Grounding a 480v 3p stepup transformer .

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pugpen

Member
Location
wisconsin
I ran into a very odd transformer setup in our plant last week and wanted to run by everyone here as it looks totally wrong . they have a 480v three phase cnc machine fed by a 480-208 v transformer ran backwards . everything seems to work ok but the machine ground is ran to the transformer cabinet ground and then back to the panel . how should this be done ? the transformer is a 480 delta to 240 Y ran in reverse . one of the electricians says we should ground the neutral and another says we should ground any of the conductors on the secondary .
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Since the 480 side is Delta it is common to ground one of the phases which creates a corner grounded Delta system. The X0 is left floating on the 208 volt side and has no connection. If you leave the 480 side ungrounded then you'll need to provide ground detectors as in 250.21(B).

Welcome to the Forum. :)
 
Two additional comments: It is very important to leave the XO on the 208 side floating. Also, AC drives often need to be fed from a 480 wye system, although many CNC have a isolation transformer built in in which case it wouldn't matter.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
The first electrician is completely wrong. The neutral in this particular reverse fed transformer is on the primary side. Grounding the neutral will do nothing to ground the 480V side, and will likely cause problems.

The second answer is plausible; corner grounded delta systems are known and do work, however you will need to confirm that the load (the CNC machine) can tolerate corner grounding.

A 3rd answer is to add a 'grounding transformer' to the secondary side. This is an autotransformer which derives a neutral; you then ground this derived neutral.

The best answer is: replace the transformer with a proper delta:wye 208:480 transformer.

-Jon
 

pugpen

Member
Location
wisconsin
I have looked into getting the proper transformer but it is a month wait . what can I do to make it safe until we get the new transformer ? the cnc machine has an isolation transformer built in to it . looks like corner grounding the delta secondary would be a good start but can I just go from one of the phases to the ground lug on the cabinet that goes back to the panel or should I drive a new ground rod . I watched a few hours of mikes videos on grounding and iam still a bit confused . Thanks for all the help .
 

kwired

Electron manager
Location
NE Nebraska
I have looked into getting the proper transformer but it is a month wait . what can I do to make it safe until we get the new transformer ? the cnc machine has an isolation transformer built in to it . looks like corner grounding the delta secondary would be a good start but can I just go from one of the phases to the ground lug on the cabinet that goes back to the panel or should I drive a new ground rod . I watched a few hours of mikes videos on grounding and iam still a bit confused . Thanks for all the help .
Can that isolating transformer built into it be reconfigured for 208-240 volts input? Might not even need the transformer in question. Otherwise probably nothing wrong with supplying with corner grounded system.
 

kwired

Electron manager
Location
NE Nebraska
The machines transformer will not take 208 .
Isolation transformer put in for the sake of drives I would imagine in the event it were powered by a delta source - corner grounded, ungrounded, or even system with a high leg. Surge protection on drive input is set up for nominal 277 to ground, get above that and it will be clamping non surge voltages and burn out the surge protection.
 
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