xo not bonded

[tim89s]

I realize you bond xo on the secondary side of a transformer. Let's say you have a delta wye
transformer and all your loads on the secondary are three phase. What would happen if you
didn't bond xo to ground. I have heard that no fault would occur since no reference to ground was made. I am confused, you would still have the ground from the primary side tied to the
ground of the secondary. Wouldn't the primary side ocpd trip? I understand the xo bond gives
you a lower impedance to ground but I would still think the primary side ocpd would trip in the event of a ground fault.

Please help to clarify
Thank you

 

[Gregg Harris]

I realize you bond xo on the secondary side of a transformer. Let's say you have a delta wye
transformer and all your loads on the secondary are three phase. What would happen if you
didn't bond xo to ground. I have heard that no fault would occur since no reference to ground was made. I am confused, you would still have the ground from the primary side tied to the
ground of the secondary. Wouldn't the primary side ocpd trip? I understand the xo bond gives
you a lower impedance to ground but I would still think the primary side ocpd would trip in the event of a ground fault.

Please help to clarify
Thank you

Take a look at the definition for "Separately Derived System"

 

[augie47]

As Gregg notes, the fact that there is no electrical connection between primary and secondary wiring is why it's an SDS.

In most case, if you have an XO secondary terminal you are required to ground it (see 250.20 & 250.21)
If you are not required to have an XO bond, you end up with a corner grounded or ungrounded system.
{see 250.21(B)}

 

[tim89s]

Gregg, are you telling me the primary ground and secondary ground are not tied together in the transformer.

 

[Strathead]

Gregg, are you telling me the primary ground and secondary ground are not tied together in the transformer.

Draw it out! A single ground fault on the secondary side would do nothing, within reason. There is a certain amount of inductive current to ground in an ungrounded system, but set that aside for this purpose. Regardless of where the ground occurs, the first ground would serve the same "purpose" as an intentional bond. That being to bring the bonded surface to the same voltage potential as the bonding point on the source. There is no electrical path from that bonding point through ground, though phase wiring of the primary side of the transformer and then back along the secondary side of the transformer and back to the bonding point. Ditto for any other path that travels along the secondary phase conductors and back to the bonding point. So it will sit there doing "nothing" until a second bonding point (ground fault) is created at a different voltage potential point along the secondary. At that time current can flow around that path at the voltage representative of the difference between the connection points.

 

[tim89s]

Thank you very very much Strathead, you made it very clear for me to understand.
I have been searching online for hours for an explanation that made sense to me.
Thank you again
Tim

 

[Strathead]

Thank you very very much Strathead, you made it very clear for me to understand.
I have been searching online for hours for an explanation that made sense to me.
Thank you again
Tim

You are very welcome. There are some brilliant minds here, but I sometimes find that a more simplistic view is more effective. I have trained every apprentice and even a few "journeymen" under me for years to look at any electrical circuit as a puzzle or a game. You play one "little" electron, and your goal is to get from point A, back to point A without backtracking (per se pending, say a reversing controller). To this day, that is how I troubleshoot, and you will see that, that is basically how I described your situation to you.

 

[sciguy]

Reverse feeding a single phase 480v transformer and the xo bond.

Reverse feeding a single phase 480v transformer and the xo bond.

What if you have a single phase 480v/240 50kva reverse fed transformer.How can you bond the secondary since you are reverse feeding it?The manufacturer recommended not to bond it because it could cause a ground loop?

 

[templdl]

The way that I look at it is that you will ground the X0 as the others stated. Then you must include an EGC with all of you circuits which provides a path back to the X0 that allows an OCPD to trip should a fault occur, again as explained by others.
However, it was stated that the circuits did not require 1ph loads and as such carrying a neutral along with the lines and EGC is not neutral along with the lines and EGC is not required.

 

[ActionDave]

What if you have a single phase 480v/240 50kva reverse fed transformer.How can you bond the secondary since you are reverse feeding it?The manufacturer recommended not to bond it because it could cause a ground loop?

Welcome.
If it is a single phase xformer you don't have an xo.
You would most likely bond your H1-H2 interconnection if the windings are in series to get 480V.

I don't know what the manufacturer means concerning ground loops. What is the application for this transformer?

 

[hardworkingstiff]

What if you have a single phase 480v/240 50kva reverse fed transformer.How can you bond the secondary since you are reverse feeding it?The manufacturer recommended not to bond it because it could cause a ground loop?

Doesn't the PoCo do that all the time? They feed a single-phase transformer with a phase voltage and the neutral, re reference the neutral to ground and then reference the secondary neutral to ground also?