Unearthed power system on ships 4

[sberbece]

Why unearthed power generation & distribution system (say 440 V AC/60Hz) is preferred on ships, rather than earthed power system (common neutral bar).

The only advantage of an unearthed system I know is that offers good continuity in operation.
On the other hand, it requires a higher insulation level, and I believe is less safe, exposing the operating personnel to electroshock.

I would appreciate to see an advice from someone working in marine industry.

Can you please advice me some bibliography regarding electrical systems on ships?

Thank you in advance.

Regards,
Stefan

 

[davva]

Check out the IET (formerly IEE) publication "Recommendations for the Electrical and Electronic Equipment of Mobile and Fixed Offshore Installations". It has an annex that specifically covers all the pros and cons of different earthing systems on system.

http://www.theiet.org/publishing/books/wir-reg/19241.cfm

 

[waross]

The issue is possible severe and rapid corrosion to both the ship and to shore installations when on shore power.
With long feeders any unbalanced load will result in a neutral current and a voltage drop on the neutral. This causes the voltage on the neutral to rise above ground potential. Any shipboard connection between the neutral and ground will shunt this potential difference and corrosive currents will flow through the ship's hull and the shore power ground grid.
The ship or load causing the unbalance may remain safe.
I worked on one ship that had a ground disconnect knife switch. They ran grounded at sea and ungrounded on shore power.
Goggle this site. There have been several discussions of this topic.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rovineye]

A momentary or permanent fault from a phase to hull (ground) will not result in a trip of the breaker, and the load can continue to operate. These grounds occur frequently, mostly due to moisture intrusion. If another phase then faults to hull there are problems of course, which is why we have ground fault monitors on all switchgear and secondaries of power transformers.

It also offers a bit more personnel safety. If the crew touches an energized conductor, while standing on the steel deck, the current flow is lessened as there is no return path. Capacitance and imperfect insulation systems mean a shock will be experienced, but the amperage could be below the fatal level.

There are some prohibitions against low voltage grounded distribution systems as well. 46 CFR does not allow tankers to use grounded distribution, because a fault could result in high hull currents that could heat up at hull discontinuities, such as non-welded plates.

NAVAL SHIPS TECHNICAL MANUAL Chapter 300 discusses basic reasoning for ungrounded systems. Distribution of that manual is limited and controlled however, so I can't send it to you.

Coast Guard NVIC 10-81 also discusses some of the rules and reasons.

http://www.uscg.mil/hq/gm/nvic/10_81/n10-81.pdf

It isn't very in depth, but if nothing else it is worth seeing just for the most brilliant quote ever by a Federal Agency

"good engineering judgment" is typically "something someone didn't have when something happened that shouldn't have,"

 

[JensenDrive]

How do the unearthed systems on ships get around the arcing ground fault issue that is reported to cause high phase to ground faults on ungrounded systems?

 

[THD]

Hi Stefan,

This topic has bothered me for some time. Why not use a common neutral bar and run it with isolation from the hull. This would prevent hull heating and welds etc. Atleast then personell who make contact with a motor that has an earth fault will NOT be shocked. The literature available on marin earthing is weak.

Rgds



Kevin Bosch
Rainbow Technologies

 

[rovineye]

JensenDrive.
A few different ways. First defense as I said is ground monitoring. A alarm sounds when a low resistance to ground is detected, which I usually set for something like 50K ohms, but the value detected is somewhat hypothetical anyway. Then the crew should quickly isolate grounded circuits. We have high resistance grounding transformers on our bus if the voltage is above 1000 volts which are sized to operate continuously on an earth fault, and we have neutral earthing resistors connected to large isolated motors. We also have cable rated for 173% of the line voltage, so we can run continuously on a fault.

THD,
What you describe would be an issue if the motor was isolated from the hull and then the motor faulted to earth. But we make sure every electrical device is bonded to ground. The resistance from cases of motors, controllers, generators etc to ground is kept to less that 0.1 ohms. That way there is no difference in potential from the motor to the hull thru the body. That is true even of resilient mounted equipment, as we add a conductor across the mount. We do the same even across hinges of panel doors when there are electrical devices such as indicator lights mounted on them.

There are many issues with running an isolated ground. Cost and weight to start with. Cable length totals on a big ship can be into the hundreds of miles in total. An extra conductor really add up. It might actually complicate the ground detection scheme since we would then need to monitor the neutral to be able to isolate that fault, but I have not really thought about it much.

 

[itsmoked]

Yeah I was wondering about how the human would make a better conductor to ground from the motor's 'now live' case than the motor being bolted to the metal ground in the first place.

Ships be a different kind of place.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[WDeanN]

“Why unearthed power generation & distribution system (say 440 V AC/60Hz) is preferred on ships, rather than earthed power system (common neutral bar).”

I can’t speak of ships, but have had a little experience with boats. One word – Reliability! It really sucks to have your power go out at 500ft below sea level!

“On the other hand, it requires a higher insulation level, and I believe is less safe, exposing the operating personnel to electroshock.”

This is the same thought I had when making the transition to the civilian electrical world. Why would someone feel safer when one touch of an energized line could make them into a load??!! Yeah, there’s a lot of capacitance in a metal boat with metal ducting and conductors surrounded by a dielectric, and believe me, it still hurts when you get a hold of one leg of that 450V, but I got hung up on 480V later, and that hurt worse. Unless you count the pain in my elbow when my arm jerked free and hit the panel door!

 

[Eleceng01]

WDeanN,

Sounds like you had the same experience as me! Yeah, that 480V in the back of the switchgear hurts. Sure miss the boat at times though! It was a great experience before engineering school.

rovineye has a great answer - tripping a motor for a phase-to-ground fault during "battle conditions" would be a big issue, especially when you are below the surface!