Electrical Accident

[texie]

Thoght this might be of interest to the forum. It's a news story about catastrophic damage to a ferry boat motor due to what appears to be human error.
http://www.msnbc.msn.com/id/49755077/ns/local_news-seattle_wa/#.UKE-m4eUaAA

 

[zog]

Wonder what real story is, the article does not make sense, sure does not sound like LOTO issue to me based on the description of events.

 

[RichB]

Could still be LOTO--If this is setup like other marine vessels then there is most likely a buss or somwe other type of cross connect for power--as these boats run one motor room for going in one direction and the other for reverse direction. If this wasn't LOTO'ed then therer could easily have been a feedback from Motor Room #2 to #1. Either that or someone overrode LOTO .

All this is speculation until more info is available.

 

[rcwilson]

That class of WA State ferries have large VFD's on the propulsion motors. If I remember correctly, the VFD output is switched to either motor, End 1 or End 2, depending on the travel direction.

There were many many issues getting the drives running originally. Minor issues like the drives randomly going to full speed away from the dock as cars were loading.

It is fortunate no one was hurt.

 

[RichB]

Thanks for the info Bob--I was digging back about 30 years into memory of how some of the stuff worked back in the Navy--Never thought of VFDs in this application--Larnin has accered--kin I git gone now with pay for the day??:lol::happysad:

 

[kwired]

News media outlets are the worst place to get the technical details from. Did someone make a mistake somewhere? Obviously someone must have. Did the media understand whatever technical info they acquired? Likely not. Someone simply forgetting to install a lock somewhere is not that good of a news story either.

This only gets news attention because of how many people are effected by the outcome. If a similar event happens inside a manufacturing plant that only effects a limited number of people, we wouldn't even hear about it, unless there were injuries or deaths.

 

[Jraef]

That class of WA State ferries have large VFD's on the propulsion motors. If I remember correctly, the VFD output is switched to either motor, End 1 or End 2, depending on the travel direction.

There were many many issues getting the drives running originally. Minor issues like the drives randomly going to full speed away from the dock as cars were loading.

It is fortunate no one was hurt.

I agree, and when I lived in Seattle, I worked on one of the ferries that had an old Ross Hill AC drive system for the propulsion. Ross Hill had gone belly up at that time, but their assets had been bought by Robicon, and we were authorized for Robicon drive commissioning and repair. Didn't know squat about Ross Hill drives, but I never let that stop me...

One thing I remember noting and writing up as I started that job was that they were using Vacuum Contactors as the only form of isolation on the cross-over transfer scheme between the two motors and two drives (so that either drive could be routed to either motor). I have never like VCs as isolation devices ever since I read a white paper from Joslyn Clark that explained why that is inherently dangerous. I have never been able to locate that paper again, but the gist of it was that if there is EVER any kind of fault in the system, there can be carbon traces left in the vacuum chamber because no vacuum is perfect. If that happens, the clearances are so small that this can quickly become a conduction path and if an arc forms, it cannot be stopped other than by opening an air break switch somewhere. That system did not have an air break switch anywhere except ahead of the drives. So if one started arcing, the circuit could become energized by the other drive even though the local disconnect on the drive we were working on was locked open! I was told it was "always done that way, nobody ever complained before". The boat was in dry dock for us, so it was no problem for me to just lock open both disconnects and go ahead with the work though.

The thing that bugged me about this story though was that they went on and on about a commutator, which means these were DC motors. I was not aware that any of the ferries had DC propulsion systems on them. But after a little research, apparently the Jumbo class and Super class ferries have DC systems on them. The M/V Walla Walla is the only Jumbo class ferry in their fleet. If they patterned the power system around the older AC units and used a similar scheme, I can see how something like this could happen.