Bond/Ground Problem

[storeytime]

We're having a problem on a CatScan trailer hook up. The trailer requires 480 volt, 200 amp, 3 Phase power. The hospital only has 208/120, 3 phase. The hospital pulled a transformer out of an old CT scan trailer and wanted us to hook it up to a new trailer. The trailer has a step down transformer for its 208/120 volt needs. It is 480V Delta primary, 208/120 volt Y secondary. The X0 is bonded at that point. The hospital transformer is fed from a 200 amp disconnect. The neutral from the hospital power is not bonded at that point. (A,B,C and ground feed transformer with no attachment to hospital neutral) The ground rod and GEC are at the disconnect. The secondary of this transformer is 480V, 3 ph, Y. (X1,X2,X3,X0 and ground wire to trailer-bonded at transformer) We have 480 volt phase to phase but are getting low voltages off the secondary trailer transformer on 2 of the phases. The trailer tech told us he didn't need the neutral from the 1st transformer(only needs 480 to trailer), so we tried removing the X0. Then we got fluctuating voltage with no 277 to ground. So, we're going like this-
Hospital 208Y to transformer --- 480Delta out of transformer --- to stepdown transformer Delta primary 208/120 Y secondary. I know that we've got a messed up bond somewhere. I also know this is long and confusing but if anybody can make sense of it and offer a suggestion, I'd sure appreciate it.
Tim
PS - We were told that the trailer came from another sight yesterday and the trailer low voltage transformer was producing the correct secondary voltage at that time.

 

[LarryFine]

I'd first try running a temporary neutral to the now-primary X0.

 

[steve66]

Then we got fluctuating voltage with no 277 to ground. So, we're going like this-
Hospital 208Y to transformer --- 480Delta out of transformer --- to stepdown transformer Delta primary 208/120 Y secondary.

It looks like the 480V is only between the two transformers, and they are both delta on the 480V side. How would you get 277V to ground with a delta??

Also, 200A doesn't sound like a lot for a 208V feed to a CT. That would give you about 87 amps at 480V.

 

[charlie b]

Re: Bond/Ground Problem

For one brief period, I worked for a company that manufactured trailers for CT, MRI, and other medical suites. Part of my job was to coordinate with the hospitals who were going to use the trailers, to make sure they had the right facilities (e.g., level parking spot, access roadway with room to turn the truck/trailer, power, receptacle for power hookup, water, phone, etc.). The real answer to your question is that the hospital needs to spend the money to set up a permanent hookup that meets the needs of the trailer.

In the mean time, you have me confused with two statements:

We have 480 volt phase to phase but are getting low voltages off the secondary trailer transformer on 2 of the phases. . . . We were told that the trailer came from another site yesterday and the trailer low voltage transformer was producing the correct secondary voltage at that time.

If you had 480 phase to phase on the output of the hospital's transformer, and therefore also at the primary of the trailer's transformer, how can the secondary of the trailer's transformer be anything but the proper 120V on each phase? Also, how can that change from one location to another?

The only thing that occurs to me is that the trailer loads are not balanced, that two of the phases were heavily loaded and the other was lightly loaded. But I will take a guess that you took the measurements with no significant loads running.

Can you describe the condition of the trailer when you took the measurements?

 

[storeytime]

Thanks for all your replies. This morning we convinced the hospital to set a new 480 volt service for the trailer and do away with trying to feed the trailer from the hospital's existing 208 boosted to 480, which is what I suggested in the first plaec. Now they'll spend twice as much to do what I proposed in the first place. America, what a country.