I'm trying to ensure I understand the code because I think there are a number of problems at our installation.
We have a large number of detached buildings at our facility. Utility power (480V, 3-phase) originates for us at a large ground-mount transformer that feeds our service entrance panel which is in a stand-alone shed on the property. From there, feeders bring power to our distribution panel in a utility room attached to our main building. From there, power is distributed to several step-down transformers (480V -> 240V) spread about the property. At each transformer shed is a large breaker panel, and many of that panel's branch circuits typically feed a sub-panel for a row of detached buildings. Those "row" sub-panels, in turn, feed a very small sub-panel in each detached building (typically consisting of a 240V powered door circuit, a 120V lighting circuit, a 120V receptacle circuit, and sometimes a 240V receptacle circuit).
There are significant metallic pathways (electrical conduit and/or fire sprinkler pipes) connecting each of the detached buildings in a row. Also, virtually all of these buildings are of full metal construction. The newer metal buildings are bolted to a concrete foundation, although I've not seen any evidence that they were bonded to the rebar for a ufer ground. Older metal buildings are typically set on top of asphalt pavement and staked in place. All electrical circuits are either 4-wire or 5-wire (because of 3-phase). In other words, there is always both a neutral and a separate ground wire between each sub-panel and the upstream source.
So I want to be clear about the following:
1. Where should the neutral be bonded to ground vs where should it be isolated?
2. Where should ground rods be required?
We have a large number of detached buildings at our facility. Utility power (480V, 3-phase) originates for us at a large ground-mount transformer that feeds our service entrance panel which is in a stand-alone shed on the property. From there, feeders bring power to our distribution panel in a utility room attached to our main building. From there, power is distributed to several step-down transformers (480V -> 240V) spread about the property. At each transformer shed is a large breaker panel, and many of that panel's branch circuits typically feed a sub-panel for a row of detached buildings. Those "row" sub-panels, in turn, feed a very small sub-panel in each detached building (typically consisting of a 240V powered door circuit, a 120V lighting circuit, a 120V receptacle circuit, and sometimes a 240V receptacle circuit).
There are significant metallic pathways (electrical conduit and/or fire sprinkler pipes) connecting each of the detached buildings in a row. Also, virtually all of these buildings are of full metal construction. The newer metal buildings are bolted to a concrete foundation, although I've not seen any evidence that they were bonded to the rebar for a ufer ground. Older metal buildings are typically set on top of asphalt pavement and staked in place. All electrical circuits are either 4-wire or 5-wire (because of 3-phase). In other words, there is always both a neutral and a separate ground wire between each sub-panel and the upstream source.
So I want to be clear about the following:
1. Where should the neutral be bonded to ground vs where should it be isolated?
2. Where should ground rods be required?