strike. All bets are off with a direct strike. I also think that the system in the stock T37 is neither adequate for areas with high lightning strikes nor up to current ABYC code.

On the issue of the bolts presumably bedded in the lead keel, I can only surmise that the sheer volume and exposed area to water of the lead keel is what provides the ground. Lead is not a great conductor (12x less than copper) but area makes up for it. But, this doesn't resolve the issue of bottom paint on the keel acting as an insulator - maybe that effect is minor. Is there gelcoat or fiberglass on the keel? I didn't think so.

I think that Timshel's use of externally bonded/bolted copper near the vertical center of the mast is a better solution. See conductivity ratings of metals here: https://www.bluesea.com/resources/108/E ... _Materials

I am going to fix an air terminal to the top of the mast when the rig comes out this spring (might literally be the first time the rig comes out - yikes!). It seems like a very simple solution and why more boats don't have one is a mystery to me.

The main issue for me is one of separating the DC ground, the lightning ground, and the RF ground. Honey is clear on not having separate DC grounds to the engine and keel. That is why I'm considering running all my lightning grounds to the forward bolt (incl the fuel tank), the RF ground (really drain wires for electronics) to the rear bolt, and leaving the engine/shaft for just the DC ground. This way they are all separate and I also don't have an additional 7ft long primary conductor running in my bilge between bolts.

Someone smart on this please tell me my best option! Too much work to do on this boat to fret over this.


 
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 Post subject: Re: RF Ground?
PostPosted: 12 Jan 2021 13:28 
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Able Bodied Seaman

Joined: 06 Aug 2020 09:02
Posts: 42
This piece by Jim Cote (PYS) is pretty good but still not as comprehensive as I would like: https://www.boatus.com/seaworthy/magazine/2016/january/lightning-protection.asp

He mentions this: "Also, if the down conductor is connected to the bonding system rather than directly to a dedicated grounding terminal (ground plate), the lightning strike can energize the entire bonding system before discharging into the water. Another common mistake is to secure the lightning down conductor to other wiring. The high current from a strike through the down conductor can result in voltage surges in these adjacent wires, leading to additional damage in equipment that would otherwise be completely unaffected by the lightning strike."

Additionally, the installation instructions for my Raymarine AIS700 say this: "The Grounding point must be connected to your vessel’s RF ground; this is NOT an optional connection. Do NOT connect to any point that is connected to your vessel’s 0V Negative battery terminal."

I think they are conflating the RF ground with the lightning ground.

From what I see, the T37 bonding system is minimal. Our thru hull valves are isolated so should not be bonded. I do, however, see two copper wires on the fuel tank. One runs to the DC negative bus bar on the engine tray. I'm not sure where the other runs because it's hard to trace. It runs forward somewhere. What else might need bonding?

Anyway, what I am not sure of is whether those two bolts in the bilge are legitimate lightning grounds. The would need to be tapped into the keel lead with some minimum exposed area. But, what else would they be? They are copper and are about 3" long. If they are lighting grounds, why two? If they are grounds, then they should not terminate at the DC ground but they do.

It is a bit frustrating to me that after 40+ years of avid boat ownership that these basic questions remain for the T37.


Attachments:
File comment: ground connections at fuel tank
20210104_174952.jpg
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 Post subject: Re: RF Ground?
PostPosted: 12 Jan 2021 20:01 
Offline
Skipper

Joined: 13 Feb 2011 21:19
Posts: 249
Location: Canyon Lake, TX
Just a guess. No basis in fact.

The forward tank ground goes to the deck fuel fitting? Thereby grounding the fitting to the tank.

_________________
Wayne
Master and Commander of the Sailing Vessel Impetuous
Rider of Waleli Honda 1800 VTX
Subservient to no man except SWMBO
Any day without dock lines is a GOOD day!


 
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 Post subject: Re: RF Ground?
PostPosted: 13 Jan 2021 07:38 
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Able Bodied Seaman

Joined: 06 Aug 2020 09:02
Posts: 42
WayneH wrote:
Just a guess. No basis in fact.

The forward tank ground goes to the deck fuel fitting? Thereby grounding the fitting to the tank.


Quite possibly so. Would prevent static discharge i guess. But isn't the fitting isolated from the tank?

I'm going to suggest here that many T37 owners are blowing out their electronics from near lightning strikes because our stock lightning ground system is tied into the battery negative. Someone tell me I'm wrong.

I still need to confirm that the keel bolts are ground points. I guess I can test for that with an ohm meter?

$4k in New electronics and im not about to fry the whole thing because I hooked it up wrong.


 
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