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Old 04-12-2013, 05:27   #1
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Chassis Ground Question

I'm trying to educate myself on boat electrics, after lightning wiped us out on our first trip in the 28 year old catamaran we bought last year and sailed from Jacksonville FL down to the Turks and Caicos. We lost a lot of stuff due to that lightning strike.

A couple things I've noticed while working my way through systems added by three previous owners over 27 years. A lot of the systems added, and that got fried, were not fused at all. The other thing that I have discovered is that very few of the added things on this boat had chassis grounds. The VHF, two MPPTs, the radar....all these got smoked by the lighting, and none of them had the chassis grounded even though all of them plainly have ground lugs on the chassis.

SO, what's the correct way to ground them, to the boat's negative electrical supply, or is it better to ground them to the sea via the strapping and anodes?
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Old 04-12-2013, 07:05   #2
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Re: Chassis ground question

The West Advisor article on grounding is pretty informative. It explains how to avoid galvanic corrosion issues. Specifically, the boat's negative supply should be treated as a dc return and not ground even though the engine and dc negative will be connected ground at the engine or other single source.

I work on land based communications systems and grounding is a big deal. We routinely have towers that take direct hits and suffer very little damage if any. Some of the principles can be utilized on a boat to add to the protection.

First, understand that a direct hit will probably cause damage to electronics even if grounded. The ground is primarily to keep you or someone else in proximity to the device from becoming a path to ground for a lightning surge. It will also help with EMP caused by nearby strikes that induce current on nearby wires. This induced current probably causes more damage than direct hits.
Second, twist the positive and negative power leads together the entire length from the supply panel to the device. If an EMP current is induced on this line, the twists may help cancel or reduce it.
Third, install surge suppressors on antenna cables and at the shore power supply. The rf suppressors are like barrel connectors that must be grounded to work and installed near the radio. The shore power protector is for surges on the commercial power system. This might be the most common source for damaging surges. Google whole house surge protector. Raycap is a company we use extensively for AC power protectors. It's not terribly expensive, but is very effective. You might consider protecting any other links to the outside world such as telephone, cable, dsl, etc. They make surge protectors for those links as well.
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Old 04-12-2013, 13:55   #3
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Re: Chassis ground question

It is rare to see DC items chassis grounded. But chargers and inverters do need chassis grounds in case of internal fault. The chassis ground goes to the negative bus which is connected to the engine block at a single location.
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Old 04-12-2013, 22:44   #4
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Re: Chassis ground question

the electronis have RF grounds for noise and interference. ideally you'd have a RF ground bar on the boat. everything gets wired to the rf ground bar, then that bar gets grounded to the main DC neg bar.

most often they are left off...

chassis grounds are different and normally only on chargers and inverters. things with AC


nothing is going to stop stuff frying from lightning... they even took the lightning wiring code out of the ABYC regs recently. because there is just no way to study and test it.
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Old 05-12-2013, 04:05   #5
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Re: Chassis ground question

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the electronis have RF grounds for noise and interference. ideally you'd have a RF ground bar on the boat. everything gets wired to the rf ground bar, then that bar gets grounded to the main DC neg bar.

what would be achieved by such a rf bus bar being connected to DC negative ??? Since there is no guarantee that DC neg has a good path to ground. ( and there are many points to argue that DC negative should never be grounded)


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Old 05-12-2013, 21:27   #6
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Re: Chassis ground question

in north America the RF ground, ac ground, bonding system all tie into the dc neg which is the ground of the boat.
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Old 05-12-2013, 21:49   #7
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Re: Chassis ground question

RF ground is always kept separate from the DC return. In fact some tie the rf ground from the HF tuner to the ground plate/through hull/counterpoise using capacitors so that there is no DC current. The ground lugs of radio chassis etc. are not used on a boat whereas they may be used in a stationary radio shack where they can be tied in directly to a ground rod.
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Old 06-12-2013, 01:02   #8
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Re: Chassis ground question

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Originally Posted by smac999 View Post
in north America the RF ground, ac ground, bonding system all tie into the dc neg which is the ground of the boat.
You really can't say that!

Ideally all ground/bonding systems are autonomous unto themselves. this means that each and every of the separate grounding/bonding systems are separate of each-other.

In other words never the 2 shall meet......except in one instance.

All shall tie to sea water at only one source.

DC ground is not "Sea Water' AND THAT'S FINAL!!!

Llloyd


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in north America the RF ground, ac ground, bonding system all tie into the dc neg which is the ground of the boat.
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Old 06-12-2013, 02:10   #9
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in north America the RF ground, ac ground, bonding system all tie into the dc neg which is the ground of the boat.
(A) it's its not and (b) that's just a particular " custom & practice ". You still didn't answer my question merely repeated " code"

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Old 06-12-2013, 05:53   #10
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Re: Chassis ground question

This is confusing me a little. I thought that the actual flow of electrons was from negative to positive, ( hence the very terms themselves) and that would make the positive wiring the "dc return".

If the "dc return" (negative) is not a ground, but is connected to ground at the engine...then it definitely IS a ground....

this is kinda why I asked the question.

The grounding lugs on the VHF, radar, MTTPs, PWMs, are not to be used?
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Old 06-12-2013, 23:34   #11
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Re: Chassis ground question

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what would be achieved by such a rf bus bar being connected to DC negative ??? Since there is no guarantee that DC neg has a good path to ground. ( and there are many points to argue that DC negative should never be grounded)


dave

the RF ground wire takes the equipment case to the reference ground voltage of the source through a non current carrying wire.

it's the same principle of grounding the drain wire on a shielded cable. it's only connected on the source end and to the source neg.

if you connected the case rf ground to another source / point, you'd still have a floating voltage between the inside components and the case itself. same as you did before you connected anything. probably achieving nothing.
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Old 09-12-2013, 17:07   #12
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Re: Chassis Ground Question

Man, I am totally confused after reading all this...just when I thought I had it all under control!
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Old 09-12-2013, 17:21   #13
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Re: Chassis Ground Question

The only thing I know is that in a direct lightning strike, nothing much protects you. Been there twice.
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Old 09-12-2013, 17:44   #14
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Re: Chassis Ground Question

Maybe this will help: Grounding

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Old 09-12-2013, 18:09   #15
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Re: Chassis Ground Question

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The only thing I know is that in a direct lightning strike, nothing much protects you. Been there twice.
Us too. All those years with no issues and then both the boat and the house got hit within a year of each other. In both cases, the strike was to a vhf antenna. The hole in the roof is fixed on the house, but I'm still trying to sort the boat out.

Hence my intense interest in these things. Neither radio chassis was grounded. Both were destroyed.
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