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Joined: 26 Sep 2005
Posts: 10
Location: Richmond, VA

PostPosted: Thu Sep 13, 2007 4:22 pm    Post subject: Reply with quote

I get the reading by touching one probe to the shaft and the other to the raw water intake thruhull (which is tied into our bonding circuit)
Terrell & Beth
T37 #374
Hy Brasil
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Joined: 28 Oct 2005
Posts: 225
Location: Lebanon, NH

PostPosted: Mon Sep 24, 2007 3:12 am    Post subject: Reply with quote

PMFJI, I'd make sure that the voltage you are seeing is "real" before worrying about it. One thing about modern fancy "digital" voltmeters is that they have essentially infinite internal impedance, meaning that they do not load the circuit they are testing. This is handy for accurate readings, but what you want to know is this: is the impedance low enough for real current to travel between those two points? You want to put a load on the tips of your voltmeter when taking this reading. Pretty much any load will do; a 1K-ohm resistor, or even a lightbulb will do nicely (not an LED, though Wink )

When you say "the engine circuit is energized" what circuit are you referring to? There are basically three places that +12v should connect to your engine: (1) The alternator (2) The starter solenoid (3) The ignition switch. The first two are always connected with heavy gauge wires; in a typical house bank / starter bank system, (1) and (3) go to the house bank and (2) to the starter bank (this requires some means of charging the starter battery, usually a switch or an isolator.) The ignition switch should cut off all current flow through (3) when in its off position. (1) will have no current drain due to the rectifiers found in an alternator; (2) will have no current drain due to the fact a starter solenoid is an electrical relay. If you actually see current flow in an engine circuit when your battery master contactor is energized but the key is off, you have an electrical fault that needs to be corrected. I'd wager a guess that an aftermarket alternator regulator has been installed and its ignition lead is wired to the wrong place, and that you have current flow in the field coil of your alternator when the engine isn't running. This isn't good. It can be easily tested for several ways; by testing the voltage on the field coil lead, testing the current through it, seeing a spark when disconnecting it, or (my favorite) seeing if a steel wrench is magnetically attracted to the pulley shaft on it.

At any rate you should not see any "circuit", i.e. current flow, that lifts the engine ground 0.7v above the battery ground.

The bonding circuit (throughhulls, mast, shrouds, etc) should connect to the battery ground at one and exactly one point, to avoid ground loops that drive radios crazy. Some are of the opinion that they shouldn't actually connect at all, but I believe the AYBC has a different opinion; I haven't read their document though; I'm sure our surveyor on the list can comment on that. In my case the only place they connect is at the cutlass bearing bolt which connects to the rudder zincs through the system Rich described.

Regarding lightning protection, it is a myth that lightning damage is unavoidable; if it were, your cell phone service would be a lot less reliable. I wrote a whole article on it for a ham magazine once, but the basic tenet is that lightning can be viewed as an RF (not DC) energy pulse. The frequency centroid is actually about 1MHz. People make the mistake of thinking that lightning is DC and that's why they fail to protect against it. So what's the difference? Bonding circuits are designed for galvanic protection and not lightning protection; they usually use round wires, but RF energy travels on the surface of a conductor, not through its core. Thus adequate lightning protection is achieved through the use of 4" flat ribbon grounds, not round wire. Is this practical considering that shrouds are round? Probably not; but as long as you have a metal mast, you can at least ground that with a ribbon to your ground shoe. The trick is that ribbons are always copper, and the connection to an aluminum mast guarantees galvanic corrosion, with the mast the loser. It can't be avoided; if you insulate it it's not bonding anymore. I've wondered about using a gas discharge tube to make the connection but haven't bothered to try it myself. It's much easier to do in your home ham shack. But I digress...
s/v Kamaloha
1987 T37 #542
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