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Mast electrical question

Ryan L

s/v Naoma
I'm trying to disconnect my mast from the boat's DC ground but am having difficulty figuring out how the mast is making a connection to ground. Anyone have any ideas? Here's what I've done so far:

1) Cut the antenna wire for the VHF. With the antenna connected I'm reading approx 13 volts (charing from show power). When it's disconnected I'm reading approx 10.5v

2) I disconnected the ground wire on my VHF, wind instruments, tricolor, loundspeaker, and deck/steaming light. When tested I was still reading mid 10 volts.

Any ideas?

Thanks in advance...
Ryan
1988 38-200
"Naoma"
 

clp

Member III
Somewhere in all that, you've probably got a shroud or stay 'leaking' to ground. The VHF coax was your best ground going, but the removal of the PL259 broke that circuit. Even then it's curious. The conventional wisdom is grounding stays for the lightning strikes I think. But the resistance you describe sounds like it is leaking backwards through a bulb somewhere, masthead or anchor light, creating a 'false' ground. The mast may not be grounded at all. Simple cure, create a ground leg.
 

Randy Rutledge

Sustaining Member
Is the voltage from DC ground to the mast? Are you reading AC or DC voltage? All lights and other fixtures should be two wire with no electrical contact to the mast, If you are reading a positive DC voltage there is a short to the mast. Turn all DC breakers off and see if you are still getting a reading, shouldn't be any DC voltage. IF you are getting an AC voltage reading you should disconnect shore power, turn off any inverter and check the reading from the ground pin (L shaped) on your shore power cable to the mast, if still there, there is stray voltage from an electrical problem in the marina and your mast is grounded to the water through the prop shaft, the keel or a ground plate on your hull.

If you are getting an AC reading with shore power disconnected do not go swimming stray current can kill Notify your marina management.
 

Tom Metzger

Sustaining Partner
I suspect you are using a digital voltmeter. If you can, measure it with an analog voltmeter. The digital meter has a very high resistance so the mast acts like an antenna.

One thing you can try with a digital meter is to put an incandescent light bulb across the meter leads while making the measurement. If the reading goes to zero, as I suspect, it is the meter's resistance causing the reading. You can also use a heater or any other resistive device.
 

Ryan L

s/v Naoma
Next steps

Thank you all for the suggestions, I'm going back to day with new hope...

clp- I was kinda thinking along the same lines except I'm unable to find a connection from the mast/shrouds to salt water ground. My ultimate intent is to be ground the mast to the keel bolts but since the mast is apparently already connected to DC ground via the coax (and possibly the other mystery connection mentioned in my thread) I did not want to great two grounds with different potentials. I'm gonna have to think about that more. Maybe put a high current breaker on the lightning ground wire so can be turned on when storms in the area but otherwise left off?

Randy- It's almost certainly DC. There are no connections between the mast/shrouds and salt water ground except possibly through the common DC ground via the mystery connection is described in my post. Plus the boat's AC system has a galvanic isolator. Regardless I take your suggestion to heart and think it's a good idea. I will test today with shore power disconnected.

Tom- Excellent idea. I will try the light bulb trick today. I tested on other metal items with no DC connection (miscellaneous screws etc) and consistently read between 1 and 2 volts so was surprised by the 10+ volt reading of the mast.

I'll post update tonight. Thank you for the help and I'd be grateful to read any other thoughts?
 

Joliba

1988 E38-200 Contributing Member
Ryan,
Just curious...are you also getting the same reading off of the shrouds? How about your anchor chain? Windlass?could the hidden ground contact be through the windlass ground?
Mike
 

Ryan L

s/v Naoma
Update

Today I started with an analog meter that showed close to zero volts. Then I switched back to the digital meter and got something between 1 and 2 volts (basically nothing). When I reconnected the VHF antenna it spiked back to 13ish volts. In other words, the antenna seems to be the only thing connecting the mast/shrouds to ground.

But I'm positive (and my wife was there to witness) that yesterday, with the exact same setup (including the antenna not attached to the mast) I was getting readings just over 10 volts.

I reconnected all the other mast electronics and got the same results - nothing with VHF antenna disconnected, 13ish volts with the antenna connected.

I'm stumped. Anyone got any theories? Is it possible the mast was picking up enough RF energy that it was reading 10 volts (is that what you meant Tom?)

Thanks...
P.S. Mike, I did not get a chance to check windlass, anchor chain, shrouds etc. Mainly b/c I forgot... :) Thanks for the excellent suggestion!
 

Tom Metzger

Sustaining Partner
is that what you meant Tom?

Yes, but it's not really what is considered "RF". It goes to zero with the analog meter because the meter requires much more energy, a meaningful amount.

Now that you know there is no energy you can measure what you really want to know, resistance, with an ohm meter.

Meters 101: Digital voltmeters are very good because they require very little energy to get an accurate reading from low energy circuits. Digital voltmeters are very bad because they require very little energy to get an inaccurate reading from high energy circuits.

Or, as Murphy said: Nature sides with the hidden flaw.
 

Ryan L

s/v Naoma
Yes, but it's not really what is considered "RF". It goes to zero with the analog meter because the meter requires much more energy, a meaningful amount.

Now that you know there is no energy you can measure what you really want to know, resistance, with an ohm meter.

Meters 101: Digital voltmeters are very good because they require very little energy to get an accurate reading from low energy circuits. Digital voltmeters are very bad because they require very little energy to get an inaccurate reading from high energy circuits.

Or, as Murphy said: Nature sides with the hidden flaw.

Thank you, good info! If I'm just trying to determine if there's a connection to DC ground what's the difference (in terms of determining ground or no) between measuring volts (battery to meter to mast) vs ohms? I measured 13v (with meter connected to battery and mast) cut the wire to the antenna, measured 1v. DC ground found and eliminated. Right?
 

Tom Metzger

Sustaining Partner
Thank you, good info! If I'm just trying to determine if there's a connection to DC ground what's the difference (in terms of determining ground or no) between measuring volts (battery to meter to mast) vs ohms? I measured 13v (with meter connected to battery and mast) cut the wire to the antenna, measured 1v. DC ground found and eliminated. Right?

OK... Meters 001: You want to know if there is a connection between the mast and DC ground. If there is a solid connection you will read 0 volts. That's ZERO volts. If you have no connection or a poor one you may read a voltage, depending on your meter. An ohmmeter will show how good the connection is. ~0 ohms if it is a solid ground. If there is a less solid ground the ohmmeter will read the number of ohms in the connection. This reading is meaningful for what you want. The voltmeter just told you that it was safe to connect the ohmmeter. The ohmmeter has an internal fuse which will blow if you test a circuit that is energized.

You are looking for an infinite reading on your ohmmeter using a high scale.
 
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