Neutral bus voltage question

sfergson727

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TECHNICAL Contributor
Oct 12, 2007
12,916
Washington Park Marina, Michigan City
Boat Info
'89 Sea Ray 390 Express
'00 Sea Ray 185 BR
Engines
mercruiser 7.4L inboards
Mercruiser 4.3L/Alpha 1
Hi everyone!

I was doing to electrical investigating on my boat and ran into something totally unexpected (by me, but not my dealer) The boat had been attacked by some stray electricity on my running gear last year, and after checking the galvanic isolators were good, I tried an experiment.

Condition: Shore power Line 1 was powered, Line 2 disconnected from boat, thus unpowered.
I hooked one end of the voltmeter to my electrical outlet hot side of circuit breaker which is powered by line 1. I touched the other lead to the meter to the neutral bus for line 1, 120 Volts, which is what I was expecting.

However, I touched the lead to the neutral bus for line 2, and got a reading of 113 volts, which is not what I was expecting. Shouldn't it have been zero?

My dealer told me that Sea Ray ties the whole neutral system together, and if that is true, why have separate neutral busses? Personally, that explanation just does not hold water with me.

Thanks in advance for any thoughts....
 
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Any stray voltage going to your running gear suggests to me that you do not have a direct path to ground through your power cable back to the source, thus you are completing the circuit back to ground when you come in contact with your running gear. Having your Shore-1 breaker in the on position and all of the branch circuit breakers in the off position, take a voltage reading from your running gear to ground. You should get a zero reading. Now turn on your Shore-1 breakers one at a time and take the same reading until you locate the circuit that is causing the problem and go from there. Is your incoming ground wire bonded to your neutral bus?
 
It should have been zero. When connected to shore power and switch positioned to shore power the neutral buss runs uninterrupted back to the marina's main disconnect panel (that is code). When you switch from shore power to generator the boat's neutral is bonded to the boat's ground network through the generator's frame as you have moved from the utility source power to the generator source power. Per NEC and ABYC the neutral shall be bonded to the ground only at the service entrance. The two independent neutral busses when on shore power should not be bonded together on the boat unless on the generator as that would promote malfunction in your GFCI circuit breakers. Now with all that said if you are using a 50 amp Y adapter the neutrals will be bonded within that Y adapter; even so at that point the hot and neutral in any of the boat's circuits should remain balanced and not trip any GFCI. So there appears to be one or more things going on - 1. there is an interconnect in the neutrals somewhere or 2. Somewhere the neutrals are bonded to the boat's ground which needs to be fixed ASAP. Just to check you shouldn't / don't have the capability to separately switch line 1 and line 2 in the shore/generator transfer breaker do you?
The galvanic isolators really have nothing to do with the neutral wiring.
 
Your two AC Lines should have both the hot and neutral wires completely isolated and only the grounds, going through galvanic isolators, are connected together. The is necessary to keep dockside other upstream GFCI breakers from tripping on two-line boats.

The voltage you saw was possibly just capacitive coupling from the long runs of the wire side by side but should be investigated. Most modern multi-meters have such high input impedance (>10Megohm) that they will pick up false reading in situations like this.
 
Your two AC Lines should have both the hot and neutral wires completely isolated and only the grounds, going through galvanic isolators, are connected together. The is necessary to keep dockside other upstream GFCI breakers from tripping on two-line boats.

The voltage you saw was possibly just capacitive coupling from the long runs of the wire side by side but should be investigated. Most modern multi-meters have such high input impedance (>10Megohm) that they will pick up false reading in situations like this.

I had just disconnected line two a minute or so before I took the reading. I'll try it again individually now that the boat has been sitting depowered for a while. I am still in the storage building, btw.

I'll try the other suggestions also, and let you guys know if I find anything more. I've checked all the grounds, they are in good shape and everything is bonded as it should be.
 
Here is what my props looked like last year when I hauled the boat mid season. I was able to clean the props up with Zing, and they didn't have significant corrosion, just the white buildup that was evident on all my exposed metals.

