XO bonding jumper

[joeyww12000]

Ive asked alot of questions on transformers lately but I want to be sure. I appreciate the input. My question is there is a thin copper strip that connects XO to the transformer case at my seperately derived system "480 to 120/208". The system equipment ground that goes back to the main service entrance is tied to the groundbar which you know is tied to the GEC which is bonded to the grounded conductor and also building steel Do I need a Seperate grounding conductor from my transformer to building steel also? The reason Im asking is my boss is giving me mixed answers and I am trying to make sure I am doing the right thing here. My understanding is I need to bond my transformers XO to its case even if the copper strip is bonding it already, then take another grounding conductor from the same point to the building steel, using bonding bushings on both ends of my conduit where my grounding conductor enters and exits.

 

[iwire]

First off I doubt that the copper strip you see has any connection to XO. Usually that copper strap is just bonding the transformer core supports to the enclosure.

You will need to install a bonding jumper and run an GEC to building steel.

Here is how I do it.

On the XO connection.

1) The grounding electrode conductor out to a grounding electrode (usually building steel) See 250.30(A)(3)

2) The system bonding jumper running to the enclosure. (see 250.30(A)(1)

3) The neutral heading out to the panel or disconnect.

On the Enclosure.

1) The incoming EGC from the 480 feeder

2) The out going EGC to the panel or disconnect

3) The System bonding jumper heading to XO.

 

[tryinghard]

?My question is there is a thin copper strip that connects XO to the transformer case at my seperately derived system "480 to 120/208"...Do I need a Seperate grounding conductor from my transformer to building steel also?

The secondary is a different system (no physical connection to the primary just inductance). A couple of things have to happen now that it?s a separately derived system (SDS):

• 
  [1] It has no physical connection from conductive items to its new grounded conductor (neutral) at XO so a bonding jumper must be installed to enable this path.
  [2] The earth grounding must be re-established with one of the seven plus listed in 250.52, building steel is most common inside a metal building.

Your equipment ground with the primary only connects to the case frame (ground bar) at its destination (transformer); secondary downstream shorts have no reason to travel on this path.

Shorts will travel to source only and the secondary is the source not the primary or earth. So you must establish an intentional effective ground fault path to enable overcurret protection to function, this is done with a bonding jumper from XO to case frame/ground bar.

 

[benaround]

Joey,

Here is some more ammo for your boss, the primary EGC ( 480v) is normally

not large enough for secondary side GEC.

 

[joeyww12000]

Is this how you terminate your transformers now still?

 

[infinity]

I agree with Bob, that thin copper strip is not the system bonding jumper. Typically a transformer has rubber vibration pads between the core and the casing, that strip bonds the two together. I've never seen a transformer that had it's XO factory bonded.