Supply side bonding jumper

[TJRasor]

Under what conditions would you hook the supply side bonding jumper from the service disconnect panel, to an insulated neutral bar on a temp power skid like this? Seems like it should be bonding the metal enclosure. This is just a google image of a very similar skid.

 

[joshfromoshoto]

Under what conditions would you hook the supply side bonding jumper from the service disconnect panel, to an insulated neutral bar on a temp power skid like this? Seems like it should be bonding the metal enclosure. This is just a google image of a very similar skid.
View attachment 2578260

I don’t have a code book with me so someone please correct me. But I believe you would need to bond the secondary side of the transformer as it becomes a separately derived system. The main feeder for the entire skid should be bonded already.


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[ActionDave]

Under what conditions would you hook the supply side bonding jumper from the service disconnect panel,

I don't understand what is meant by this. Service and service disconnect have a particular definition in the NEC and are not part of what is shown. I'm not a nit picker, but we need common terms used according to the definitions or communication breaks down. You can't call a candy bar a milkshake and then ask what to do with the wrapper.

to an insulated neutral bar on a temp power skid like this? Seems like it should be bonding the metal enclosure. This is just a google image of a very similar skid.

If we are talking about the neutral or X0 on the secondary of the transformer if you bond it inside the transformer then the neutral bar in the panel would be floating. You could legally bond the X0 in the panel. Electrically it doesn't matter, the whole skid is one big neutral once you bond the secondary.

 

[TJRasor]

First, i truly appreciate the frankness of your reply. Im just trying to understand why the grounding conductor from the main service panel to the skid is connected to a floating neutral in the skid. Obviously its a 277/480 supplied panel with a 120/208 on the other side of the transformer. For sure if theres a fault on a light or anything on the downstream side the breaker would trip, and as long as all the equipment grounding conductors in the 277/480 panel are also hooked to the floating neutral bar, theres not a problem. But what if the transformer itself has a short? It doest have a effective groumd fault current path. Thats my question. I hope this helps clarify.

 

[ActionDave]

First, i truly appreciate the frankness of your reply. Im just trying to understand why the grounding conductor from the main service panel to the skid is connected to a floating neutral in the skid.

Connected where? If we assume a normal application the 480V panel on the skid would have three hots, a neutral and an Equipment Grounding Conductor feeding it. If the 480V panel is only feeding the transformer then you could get away with three hots and an EGC. The EGC run with the feeder wiring should be bonded to all of the metal parts of the skid.

Obviously its a 277/480 supplied panel with a 120/208 on the other side of the transformer. For sure if theres a fault on a light or anything on the downstream side the breaker would trip,
and as long as all the equipment grounding conductors in the 277/480 panel are also hooked to the floating neutral bar, theres not a problem.

Are we talking about a 277/480V fault or a 120/208V fault?

But what if the transformer itself has a short? It doest have a effective groumd fault current path. Thats my question. I hope this helps clarify.

A picture is worth a thousand keystrokes. Depending on the location of the fault it should flow back to the XO or back to the service of the building.

 

[electrofelon]

The secondary of the transformer would need to be hooked up to a grounding electrode. They're probably using that jumper to connect the transformer to the building grounding electrode system. It's probably not entirely correct but functionally fine.