Shipboard Distribution and Shore Power

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87CR250R

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Location
Oakland, CA
I work a a maintenance mechanic and maintain a fleet of ship docking tug boats. Most of the electrical work I deal with is troubleshooting motor controllers. I have had several failures recently that have brought to light what I believe to be a potential safety problem. In summary, our 480v generation and distribution system on the boat is set up for Delta configuration. As I have found out recently, our shore power transformer is 480v Wye. I think this is a problem but I am having a hard time convincing the company that this is a safety issue.

For a more detailed description, each boat has two 150-175kw generators wired 480v Delta. The generator chassis is bonded to the hull and we take 3 leads from each into the switchboard. In the switchboard all of the separate chassis have grounding wires that tie them into a common terminal strip which is bonded to the hull. There are 3 small control transformers wired into Wye configuration to create a neutral tied to ground. These form our ground indicating circuit for the 480v system. This is all pretty standard. There is a single 45kva 208Y/120 transformer for lighting loads. This system bonds the neutral to same common ground terminal strip one half of the boats in the class and to a separate on the other half. There is a current transducer on the neutral to ground connection for a ground fault indication.

Everything I have described about the distribution system is standard. The lighting systems in the past have typically been Delta but modern boats have been using Wye more and more frequently. So the issue I have is once the boats are tied into the Wye shore power. The shore power is a 4 conductor cable, 3 hot and a ground that connects to the switchboard. The ground from the shore power is tied into the same terminal strip as the 480v Delta grounds. The first issue I have seen is that if you do have a ground fault on shore power, this connection has rendered the 480v ground indicating lights inoperative (they all stay lit). I'm not sure why this is, yet. But the biggest issue I see is that I have had ground faults in equipment that cause the equipment to energize at 277v through the ground connection to the neutral in the shore power transformer. I first experienced this on a hawser winch with a 480/240v brake solenoid. A screw in a junction box had pierced the insulation on a wire. The winch failed to operate on generator power because the brake would not release and the motor could not overcome the brake. The boat comes in for me to troubleshoot, we plug in, and everything works normally. The screw pierced the wire in such a way that power from L1 could flow to ground through one half of the solenoid (which effectively rewired it for 240v with half the power available). So, on shore power, L1 to ground generated 277v which was enough to drive the solenoid.

More recently on an older boat, I had a similar problem with a chaffed wire in a pressure switch controlling an air compressor. Upon energizing the controller (with the switch in the off position), the controller began buzzing loudly. Because of the chaffed wire, even with the switch off, 277v was still being applied across the m-coil in the contactor causing it to buzz but not start the equipment.

These boats are set up so equipment wont trip if there is a single ground fault. Instead, the indicator lights will show problem and it is up to the engineer to diagnose and repair the problem when conditions allow. But having equipment energize itself through the ground circuits, not trip breakers, and render the ground fault indication system inoperative really bothers me. Trying to tell my company to buy a new Delta 250KVA transformer is going to be a difficult sale but what other options are there?
 

Joethemechanic

Senior Member
Location
Hazleton Pa
Occupation
Electro-Mechanical Technician. Industrial machinery
Your shore power has a N-G bond, correct?

What happens if you lift the N-G bond from your shore power's secondary?
 

Joethemechanic

Senior Member
Location
Hazleton Pa
Occupation
Electro-Mechanical Technician. Industrial machinery
BTW, your ground fault indicator lights, full brightness, or dim? And are you in salt water or fresh?
 

87CR250R

Member
Location
Oakland, CA
That was one of my first thoughts but by lifting the neutral-ground connection in the transformer we lose the ground fault trip protection for the yard because the yard does not have any ground fault indication system. We don't use 277 for anything I would just have the transformer reconnected in Delta but the nameplate says 480Y.
 
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Joethemechanic

Senior Member
Location
Hazleton Pa
Occupation
Electro-Mechanical Technician. Industrial machinery
Can you do it at a time when the place is closed and no yard personnel are present just to see what happens? or maybe you could try a small isolation transformer in your shore power cord with an ungrounded secondary. It wouldn't have to be big enough to operate all the equipment. Just big enough to see if your fault indicator went out and for you to do some tests.

You really can't take your wye shore power trans and reconnect it as a delta because you would end up with nothing but 277 three phase.
 

Joethemechanic

Senior Member
Location
Hazleton Pa
Occupation
Electro-Mechanical Technician. Industrial machinery
Hey,

There isn't a voltage potential between the pier you are working on and the hull of the tug is there? I mean a real one, not some craziness from a digital meter.

Get a 25 watt bulb and attach one side to the hull and the other to some kinda ground rod stuck in the dirt. If the bulb lights you got a serious problem, but even if it doesn't, put a volt meter across the bulb and see if it reads anything.

Be careful, you could have a seriously dangerous situation there. You know this could even be a screw up on somebody else's boat.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
That was one of my first thoughts but by lifting the neutral-ground connection in the transformer we lose the ground fault trip protection for the yard because the yard does not have any ground fault indication system. We don't use 277 for anything I would just have the transformer reconnected in Delta but the nameplate says 480Y.
If you deried to change your shore power to ungrounded 480V, all you need do is lift the N-G connection (float the neutral) and add ground indication/alarming.
The connection of the coils is not important. Depending on its setup, your existing shore GF may actually continue to function correctly.
 

87CR250R

Member
Location
Oakland, CA
No, I don't see any voltage difference from the boats to the pier. Every once in a while we get a hot hull that sees a lot of electrical action on the exterior. Our boats with copper-nickel heat exchangers outside of the boat tend to go through zincs more quickly but nothing really stray current related.

The issue here isn't that the boats are hot, none of them currently have any significant ground faults. I've got one intermittent ground on a 24 VDC alarm panel but I know the cause and will be fixing it shortly.

My real issue here, is it safe to power up a distribution system designed to work on Delta with a Wye transformer. Floating the neutral is a possible solution but I don't think that's kosher with the NEC for our shore side power.
 

Joethemechanic

Senior Member
Location
Hazleton Pa
Occupation
Electro-Mechanical Technician. Industrial machinery
Btw,


Lifting the bond may not be the fix to this situation. But it probably will give us a better idea where to look.
 

87CR250R

Member
Location
Oakland, CA
That video reminds me of a problem I've heard of some years back. I learned about electricity working on automotive things. You know, red is positive and black is negative. It wasn't until college that I even started to touch residential electricity. I've always had an understanding of AC power, scientifically, but never practically. It was shocking to me to find out that most people learn about electricity working on their houses where black is hot and white is neutral and that they actually think DC wiring is difficult to understand. I never figured that out. But basically, the issue with boats, is that people are landing the black hot 120v wire on their 12v negative black wires. Since the negative is bonded to the engine block in most 12v marine systems, the engine block, transmission, propeller, and through hulls all get energized at 120v. Internally, on the insulated fiberglass boat, this causes no problems. Externally... well, you saw the video too. You will notice that if you work on boats about 8 years and newer, many have yellow negative ground wires and maintain the same red positive and black/white hot/neutral just to prevent problems caused by the DIY homeowner.

This does make me think. What if you have a ground fault on a boat connected to shore power with a bonded neutral. We bring divers to our boats all of the time. The divers we hire spend the rest of their time scraping bottoms and replacing zincs in marinas. I'll have to ask them if they've ever been shocked. Of course everything is salt water here.

But again, my issue here is, according to NEC, grounding systems are intended to create a low impedance fault that will draw high current in the event of a ground fault in order to trip the overcurrent device. Delta systems can't do this so they use a ground indicating light so the engineer can diagnose and correct a ground fault before it causes an equipment outage (important on a boat that can't simply shut down). Our yard power uses metal conduit and metal boxes in order to generate low impedance paths to the ground in the main power panel which is bonded to the neutral. My concern is that by lifting the neutral I will break this low impedance path.

Now, how about GFCI? That would certainly protect personnel and one major problem I have seen is equipment powering itself through the ground in the event of a ground fault. The current levels are low enough not to trip a breaker but GFCI would trip the breaker.
 
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