ungrounded

[ajayl]

I just recently started my career (electrician) and needhelp understanding this question.
I just worked on a drill press
The disconnect is feed with 480v grounded system this feedsa buck transformer on the drill press. This transformer is a delta primary(480V 3phase) delta secondary (240V 3phase) ungrounded system. After the secondary it goes to another disconnectand then into the drill press motor. The equipment grounding conductor is runto the drill press motor.
My question is if there was to be a ground fault at themotor would it pop a fuse or since the buck transformer is delta and ungroundedthere would not be a ground fault.
I work with an experienced electrician that says it willblow a fuse because it will return on the 480v grounding conductor.
But current must return to its source which is the secondaryungrounded system. No return for the current.

Any help please and thank you

 

[GoldDigger]

But current must return to its source which is the secondaryungrounded system. No return for the current.
Any help please and thank you

On an ungrounded system the first fault to ground will not carry any current at all, but can be detected by a proper ground detector circuit. The second fault to ground is the one that will cause current to flow, but it may or may not flow through the grounding conductor, depending on where both faults are located. For example, two faults from windings to motor casing may trip the OCD but not depend on the presence of the grounding conductor to do it.

 

[Smart $]

I just recently started my career (electrician) and needhelp understanding this question.
I just worked on a drill press
The disconnect is feed with 480v grounded system this feedsa buck transformer on the drill press. This transformer is a delta primary(480V 3phase) delta secondary (240V 3phase) ungrounded system. After the secondary it goes to another disconnectand then into the drill press motor. The equipment grounding conductor is runto the drill press motor.
My question is if there was to be a ground fault at themotor would it pop a fuse or since the buck transformer is delta and ungroundedthere would not be a ground fault.
I work with an experienced electrician that says it willblow a fuse because it will return on the 480v grounding conductor.
But current must return to its source which is the secondaryungrounded system. No return for the current.

Any help please and thank you

You are thinking correctly... but subject to detail clarification. You called this a buck transformer. It does "buck", but typical buck transformers are connected as autotransformers. A 480V:240V 3? transformer is usually an isolation transformer, and called a step-down transformer where primary voltage is higher than secondary.

Additionally an ungrounded system such as the secondary you describe is required to have a ground [fault] detector [250.21(B)]. (Sidenote: A downside to that requirement is, it doesn't require any action when a ground fault is detected.)

 

[Smart $]

PS: It is more common that a 240V delta secondary is connected so the midpoint of one phase winding is grounded. For some situations, an ungrounded system is utilized such that the first line-to-ground fault does not trip the OCPD, as it is desirable for the equipment to continue to operate. It is assumed by the designer the ground fault detector will be monitored and any ground fault will be remedied ASAP (...or something like that ).