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Any stray voltage going to your running gear suggests to me that you do not have a direct path to ground through your power cable back to the source, thus you are completing the circuit back to ground when you come in contact with your running gear. Having your Shore-1 breaker in the on position and all of the branch circuit breakers in the off position, take a voltage reading from your running gear to ground. You should get a zero reading. Now turn on your Shore-1 breakers one at a time and take the same reading until you locate the circuit that is causing the problem and go from there. Is your incoming ground wire bonded to your neutral bus?
I don't know if the incoming ground wire is bounded to the neutral bus. I have not touched that part of the electrical system, so it is OEM. The dealer says Sea Ray bonds all the grounds together to include the neutral buses.
 
I don't know if the incoming ground wire is bounded to the neutral bus. I have not touched that part of the electrical system, so it is OEM. The dealer says Sea Ray bonds all the grounds together to include the neutral buses.

That makes sense, but do you actually have a confirmed ground connection coming into the boat via your shore power cable? If you do, then any stray voltage should be drained to ground through this ground conductor. If this ground path is intermittent or permenantly compromised, then you now have a situation that can potentially set up "ESD" (Electric Shock Death) http://marinesurvey.ca/hot-docks/ scenario where you are inadvertently introducing stray voltage into the water via non-current carrying parts such as your running gear. The fact that you received a shock while touching your running gear is definite cause for further investigation while your boat is out of the water.
 
I don't know if the incoming ground wire is bounded to the neutral bus. I have not touched that part of the electrical system, so it is OEM. The dealer says Sea Ray bonds all the grounds together to include the neutral buses.
The incoming (shore power) ground wire(s) must not be bonded to the neutral buss that would be an NEC and ABYC violation. The only time the neutral and ground are bonded together are when the transfer switches are positioned to Generator as they are bonded together at the generator. The AC ground buss, DC battery negative buss, ship's bonding system, and ship's lightning protection system must all be tied together. If your marina has GFCI in either their main distribution panel or on the dock pedestals and your boat has neutrals that are mixed up you will probably be tripping the marina's GFCI system. See schematic below which is typical Sea Ray
upload_2019-4-5_6-54-58.png
 
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The fact that you received a shock while touching your running gear is definite cause for further investigation while your boat is out of the water.

I'm not sure where you are getting this from, I didn't receive a shock of any kind. I guess my initial description must have given you that idea. The boat was attacked, when I said I was attacked, I meant the royal I (Lebowski)

So if I take the leads of my voltmeter to the prop and the garboard drain, is that how to test for current? (with the boat plugged into shore power of course)
 
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The incoming (shore power) ground wire(s) must not be bonded to the neutral buss that would be an NEC and ABYC violation. The only time the neutral and ground are bonded together are when the transfer switches are positioned to Generator as they are bonded together at the generator. The AC ground buss, DC battery negative buss, ship's bonding system, and ship's lightning protection system must all be tied together. If your marina has GFCI in either their main distribution panel or on the dock pedestals and your boat has neutrals that are mixed up you will probably be tripping the marina's GFCI system. See schematic below which is typical Sea Ray
View attachment 67431
I don't have a generator onboard, if that helps....

One thing to note, I have never had any issues with electrolysis until last season. Something has changed, either at my slip, with a neighboring boat, or my boat...just figuring out what...that is the pain in the rear right now.
 
I don't have a generator onboard, if that helps....

One thing to note, I have never had any issues with electrolysis until last season. Something has changed, either at my slip, with a neighboring boat, or my boat...just figuring out what...that is the pain in the rear right now.
If your galvanic isolators are correctly working and your boat's bonding system is in good repair you should be OK. Talk to the marina; they should have a capability to determine what is going on in the water; when I got the 52 and moved to a new slip we tested the slip for electrical potential. Older boats and boats that are not maintained well tend to be culprits if slipped next door.
 

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