Factory bond strap in dry xfmr

[farmantenna]

Inside a 3 phase 45 kva dry type typical 480v-208/120 step down transformer (the one I examined) there was a factory installed bonding strap from the enclosure threaded stud to the core support structure. Is there an internal factory connection from that support structure to the XO terminal?

I ask this because I had to investigate why a FACP and a few FAPS (all the same fire alarm manufacturer and everything existsing for decade+) several feet from this transformer all a had a alternating normal and power loss trouble when all readings were proper for 30 minutes when I was there. But inside the transformer although the GEC conductor ( to building steel) was connected to XO with panel neutral, the panel's EG conductor was going to the xfmr's enclosure stud. The panel's neutral and ground bar were separated. All conductors were in flexible metal conduit. I think there should be an additional bonding conductor between the neutral lug and the enclosure stud but I don't know if the xfmr already has it. I don't think it does.

 

[Dsg319]

I’ve never seen a transformer with a factory XO bond, not to say that is not typical, just not in my case.

Take an ohm meter to it.

 

[augie47]

What you are probably seeing is an internal strap connecting to the core and not the XO MBJ. You still need to bond X0 to grounding, etc
That strap confuses a lot of folks

 

[roger]

What Gus said.

 

[infinity]

What you are probably seeing is an internal strap connecting to the core and not the XO MBJ. You still need to bond X0 to grounding, etc
That strap confuses a lot of folks

I agree, I have never seen a factory bonded neutral. The strap bonds the core since it's sitting on rubber isolation pads.

 

[solarken]

I recently had the same question when I opened a similar transformer to make a solar interconnection. As others have said, that bonding strap is just to bond the core, not the XO neutral. With no external neutral conductors connected to XO, I believe you will find XO to ground will measure open circuit.