when is transformer grounded like a service? I mean, when would you have a double barrel lug on xo with both a ground and nuetral terminated on it.
Transformers in electrical rooms I've worked on (eg:90kva) 480/277 stepdown to 208/120 did not have the ground tie in with the neutral. However, I've seen temp skids and certain roof top transformers with had the ground go through a grounding bushing then to the frame of the trans. and then up to the neutral terminal (xo) along with the nuetral.
Is this done if lighting strike are a concern?like on a roof? or an outdoor temp skid. service ground VS equipment ground.
If the transformer is supplying a system that is required to be grounded (one that has a neutral conductor), the grounding electrode conductor (GEC) can be connected to the neutral either at the transformer XO terminal, or at the neutral buss of the first switch controlling the system.
Ed is correct. I would just add on thing. Have a look at 250.30 (A)(1) in the 2002 code. It states that "...it shall be made at the source of a seperately derived system that has no disconnecting means or overcurrent devices." If you are feeding a panel that doesn't have a disconnecting means, you must leave the "strap" (bonding jumper) in it that comes from the factory, or put one in it if it doesn't have one. I've never seen a wye type transformer that didn't come with a factory bonding jumper. If your panel has a disconnecting means, and you want to bond the neutral at the panel, it is perfectly legal, but most installations that I've seen, and done, have the bond on the secondary side of the transformer. My reasoning, you may ask? I figured, why should I remove, and replace something that the "factory" has done for me. I couldn't tell you if there are any advantages or not. I will say this, "If you have a factory installed bonding jumper that aint broke, then don't fix it."