480V Primary 208V Secondary Transformer question

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ActionDave said:
That is too bad, you really should have. Right now you are not coming off as someone wanting to be helpful, more like kicking a guy when he's down.
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I wouldn't do that.

I'd just lay on them so they can't get up:)

I do weigh around 300 pounds.
 
kwired said:
...Still amazed that any loads utilizing the neutral on the 480 volt side work at all....
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Agreed. If that has been explained, I missed it. I wouldn't believe that a small, modern ungrounded system would support much more than a few milliamps of combined L-G capacitive/leakage current.
 
kwired said:
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Still amazed that any loads utilizing the neutral on the 480 volt side work at all. Is possible you maybe have a neutral terminal in the equipment as standard but maybe there is no 277 volt loads on this particular unit?
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big john said:
Agreed. If that has been explained, I missed it. I wouldn't believe that a small, modern ungrounded system would support much more than a few milliamps of combined L-G capacitive/leakage current.
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...this Neutral is being used inside of the MRI control Cabinet, ...
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Glancing at some MRI schematics I found in some patent docs, I'd say there were no actual 277V loads, the N circuit was likely a requirement because the MRI machine was designed for a grounded Wye system. The thing is basically just a bunch of power supplies going to RF transmitters and receivers, a few servo amplifiers and a PC controlling it it all. Everything I saw was Line to Line loads.

The lucky thing is, apparently nothing has happened on the 480V side of this yet, because if it did, some of the components in that system are undoubtedly NOT rated for the potential 480V line to ground potential you now have. For example there are likely MOVs on the main input power supply that are going to be connected in a Wye and likely referenced to ground. If you ever had a grounded fault on the 480V side, those MOVs would instantly vaporize and may coat everything around them with conductive material, leading to a flash-over. I've seen that happen in large VFDs that have the same sort of arrangement. Everything is fine, until it isn't, then the parts that are suppose to protect your equipment actually turn into little bombs and make it all worse.

Now, when you get that new transformer, it might be a good idea to snoop around a bit in the main incoming cabinet and look for the MOVs that are likely there. If they are all a nice blue, pink or red color, they are likely OK. Not if they are grey, brown black or obviously vaporized, you may want to pay for a PM call from the mfr and get them replaced post haste.

Slo-mo video of an MOV failing "inappropriately"...
https://www.youtube.com/watch?v=fzSjK9Nz44w
 
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