hardworkingstiff
Senior Member
- Location
- Wilmington, NC
Responded to a request today to look at a dock. Parallel 3/0 type W cables feed from the service disconnect (480/277V) on shore down to a floating dock and then a few hundred feet to a MDP. The dock is installed parallel to the shore line and at low tide, the muddy bank is visible on the land side of the dock (I guess this info doesn't matter, but ..?). From the MDP, type W cable feeds to single-phase transformers and then to power centers to provide power to the boats. From the MDP to the farthest transformer is about 600'.
The trouble call is that the customer had someone get a shock when they reached into the water from the dock. The electrician on site (certified boat electrician working on getting his EC license) said he found 80-volts from a SS deck screw to the river water. The EC that installed the system (about 6 years ago) has been checking it out for a week and can not find a problem.
I could not get 80-volts today, but I did see about 30-volts from the deck screw to the river water using a DMM. The voltage went away when the voltage was checked in parallel with a small load (light bulb).
I also found voltage (from 17-volts to 41-volts) from the river water to the fresh water outlet on power center. That outlet is fed by plastic pipe and has brass ball valve and tee on the outside of the pedestal (connected to the plastic pipe feeding to the pedestal). There was no voltage reading from the river water to the ground on the pedestal 50-amp receptacle. The voltage went away again when we checked it in parallel with a light bulb load.
When the circuits in the MDP are opened, the voltage goes away. The voltage steps up a little at a time as you close the breakers. The (2) farthest transformer circuit put the most voltage on the location we were checking.
I told the customer I don't know how to get rid of this voltage, but I doubt it is an issue. I also told him I was concerned about the people getting shocked. They are going to try to recreate the issue.
The odd thing I noticed which I'd love to learn why is the following:
At one specific location (I just picked a spot) I measured from a disconnect mounted on the side of the transformer (connected to the load side of the transformer) to the river water and to a deck screw. Here are the measurements:
L1 to L2 = 244V
L1 to N = 122V, L1 to G = 122V, L1 to river water = 122V
L2 to N = 122V, L2 to G = 122V, L2 to river water = 122V
N to G = 0V (same to the river water, 0V)
L1 to deck screw = 113V
L2 to deck screw = 129V
N to deck screw = 32V
G to deck screw = 32V
OK, so here come the questions.
Can the W cable impress a "ghost" voltage onto a plastic water line? How can a SS deck screw (wood dock with some galv. hardware) have those "offset" readings on voltages?
I tried to keep the information succinct, but I'm sure I left something out, so ask any questions you might have.
Thanks for any input.
The trouble call is that the customer had someone get a shock when they reached into the water from the dock. The electrician on site (certified boat electrician working on getting his EC license) said he found 80-volts from a SS deck screw to the river water. The EC that installed the system (about 6 years ago) has been checking it out for a week and can not find a problem.
I could not get 80-volts today, but I did see about 30-volts from the deck screw to the river water using a DMM. The voltage went away when the voltage was checked in parallel with a small load (light bulb).
I also found voltage (from 17-volts to 41-volts) from the river water to the fresh water outlet on power center. That outlet is fed by plastic pipe and has brass ball valve and tee on the outside of the pedestal (connected to the plastic pipe feeding to the pedestal). There was no voltage reading from the river water to the ground on the pedestal 50-amp receptacle. The voltage went away again when we checked it in parallel with a light bulb load.
When the circuits in the MDP are opened, the voltage goes away. The voltage steps up a little at a time as you close the breakers. The (2) farthest transformer circuit put the most voltage on the location we were checking.
I told the customer I don't know how to get rid of this voltage, but I doubt it is an issue. I also told him I was concerned about the people getting shocked. They are going to try to recreate the issue.
The odd thing I noticed which I'd love to learn why is the following:
At one specific location (I just picked a spot) I measured from a disconnect mounted on the side of the transformer (connected to the load side of the transformer) to the river water and to a deck screw. Here are the measurements:
L1 to L2 = 244V
L1 to N = 122V, L1 to G = 122V, L1 to river water = 122V
L2 to N = 122V, L2 to G = 122V, L2 to river water = 122V
N to G = 0V (same to the river water, 0V)
L1 to deck screw = 113V
L2 to deck screw = 129V
N to deck screw = 32V
G to deck screw = 32V
OK, so here come the questions.
Can the W cable impress a "ghost" voltage onto a plastic water line? How can a SS deck screw (wood dock with some galv. hardware) have those "offset" readings on voltages?
I tried to keep the information succinct, but I'm sure I left something out, so ask any questions you might have.
Thanks for any input.