So, how come..?

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Power_Me_Now

Member
It's driving me a bit crazy...

I've read the past posts on "stray voltage" and I still can seem to "fix' my problem..

This applies to a UPS that is being used on-board a large ship.
There is a voltage reading between G and N of 66VAC with .39 Amps showing between the same..
I *thought* once I placed a load on the UPS, this "phantom" reading would go away - it didn't.
I went to a totally different UPS (identical configuration) and it also reads pretty much the same..

I'm sorry in advance for such a odd post but I can't seem to figure out where I getting this bit of power or how to get rid of it..

Any suggestions / thoughts would be most appreciated..

Many thanks,

Doug
 
Rick Christopherson

Rick Christopherson

Senior Member
In my opinion, the terms "stray voltage" or "phantom voltage" are poor terms because they imply that the voltage is not real. The voltage is real, but it may be easily defeated with a small load via Ohm's Law.

I do not know how your UPS is configured nor do I know how the NEC indicates the UPS should be handled, but that being said, the reason why you have a voltage between neutral and ground is because your UPS is behaving like a separately derived system (regardless whether the NEC classifies it to be one or not) and there is no ground/neutral bonding on the load side of the UPS. The implication is that there is no direct reference connection of the neutral conductor on both sides of the UPS.

So that explains the "why's", but as for how you should remmedy the situation, that is something you will have to dig through the NEC to determine.
 
brian john

brian john

Senior Member
Location
Leesburg, VA
Rick Christopherson said:
In my opinion, the terms "stray voltage" or "phantom voltage" are poor terms because they imply that the voltage is not real. The voltage is real, but it may be easily defeated with a small load via Ohm's Law.

I do not know how your UPS is configured nor do I know how the NEC indicates the UPS should be handled, but that being said, the reason why you have a voltage between neutral and ground is because your UPS is behaving like a separately derived system (regardless whether the NEC classifies it to be one or not) and there is no ground/neutral bonding on the load side of the UPS. The implication is that there is no direct reference connection of the neutral conductor on both sides of the UPS.

So that explains the "why's", but as for how you should remmedy the situation, that is something you will have to dig through the NEC to determine.
Click to expand...

Call it what you want in any language we need terms to identify what we are, do, need, say and see. That these terms have been utilized to identify the situation under discussion and is no different than the term ground utilized to bond boxes, neutral for the center tap transformer and on and on. It is a word assigned for that situation. At some point hopefully one term will be defined as the definite term by the NEC or IEEE.
 
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zbang

Senior Member
Location
Roughly 5346 miles from Earls Court
Power_Me_Now said:
This applies to a UPS that is being used on-board a large ship.
There is a voltage reading between G and N of 66VAC with .39 Amps showing between the same..
Click to expand...

IIRC some ships are wired with what are effectively balanced power systems, so there will be 60-ish volts N-G and another 60-ish H-G. Is the ship wired this way, or is there actually a low-resistance N-G bond somewhere?
 
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charlietuna

Senior Member
Most ships have an isolated potential to the frame of the ship--so you could have a zero resistance path to the ship's frame and not trip a breaker or fuse!
 
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achieverelectro

Member
Location
1000' above sea level
PMN:

The likely reason you're having a problem is that neutral wires are illegal aboard USCG inspected vessels. Ergo, you don't really have a "neutral"...both legs are "hot".

Believe me when I tell you that I run into this problem when beach-side electricians come aboard all the time.
Down here in the Oil Patch, a lot of shipyards are in the habit of wiring their secondaries "wye", to get 208/120 volts, and then in accordance with the CFR's, grounding the neutral at the switchboard.

Frankly, this drives me nuts as they can't seem to understand that they are using the hull of the mudboat as a conductor. The folks who sell hull sacrificial anodes and through-hull bearings,{dissimilar metals immersed in salt water}, don't seem to have much problem with this arrangement,though.

If your UPS units were from APC, there's mention in the paper documentation, and also available on their website that addresses this problem.

Basically, you need to purchase shipboard-specific UPS units.

Good Luck.
 
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
achieverelectro said:
PMN:

The likely reason you're having a problem is that neutral wires are illegal aboard USCG inspected vessels. Ergo, you don't really have a "neutral"...both legs are "hot".

Believe me when I tell you that I run into this problem when beach-side electricians come aboard all the time.
Down here in the Oil Patch, a lot of shipyards are in the habit of wiring their secondaries "wye", to get 208/120 volts, and then in accordance with the CFR's, grounding the neutral at the switchboard.

Frankly, this drives me nuts as they can't seem to understand that they are using the hull of the mudboat as a conductor. The folks who sell hull sacrificial anodes and through-hull bearings,{dissimilar metals immersed in salt water}, don't seem to have much problem with this arrangement,though.

If your UPS units were from APC, there's mention in the paper documentation, and also available on their website that addresses this problem.

Basically, you need to purchase shipboard-specific UPS units.

Good Luck.
Click to expand...
I don't understand how that is using the hull as a conductor, unless they are connecting one side of the load directy to the hull. It seems to me that if they did that the hull would be the grounding electrode, but not a conductor.
 
mivey

mivey

Senior Member
don_resqcapt19 said:
I don't understand how that is using the hull as a conductor, unless they are connecting one side of the load directy to the hull. It seems to me that if they did that the hull would be the grounding electrode, but not a conductor.
Click to expand...
Me too. Achieverelectro: could you please sketch the current paths, showing a set of sample currents?
 
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