Grounded Corner on Delta Secondary?????

[billfoster67]

Here is what I think I know about grounded corners:

Gets rid of the circulating current in the bank.

Opens all the cutouts, sympathy trip, if one can is overloaded. You can't single phase the motors.

If one can goes bad you can cut it over to to open delta easily.

You only need two cts to meter.

The customer can't steal power.

What else is there to know?

[Trampbag]

Grounded Corners?

My understanding is you can ground 1 of 3 phases. What do you mean by “corners”?

[billfoster67]

I know you can ground one phase on a straight delta secondary power bank. Is it for metering? What is the electrical theory behind it? : :confused:

[Outlaw Lineman]

It's to keep the customers from stealing 120v to power lights off a 240v 3 phase service and that is just about the only reason to corner ground a Delta Bank.

[Sno10]

Thats my take on it Outlaw and also to provide some sort of safety/protection for the consumers.

[Trampbag]

I agree grounding one phase is to keep customers from stealing 120v, on a 240v∆ 3 phase secondary with a lighting pot in either open or closed Delta bank. This is common in California, in fact is a GO95 regulation, but not so common in most other jurisdictions.

Most places use 120/208v and 240v∆ 3 phase is not used because of the inefficiency of the Delta system and the ability to operate a true delta with grounds on all phases though insulation leaks, as long as the grounding does not add up to 100% total. Actually Delta has been replaced by Wye unilaterally, 240v∆ for 120/208, 480v∆ for 277/480, 600v∆ for 347/600.

I still do't know what is meant by grounding a corner.

[Outlaw Lineman]

Corner grounding means just that...grounding 1 corner of the delta.

[Trampbag]

Ok, so a 3 transformer bank (2 - 50kva – 1 - 75kva in center) 240v∆ with x2 (on secondary side) on center transformer grounded for neutral to give 120v(x2 on the 50kva transformers not used). Delta connection on high side. Typical California connection, right?

Voltages are a-b 240v, b-c 240v, c-a 240v; on center transformer x1-x2 120v, x2-x3 120v; x2 to wild leg 240v

I don’t understand what are you calling the corner?

I’m not trying to be cute; I’m just trying to understand your terminology. I never heard it called that before.

[dbrown20]

The name simply comes from drawing a schematic of a delta with a ground symbol on one corner of the drawing hence the name corner grounded instead of the midpoint ground found on the schematic of a combination power/lighting drawing. Draw a delta on paper and place a ground symbol at one corner, you'll see. dbrown20

[Trampbag]

Ah, ok. An electrical symbol rather than the electrical hook up. Gotchya. Thanks.

Every local area has differing terminology for the same thing. That’s what makes this trade so damn hard and so very interesting.

[dbrown20]

Ah, ok. An electrical symbol rather than the electrical hook up. Gotchya. Thanks.

Every local area has differing terminology for the same thing. That’s what makes this trade so damn hard and so very interesting.

Actually it is both from the symbol and the actual hook up. Draw a delta hookup on, for example a 3 pot bank. The corner ground is placed between one of the H1 to H2 connections of the high side of the pots. So it really is grounded at the "corner". Worked once where they used a 7200 volt corner grounded primary. I believe we had 3 small subs that were hooked this way. Only took 2 cut outs and arresters for a 2 pot or a 3 pot bank. One phase being grounded didn't require anything. dbrown20

[scammy]

makes for some hard trouble shooting ,,,,,,,after a few fuses blow back on you

[dbrown20]

makes for some hard trouble shooting ,,,,,,,after a few fuses blow back on you

No worse that any other. A ground on the mid-point of a delta 120/240 secondary setup is done all the time. It just looks funny when you corner ground it. Like they say on a delta. Only one ground at a time. dbrown20

[scammy]

I meant when the customer has one,,,so he can steal light ,and they are closed for business ,,,,,and you cant figure out WHY the fuse keeps blowing,,but now we ground all our banks so they cant do this ,,,,,scammy

[spooler]

I found an interesting story about an attemped hot corner grounding on a 480v delta secondary. If you would like to read it go to www.themeterguy.com then in the upper left hand conrer,click on mysteries, when a greenish box comes up just to the right, click on the first mystery!

[wyliesdiesels]

@billfoster67- A regular 3 or 4 wire delta can be cut over to 2 cans just as easily! Corner grounding doesn't make it any easier! And corner grounding can be done on both the primary and secondary sides!

@billfoster67 & Outlaw Lineman- As far as stealing power, u got it backwards. There's no neutral, so no 120volt to steal. Its a 3phase 3 wire service! Because there is only 2 CTs and the 3rd leg is grounded, it IS possible to steal power with a corner grounded delta! It was only suppose to be for balanced 3 phase loads but some customers would hook up single phase devices between the grounded leg and another leg and steal power by only having half of the power metered!

There was 2 reasons for corner grounding- the first had to do with saving money on wire by having only 2 insulated conductors from the pole! The other reason was to provide a grounded service to a previously ungrounded service without the expense of changing the service drop/USE because the usual delta service is 4 wire.

@Trampbag- Again, there is no 120v available as there is no neutral in this type of service. If there was a neutral in this type of service u would blow primary fuses immediately upon closing them as u would be causing a phase to phase fault(from grounded phase to grounded neutral/midpoint tapped phase!)

What is meant by grounding a corner is if u draw a schematic with a triangle to represent a delta service and u ground one corner, which is one phase leg, then u will have a "grounded corner" delta!

Wild leg voltage is actually around 208v not 240v.

And no, its not just terminology but also how its actually hooked up. A #6 or larger solid CU wire was typically ran off the B phase x1/x3 transformer lugs! I wish I could find a picture of such but these don't exist on the line side of a service anymore. If a customer wants a corner grounded delta service, it has to be load side derived!