Chassis Ground Question

[colemj]

Originally Posted by Vasco

The only thing I know is that in a direct lightning strike, nothing much protects you. Been there twice.

Not true. Here is our newly installed lightning prevention system:

Mark

[El Rubio]

Originally Posted by Vasco

The only thing I know is that in a direct lightning strike, nothing much protects you. Been there twice.

Proper grounding will protect you from being electrocuted, and it will prevent severe hull damage which could sink your boat. It will limit the damage to other systems too.

[sv.antea]

I once had a system aboard Antea protecting my VHF and ham radios. The ham actually had one of those archaic Dr. Frankenstein dual blade switches on a ceramic base. One side would switch the radio's antenna terminal from the tuner to the chassis ground and the other switched the radio's "12V in" from battery to its chassis ground, thereby somewhat isolating the radio from spikes from antenna and the 12V system. The VHF I simple unplugged, both power and antenna, and plugged the antenna coax to an external connector that was shorted and connected to the boat ground. As I used either radio very little at the dock, I would leave the radios in the protected mode.
I live aboard in Florida. The past decade has been relatively quiet but in the eighties, electric storms were very frequent and violent. One time I was having a drink at the club house and actually observed lightning striking the top of Antea's mast. A shower of green sparks cascaded down to the deck. The VHF antenna was vaporized. All instruments and autopilot fried. Semiconductors and chips exploded - little chunks of black epoxy rattling in the boxes. But both the ham radio and the VHF survived. Anecdotal? Just saying. Stan

[CharlieJ]

A cloud to ground strike typically has a potential of 5,000 to 50,000V and current of 30,000A can travel 50,000 feet from the cloud to the ground, ionizing the air and making it conductive and heating the air to > 20,000C and we think we are seriously going to affect the outcome in the last 50 to 100 feet of its journey?????

[sv.antea]

Charlie J, I think your voltage is actually low (5kV will not produce a very long spark), but why so fatalistic? Lightning strike's energy is contained mostly in portion of radio frequency band. Hence the mysterious paths the current often takes, dodging interiors of Faraday's cages and staying on the surface of conductors (skin effect). A sailboat mast and rigging grounded to an external keel can apparently easily handle the current (few microseconds?). One such Faraday's cage probably was the box of my ham radio. And some say the sailboat rig constitutes a Faraday's cage of sorts as well.
I am sure there are other anecdotes of this kind. A J-24 few boat lengths on my beam in a Thursday night race was struck by lightning. No one was hurt, in fact the crew didn't believe me they were hit until they saw what was left of their Windex.
In any case, what little protection we can provide may not save the chart plotter from direct hit, but may from the spikes (EMP) induced in conductors aboard your boat when a boat down the dock from you gets it. Stan

[Wotname]

Please remember that a lightning strike produces several but very different effects.
For instance, there is the current flow produced when the very high voltages ionise the air and thus makes it a very good conductor. This massive current flow creates lots of heat and melts or burns things that are part of it's path.
Then there is the electromagnetic radiation that is produced from the current flow, this has the potential to fry electronics at quite some distance and induce voltages and potential current flows in nearby metal objects.

These are separate issues and require different types of protection or at least mitigation.

There is no silver bullet.

[Wotname]

Originally Posted by Canibul

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....

Yep, electickey can be confusing and when terms like ground returns, earth loops, single ground points, RF grounds, ac ground, dc ground, shield grounds , isolated circuits, floating voltages, current flow, bonding, equipotential earths and so on get tossed around in the conversation, even well qualified practitioners of this dark art can get confused so really the lay person has no chance of staying abreast of the conversation, let alone fully understanding it.

But help is on it's way Canibul with my simple to follow explanation.

When discussing or thinking about low voltage (12 or 24 volt) DC boat circuits, disregard and don't use any term that contains ground or earth. I would go so far as to remove the word "return" as well. Leave these terms for the others to use. There are only two wires to consider, the positive one and the negative one.

