Corner grounded delta transformer connections and grounding

[Electron_Sam78]

I've got a separate structure with a 1ph corner grounded delta 480V that is being stepped down to 240/120V (center tapped/grounded secondary). I'm replacing the existing panel and noticed some code deficiencies that I'm going to correct when I replace it. For one thing the corner grounded system is entering the building and the grounded phase is fused inside the disconnect. Another is that the grounded phase of the corner grounded delta is not connected to a grounding electrode or any equipment grounding connections. The CB panel fed by the transformer secondary (240/120V) has a grounding electrode where the secondary center tap (neutral) is connected but as stated the corner grounded delta supply is not connected to it.

My proposed fix is:

1). Get a new disconnect with a neutral bar and run the incoming corner grounded phase into the neutral bar. I will also connect the neutral bar to the metal enclosure of the disconnect and a grounding electrode (no equipment ground was run with the supply).

2).Connect the neutral bar in the secondary fed panel to the same grounding elecrode the supply system is connected to as well as the metal panel enclosure.

3). Make sure all metal equipment frames and conduits are bonded together and grounded

I believe this corrective action is right according to code (2008) but let me know if I need to change something

 

[augie47]

You refer to it as a "corner grounded" system. Is it actually grounded at any point (such as at it's origin). If not, it may simply be an ungrounded system and there is no requirement to ground one phase.
{You might refer to 250.21(B)}
As far as your transformer, note that 250.30(A)(3) requires the system bond and grounding electrode conductor be connected at the same location (both at the transformer or panel)

 

[Electron_Sam78]

It is a corner grounded or grounded B phase system from the utility.

 

[Electron_Sam78]

The separate structure is using a single phase of a three phase system

 

[augie47]

In that case I would agree with your pakn for the 480 volt. Since the system is grounded at some point, I would think you should handle it as planned.
As far as the transformer, I would complete my systej bonding and grounding at tbe same location.

 

[Electron_Sam78]

ok just to make sure I have this straight, I'm going to have:




1ph of a 480V corner grounded delta (1 ungrounded leg and 1 grounded leg) with no equipment grounding conductor run with circuit from originating source. At a separate building this enters into line side of a fused disconnect where:

1. one ungrounded leg is fused;
2. one grounded leg is running through a neutral bar;
3. an equipment bonding jumper connects neutral bar to disconnect enclosure [due to no equipment grounding conductor];
4. a grounding electrode conductor is run from neutral bar to electrode;
5. an equipment grounding conductor is run from the neutral bar with the load wires to the transformer.

Exiting the disconnect into the transformer will be:

1. a fused, ungrounded conductor connected to primary side
2. a non-fused grounded conductor connected to primary side
3. an equipment grounding conductor (as mentioned above in item #4) connected to transformer metal frame

Exiting transformer will be:

1. two ungrounded conductors from the secondary connections
2. one (to be) grounded conductor from the secondary connection center tap
3. one equipment grounding conductor connected to metal frame of transformer

Circuit breaker panel will have the following:

1. two ungrounded conductors from transformer to line side of main breaker
2. one grounded conductor from center tap of transformer secondary connected to neutral bar
3. one equipment grounding conductor coming from transformer frame to equipment grounding bar
4. one system bonding jumper from neutral bar to equipment grounding bar
5. one equipment bonding jumper from panelboard enclosure to equipment grounding bar
6. one grounding electrode conductor from neutral bar to electrode (same electrode that 480V supply is connected to)

 

[augie47]

It is common practice here to install the system bonding jumper at the transformer and connect the grounding elected conductor at that point, but if I read correctly, your install meets 250.30.
Most interesting install Should confuse untrained personnel to no end.

 

[Electron_Sam78]

It's confusing enough for me! I just came across corner grounded delta for the first time last year and it's been years since I had to mess with transformer connections so this is a fun one! I appreciate the input!

P.S. There's just not enough room in the Xfrmr for all those grounding connections. It's only a 3KVA!

 

[George Stolz]

Also, keep 240.85 in mind.

 

[Electron_Sam78]

Roger George! :grin: