Harris Institute of Technical Training

My first experience with a corner grounded delta here. I've got a bunch of questions!
1. Where the the ground typically established. At the bank? At the CT cabinet? Source or Load side of the CTs?
2. Who is responsible for grounding the delta? The utility or the consumer?
3. Does the grounded leg need to be bonded to the case/equipment if there is a separate driven ground? At each termination? Sub panels, sub transformers etc.
4. Does the grounded leg need to be grounded from the system neutral or would a driven ground work just as well? We are currently using the system (primary neutral)

This installation is at a government campground.480v corner grounded delta to a 400amp panel. A lot of the load is to lighting and outbuildings. At these locations they go to a 3 phase dry transformer to step down the voltage. Each run out of the panel has a ground wire (1/0) that accompanies the 3 delta legs. The electrician informed me that they treat the grounded leg as a phase, and use the ground wire to ground the case/sub panel. The CTs and main panel is all new equipment, replacing the old corner grounded system.

I have done some research on corner grounded deltas but I still find this setup very confusing. It seems like they are treating this as 4 wire delta and they essentially have two grounds. Kind of like two hot legs of 480, one neutral (grounded 480 leg) and a ground (undersized green labeled conductor that is tied to a driven ground). I expected to find a 3 wire service where the grounded leg was tied to the case at each termination.

Does this seem out of the ordinary to anyone? Anyone with some experience with corner grounded delta secondaries be willing to field a phone call from me?

1. The corner ground is at the utility's tramsformer bank.
2. Utility
3 No. It creates a grounding loop and a greater hazard for Ground Potential Rise. (The equipment still must be bonded) At each drypot the secondary neutral needs to be attached to a driven ground rod as well as bonded at the secondary subpanels.
4. Keep it off the primary neutral. As it can be energized at primary voltage.

Thanks Ed for the reply. We completed this job a month or so ago and we did it as you described. When energized the grounded leg to the case ground was 20volts or so. My instinct was to tie the case ground to the grounded leg to get rid of the potential difference, but I did not.

I have a 240 volt 3 wire 3 phase form 12s meter on a three wire 240 volt service ungrounded for three phase load only. However, the customer is using one leg for lighting and causing the meter to go backwards. Is there a way for the customer to have single phase load by using drypack transformer and still meter correctly.

Meter Tech Frank

Meter Tech Frank,
What I can derive from your question is that the customer is using one leg, but they must have another voltage that they are using this with to connect the lighting load. This metering problem can be resolved with a drypack transformer that is connected across any of the 240v voltages from the 3 wire system. The 12S meter will correctly meter the 3 phase 3 wire load and will also correctly meter any phase to phase connected load. So, that is my suggestion. Have them use a drypack transformer connected phase to phase to remedy this problem. If you have any further questions give me a call or respond here.
Brad
PS: as an FYI, We also posted your question as it's own topic on the Q&A page for others to find. The Question Topic is titled: 240v - 3wire/3phase - form 12S