grounding xfmr?

[barbeer]

Need opinions-

200amp 3ph 120/208 volt MLO panelboard has no ground bar and is using EMT feeding panel for grounding conductor.

100 amp 3ph 208 volt primary from above panel, no grounded conductor to xfmr.

480 volt 3ph secondary to paint booth.

No grounded conductor means non SDS.? therefore no electrode system needed at xfmr.

Somehow I feel a little uncomfortable asking that they bond the xfmr enclosure to EMT that runs from front of warehouse to back along steel framework- help me not feel this!

 

[raider1]

Unless the transformer is an auto-transformer, the transformer will be an SDS.

You will need to ground and bond the transformer in accordance with 250.30.

Chris

 

[RUWired]

The transformer and the raceways are required to follow the equipment grounding rules of article 250.The secondary side needs to follow article 250.21. Ground dectectors are required in that article.Also 250.20(B) says where the system can be grounded. With a delta secondary, It cannot be grounded.

250.21 Alternating-Current Systems of 50 Volts to 1000 Volts Not Required to Be Grounded
The following ac systems of 50 volts to 1000 volts shall be permitted to be grounded but shall not be required to be grounded:
(1) Electric systems used exclusively to supply industrial electric furnaces for melting, refining, tempering, and the like
(2) Separately derived systems used exclusively for rectifiers that supply only adjustable-speed industrial drives
(3) Separately derived systems supplied by transformers that have a primary voltage rating less than 1000 volts, provided that all the following conditions are met:
a. The system is used exclusively for control circuits.
b. The conditions of maintenance and supervision ensure that only qualified persons service the installation.
c. Continuity of control power is required.
d. Ground detectors are installed on the control system.
(4) Other systems that are not required to be grounded in accordance with the requirements of 250.20(B).
Where an alternating-current system is not grounded as permitted in 250.21(1) through (4), ground detectors shall be installed on the system.

 

[iwire]

With a delta secondary, It cannot be grounded.

You can choose to ground a delta secondary and I would unless there was some compelling reason not to.

 

[RUWired]

You can choose to ground a delta secondary and I would unless there was some compelling reason not to.

Would'nt grounding one leg of the secondary side be the same as having a ground fault.I realize that having a second fault would cause a phase to phase fault. With the dectectors you'd be able to monitor all three phases with out having to go with the grounded delta?
Rick

 

[mpd]

look at 250.30 (B)

 

[johngil]

Is this a delta delta transformer?
If so, how would you consider it a separately derived system?
A neutral is not being created, so I guess I don't understand the concern. An equipment ground is all that is required. Please tell me why I'm wrong.

 

[don_resqcapt19]

John,
All transformers, other than auto transformers (where the primary and secondary windings are physically connected) are SDS. Separately derived systems require a grounding electrode with a few exceptions. This is not one of them.

 

[dnem]

Rule #1
Current is not "searching" for ground, it's "searching" for a path back to a transformer winding of the transformer that generated the flow. . If the ground is part of that path, then it will flow thru it not to it. . So the question is: Is there a low impedance path ?

Would'nt grounding one leg of the secondary side be the same as having a ground fault.

Grounding one leg of the secondary side would be the same as having a ground.
Grounding one leg of the secondary side would not be the same as having a ground fault.
A fault is a low impedance path back to the transformer windings that allows excessive current flow.
A fault is not a low impedance path to the ground. . Grounding is only a problem issue if it provides a low impedance path from a phase conductor back to the transformer windings. . So ask yourself, What is the path back to the transformer windings ? . If you had a grounded center tap on a wye [neutral point], that would be your path back. . But you don't have a wye, you have a delta. . There's no path back until you get a fault in the path of an ungrounded phase conductor.

The wye doesn't have a fault path issue because the neutral center tap is the only point that is grounded. . There's no low impedance path to any of the ungrounded phase transformer windings. . All current must flow thru the load to complete a circle/loop, a path back to a transformer winding.

The corner grounded delta doesn't have a fault path issue because the grounded phase is the only point that is grounded. . There's no low impedance path to any of the ungrounded phase transformer windings. . All current must flow thru the load to complete a circle/loop, a path back to a transformer winding.

Does that explanation make sense ?

David

 

[RUWired]

Does that explanation make sense ?

David

Yes, it does thankyou.So is a standard delta delta transformer listed for corner grounding and what would be easier to install? monitoring enclosure or a special breaker enclosure so the grounded phase is not fused.
Rick

 

[dnem]

Yes, it does thankyou.So is a standard delta delta transformer listed for corner grounding and what would be easier to install?

I don't know how you could define "standard". . Go by the manufacturers instructions and listing requirements. . I would venture to say that delta transformers that can be corner grounded are common place and easily found.

monitoring enclosure or a special breaker enclosure so the grounded phase is not fused.
Rick

I assume by "monitoring" that you're talking about ground detectors and an "floating" ungrounded delta system [250.21(B)]. . That's an option, better for some applications, worse for others.

If you ground a phase that doesn't mean you would need "special breaker enclosure". . As long as you're using a multipole breaker, you can put the grounded phase thru the OCPD [240.22(1)]. . But if you use fuses, you would have to make sure that "the grounded phase is not fused".

David