480 to 120/240 system

[dm9289]

I work in an older facility that constantly needs updating and I was hoping someone could clarify somethings for me that I believe would fall under the catagory of separately derived systems.
We have a 480v ungrounded delta system that runs from several pieces of switchgear to various areas throughout the facility via overhead lines for machinery and various services. My question deals with properly converting the 480v to 120/240 for receptacles hand tools, coffee makers, lighting, water heaters, AC etc.

Our current method used during the original 1950s installation was to tap 2 legs off of the 480v from the overhead feed connect them to a transformer at floor level, then run from the transformer secondary about 5feet to a panel an Aphase Bphase and use the center tap as the neutral and connect these to the 120/240 service panel. At the service panel a ground is ran from it to the building I beam and connected via drilled and tapped hole.

Finally the questions
In the service panel should the neutral be connected to the gnd using the grounding screw in the panel like a regular residential service?


Is this method of using the I beam adequate for grounding or does a secondary gnd have to be added?

 

[Smart $]

Details determine whether ultimately compliant, but the simple answers to your questions are: Yes, bond ground and neutral just like residential; and the method of using the I-beam, provided it qualifies as a grounding electrode, is actually required if it is nearer than a qualifying water pipe electrode. No additional "earth" grounding of local system is necessary... but you may have to bond local metalic piping systems to your grounding system.

 

[augie47]

This graphic shows two of the most common methods:

 

[dm9289]

Thank your info was very helpful

 

[dm9289]

In some of our panels the neutrak is not bonded as in a residential subpanel where the neutral bar is standing on some plastic with the bonding screw not used

 

[don_resqcapt19]

In some of our panels the neutrak is not bonded as in a residential subpanel where the neutral bar is standing on some plastic with the bonding screw not used

If the voltage is 120/240 and the panel is supplied from a transformer, there must be a bond at some point on the secondary side of the transformer. It can be at the transformer and if that is the case, a bond is not permitted at the panel.

 

[Smart $]

If the voltage is 120/240 and the panel is supplied from a transformer, there must be a bond at some point on the secondary side of the transformer. It can be at the transformer and if that is the case, a bond is not permitted at the panel.

And in this case, the grounding conductor run to I beam also has to go to the transformer, not the panel.