Voltage on alternator ground?

[tkeithlu]

I use a full Balmar alternator/regulator system to charge my house battery bank from my engine. It works beautifully. However, I was checking my bonding system yesterday with the engine off and noticed a small spark from the Balmar alternator ground to the engine block. My trusty multimeter revealed house voltage on the ground, but zero current. Could this be reading through my instument panel voltage meter, or some other un-measurably small use of current? I've not noticed any current drain from the battery bank. Thoughts from someone who knows DC better than me?

[thomm225]

Is the ground connection loose at the block?

[Stu Jackson]

If your alternator is connected to the house bank, it is always LIVE. Of course, it should be fused, but Maine Sail also suggests a service disconnect switch so that when you work on the engine you disconnect the alternator. I just remove the fuse.


Tighten that connection, Sparky!


You might consider learning some more about your electrical systems, too. Good luck.


Electrical Systems 101 Electrical Systems 101

[tkeithlu]

The connection is normally tight; I'm not that inept. But, thank you for the guidance.

[thomm225]

Sometimes you have to loosen, clean, then retighten for a good connection even though it feels and appears tight especially if there are multiple wires attached.

Kinda like when your car won't start but when you tap the battery cables at the terminals/posts with a small hammer or heavy enough screwdriver etc it starts right up. (even though the cables felt tight to begin with)

[tkeithlu]

Thank you. I designed and built a rather complicated AC and DC system for our boat, with multiple sources of power for just about any task. I found this "problem" when I installed a heavier bonding cable for the starter battery for the main engine. It had not occured to me that the alternator ground would be live, given devotion to avoiding using the steel hull as a current return.

[billknny]

Originally Posted by tkeithlu

Thank you. I designed and built a rather complicated AC and DC system for our boat, with multiple sources of power for just about any task. I found this "problem" when I installed a heavier bonding cable for the starter battery for the main engine. It had not occured to me that the alternator ground would be live, given devotion to avoiding using the steel hull as a current return.

Does your alternator have an isolated ground?

That is the way marine alternators SHOULD be, but all too often are not. That's the way that you avoid using either the engine block or a steel hull as the return path for the alternator output.

[tkeithlu]

Damn! That's good. Thanks so much. I'll just run it to the battery negative rather than the block.

[RossD]

Good information here, thanks!

[kenbo]

Originally Posted by billknny

Does your alternator have an isolated ground?

That is the way marine alternators SHOULD be, but all too often are not. That's the way that you avoid using either the engine block or a steel hull as the return path for the alternator output.

I agree if you have a metal hull but it's not necessary on a fiberglass hull. Many engines come standard with 1 wire sensors that require the block for ground. Unless you have 2 wire sensors you need to ground the engine block.

I think the problem arises when you use the alternator case as the ground rather than a separate but equally sized ground wire for the alternator. I also see a lot of problems when multiple ground points/planes are created by improper wiring.

To the OP you might want to check the diodes in the alternator's bridge rectifier, especially the negative diodes.

[hellosailor]

"It had not occured to me that the alternator ground would be live,"
FWIW, I've never heard an electrician refer to a ground as "live". Live is reserved for "hot" wires, meaning they are carrying voltage above ground level. There can be various types of "ground" but they are always assumed to be connected, GROUNDED not LIVE or HOT, at all times.

If your alternator made a spark to ground when it was not running? Then some component in the alternator was passing power when it should not have been. The alternator may be wired wrong (in the control circuits) or you may have a diode failing, passing reverse current. Before you start moving the ground, it may pay to start checking the alternator for internal or configuration problems. There's still going to be something wrong--regardless of where you move the ground to.
If you disconnect the ground from the block, and insert an ammeter, either it will read zero, or it will show there's a current flow. ANY current flow when the alternator is off, means a problem.

[Captain Bill]

Check for voltage between the block and battery negative. I once had a corroded ground connection at the engine block and there was 8 volts between the block and the battery. Oddly enough the bolt head had zero volts to the battery as it was making good contact with the cable. The block however was not. While trouble shooting I was assuming that the bolt head would be the same as the block, but it wasn't. The ultimate source of the problem was a leaking exhaust elbow which had dripped on the ground connection. I cleaned the connection and replaced the elbow and all was good.

[Boatyarddog]

Originally Posted by tkeithlu

I use a full Balmar alternator/regulator system to charge my house battery bank from my engine. It works beautifully. However, I was checking my bonding system yesterday with the engine off and noticed a small spark from the Balmar alternator ground to the engine block. My trusty multimeter revealed house voltage on the ground, but zero current. Could this be reading through my instument panel voltage meter, or some other un-measurably small use of current? I've not noticed any current drain from the battery bank. Thoughts from someone who knows DC better than me?

You said "house voltage" is that 120 or 12vts.
Bonding systems are usually connected to 12 Vt.DC circuits and 120 Vt in vessels to stop or reduce corrosion of metal.
But, only the neutral.
Could there be an issue in your dockside pedestal power box? Problem with your bonding?
SV Cloud Duster

[tkeithlu]

Lots of good thinking here, which I appreciate. I'll start with checking the diodes, which didn't occur to me given a Balmar alternator that is only a few years old and is charging the house bank of six size 27 lead-acids just fine.

I found house voltage between the alternator ground wire (not to the alternator case) and the engine block, but zero current, at least to the level that my multimeter could detect.

Yes, we are talking about the 12vDC circuit, not the 120vAC circuit. Our shore power goes directly to a 1:1 transformer, isolating the boat.

[SwellGuy]

Originally Posted by tkeithlu

Lots of good thinking here, which I appreciate. I'll start with checking the diodes, which didn't occur to me given a Balmar alternator that is only a few years old and is charging the house bank of six size 27 lead-acids just fine.

I found house voltage between the alternator ground wire (not to the alternator case) and the engine block, but zero current, at least to the level that my multimeter could detect.

Yes, we are talking about the 12vDC circuit, not the 120vAC circuit. Our shore power goes directly to a 1:1 transformer, isolating the boat.

I'm curious how you are measuring current. If you are just clicking your meter to "current" and measuring the same way you measured voltage (ie, with the meter connected in parallel to the target circuit) it won't work.