Grounding Conductor - Step-Up/Step-Down Configuration

[dwcflorida]

We are currently taking a 120/GND source and stepping it up to 480/GND using a 5 KVA transformer. We are then going 1500 feet and stepping back down to 120/GND to power a hub cabinet on a roadway. We would like to avoid using a grounding conductor since any potential difference would seem to be negligible for this much of a separation. Both transformers would be grounded appropriately and we are proposing running between the two transformers with 2 wires. Please let me know if this is appropriate. Thank you.

 

[iwire]

Re: Grounding Conductor - Step-Up/Step-Down Configuration

Originally posted by dwcflorida:
Please let me know if this is appropriate. Thank you.

It is not, you must run either a grounded or grounding condutor from the source out to the transfomer.

The earth can not be used as a ground fault path.

 

[iwire]

Re: Grounding Conductor - Step-Up/Step-Down Configuration

The problem is that if your 480 volt supply shorts to the transformer enclosure there is no adequate fault path back to the source to cause the over current protection to open.

In the mean time anyone coming in contact with that transformer enclosure will be subject to a 480 volt (or more likely 277 volt) shock from the case to ground.

 

[dwcflorida]

Re: Grounding Conductor - Step-Up/Step-Down Configuration

That's fine. A ground to the source is fine. My question would be, why would a grounding conductor be necessary between the two tranformers themselves. Please see Exhibit 250.16 of the 2002 NEC (p. 190). It appears that two conductor feeders are being run between buildings. How would this be different. Thank you.

 

[pierre]

Re: Grounding Conductor - Step-Up/Step-Down Configuration

Lets not forget that you will need an "effective ground-fault current path" on the load side of the 1st transformer to the 2nd tranformer and ditto from the 2nd transformer to the 3rd., To protect against a potential ground fault. That is why you will use either of the two permitted methods, based on the parameters set forth.

 

[Zifkwong]

Re: Grounding Conductor - Step-Up/Step-Down Configuration

Does 250.21.5 allow you to leave one conductor ungrounded?