Now remember that all DC electrical devices require current flow in order to operate and this flow must have a path to flow along. Interrupt the path and the item stops working. In this simple explanation, it really doesn't matter which way this current flows or what the current is (more below on that).

The source of the current will be a battery or a solar panel or a alternator and these items have two terminals - one marked positive (+ve) and the other negative (-ve). Conventional flow is considered that the current flows from the +ve to the -ve external to the source. Of course, you can see that internal to the source, this same current must be flowing from the -ve to the +ve but mostly we don't care about where the current flows inside the battery or solar panel or what have you.

So we have a +ve lead taking the current to your desired device or load (light, anchor winch, radio etc) and a -ve lead taking this current back to the source (battery etc). The load has two terminals, a +ve one and a -ve .We connect the source to the load using +ve to +ve and -ve to -ve. That's it in a nutshell.

We usually stick the circuit protection devices (circuit breakers / fuses) in the +ve lead along with controls (switches) and distribution items (switchboard, bus, panel etc) and then lump all the -ve leads together and take them back to the battery.

We could do it differently, we could put say the switch in the -ve side or even everything in the -ve wire but we normally just don't, we stick with the conventional way.

We now have the DC device working and no mention of ground, earth or whatever. These terms do have uses and do mean things, it is just they are not relevant to understanding how low voltage DC circuits work.

Now about the direction of current flow; this keeps changing as the conventions change but really at the end of the day, they are merely conventions. If current is to be considered as the movement of electrons, then they move from the -ve terminal to the +ve terminal (exterior of the source); if current flow is considered to be "holes" or the absence of electrons, then it flows the other way. So you see it really is just convention and the current convention (no pun intended) is that it flows from +ve to -ve which is kinda easy to follow. It goes from where there is plenty (+ve) to where is none (-ve).

Now please, no complicated replies pointing out theoretical inconsistencies of the above, it is just a basic DC primer to point out how to understand a boat DC circuit without resorting to confusing terms like grounds etc. for the uninitiated.

By all means point out gross inadequacies of my explanation.

Originally Posted by btrayfors

Maybe this will help: Grounding

Bill

Nice article Bill, clear and concise, thanks for osting it.

[Vasco]

Originally Posted by colemj

Not true. Here is our newly installed lightning prevention system:

Mark

You going to sail with that?

[Vasco]

Originally Posted by El Rubio

Proper grounding will protect you from being electrocuted, and it will prevent severe hull damage which could sink your boat. It will limit the damage to other systems too.

My keel stepped mast was grounded to the external lead keel with the appropriate size wire. Everything on the boat was fried except a small Garmin GPS (G67?) and my hand held vhf which was tucked into my bib foulies. There was evidence of "treeing" (burn marks) in the paint at all the thru hulls and at the gudgeon and pintle. The thru hulls were not bonded. All lights blown, alternator shot. Autopilot shot and all instruments shot. Large error in magnetic compass. I was sailing at the time and started the motor when there were two strikes right ahead of me. Direct strike at the masthead.

What could've prevented this? One of those bottle washers? If you get a direct hit what can "limit the damage to other systems"?

[CharlieJ]

Vasco- IMHO, nothing could have prevented your damage.

There is some evidence that the system developed by Dr. Thompson at the University of Florida Marine Lightning Protection Inc. can mitigate the damage so the vessel is not severely damaged. Note that, even with Dr. Thompson's system installed, Domino's systems survived one hit/near hit but the systems were damaged some time later by another hit/near hit.

If struck or nearly struck, the vessel's systems must survive a direct surge through the wiring system, an induced surge through the wiring system and electromagnetic pulse (EMP). Surge protectors upstream of mission critical components may help mitigate the surges. EMP hardening of marine electronics will increase the survivability of those electronics. As I understand it the EU is far ahead of the USA regarding EMP hardening.