240 volt Delta corner grounded system

[mitch cochrell]

Is there a video available to educate somebody on corner grounded delta systems? I am providing electrical services at a brand new facililty in Houston. They relocated from an area of town that had 240 volt delta as the only option for three phase systems. Therefor, all of the existing equipment is 240 volt 3 phase (high leg). The engineer designed the new building with two 480v/240v Delta Delta corner grounded transformers to supply the existing equipment. We had to get UL involved due to The City of Houston ordinance to approve all equipment. The UL representative noticed sometimes you read 140 to ground on all three phases and sometimes you read 240 to ground on two of the phases and 0 to ground on the third. He thinks the company that installed the new system didn't ground the transformers properly and could be a shock hazard and available fault current issues. I have heard of this system but never seen or installed one before. Any help would be greatly appreciated. Thanks

 

[Smart $]

Is there a video available to educate somebody on corner grounded delta systems? I am providing electrical services at a brand new facililty in Houston. They relocated from an area of town that had 240 volt delta as the only option for three phase systems. Therefor, all of the existing equipment is 240 volt 3 phase (high leg). The engineer designed the new building with two 480v/240v Delta Delta corner grounded transformers to supply the existing equipment. We had to get UL involved due to The City of Houston ordinance to approve all equipment. The UL representative noticed sometimes you read 140 to ground on all three phases and sometimes you read 240 to ground on two of the phases and 0 to ground on the third. He thinks the company that installed the new system didn't ground the transformers properly and could be a shock hazard and available fault current issues. I have heard of this system but never seen or installed one before. Any help would be greatly appreciated. Thanks

Definitely requires investigation.

Transformers nameplate data regarding configuration? (picture if not too difficult)

3? delta transformers are not manufactured corner grounded TTBOMK. Corner grounding is a field-implemented configuration. I have no idea why an engineer would specify a corner grounded system to power a formerly hi leg powered system, and hi leg secondaries are probably more common than straight delta.

Corner grounding the transformer is no harder to implement than center-tap grounding. Simply move all grounding connections normally made to the center tap to the B phase (typically grounded corner).

The main difference is usually evident on the load side distribution. With one corner grounded, you can use what appears to be split single phase equipment. However, it differs in that you can use two pole breakers for a three phase load and single pole breaker for a 240V load. Alternatively, some use a 3? panel and multipole (common trip) any load using the grounded corner.

 

[jim dungar]

You mentioned corner grounded and then you said high-leg.
These are two wildly different systems even though they are based on a 'delta'.

It sounds like you have one transformer bank properly corner grounded and the other one improperly ungrounded/floating.

 

[Phil Corso]

Three systems... Op said there are 2 Xfmrs... Open-Delta?

Phil Corso

 

[mitch cochrell]

The two transformers are being fed from a 480 volt MDP. They are seperate from each other. Basically two individual ground mounted transformers feeding seperate 240 volt 600 amp panels. Sorry about that. I should have went into more detail ..... The other thing we noticed is the transformers are connected to the same ground rod. No grounding electrode conductor tapping steal.

 

[Smart $]

The two transformers are being fed from a 480 volt MDP. They are seperate from each other. Basically two individual ground mounted transformers feeding seperate 240 volt 600 amp panels. Sorry about that. I should have went into more detail ..... The other thing we noticed is the transformers are connected to the same ground rod. No grounding electrode conductor tapping steal.

Likely being separately-derived systems, the required GEC is run to the same enclosure where the system bonding jumper is located, which can be either at the transformer or the system disconnecting means.

If these are outside ground-mounted transformers, many consider them a separate structure and configure grounding as such [one interpretation of 250.30(C)]... but the minimum grounding electrode system requirements kick in... so a single ground rod is likely a violation unless provable as 25 ohms or less to earth.