3 phase open delta system

[mshields]

Have just encountered one of these for the first time (in Florida where I gather they are common).

Tenants are experiencing lots of fried equipment. So I'm studying these systems to generate questions for the electrician down there (facility is in Florida I am in Boston). The building in question has only a single phase meter center. So the three phase of the open delta three phase system is being used for other facilities. So from what I can tell, the following questions should be asked of the electrician:

1. Did he bring in the correct 2 phases (i.e. the two between which is the ground tap). But that would be pretty hard to believe and cause immediate mayhem. Seems unlikely to me.
2. Did they run shared neutrals and if they did, are neutral on circuits associated with fried equipment solidly connected to the neutral bus. This of course could be a problem independent of the type of three phase equipment.

lots of voltage checks across the board, etc.

My question for you is this. Are there problems specific to open delta 3 phase systems that we should be looking for?

Thanks,

Mike

 

[LarryFine]

First of all, the (presuming B phase is the high leg) A-N-C 120/240 section is identical to a single-phase system. So, the first thing to do is measure and compare the phase-to-neutral voltages. The high leg should read around 208v.

Second is to see if you can measure for any voltage between the neutral and a reliable earth, like a fully-metallic water piping system, or even a ground rod.

As for the presence of a high leg, if properly wired, it should have no effect on what's going on. Sometimes, the high leg is used to supply 240v-only equipment, between the high leg and either other line, in addition to three-phase motor loads, like A/C compressors.

 

[jim dungar]

My question for you is this. Are there problems specific to open delta 3 phase systems that we should be looking for?

Technically you have a center tapped delta, also referred to as a wild or high leg. The ANSI method of identifying this is to show it as 240/120V, although I prefer to add 3 phase 4 wire to the description

To call this an open delta 3-phase is nothing but local slang. Yes the wild leg system is often configured as an open delta, however not all open-deltas provide 240/120V 3PH 4W.
Similar to: all school buses may be yellow, but not all yellow vehicles are school buses.

So is your question about open-deltas in general or center-tapped deltas in specific?

 

[LarryFine]

So is your question about open-deltas in general or center-tapped deltas in specific?

The latter, from the wording of the OP.

 

[roger]

As stated by the others, an open Delta has nothing to do with it being center or corner grounded, closed or open Deltas will have a high leg if they have a center tapped winding.

Roger

 

[electrofelon]

To call this an open delta 3-phase is nothing but local slang.

What is wrong with calling it an open delta if it is an open delta? Granted that bit of information is not necessary to the electrician 99% of the time

Here is Seattle City Lights spec and diagram for an open delta connection on their MGN system. They use only two primary phases. I like the terms for the two transformers, "lighter" and "teaser"

http://www.seattle.gov/light/engstd/docs2/0125.02.pdf

 

[mivey]

My question for you is this. Are there problems specific to open delta 3 phase systems that we should be looking for?

Not in your case. Your problems are not because of an open delta. You have other issues.

That said, an open delta can have a higher voltage unbalance than closed delta. This can impact large motors if the bank is sufficiently loaded and the motor is undersized. This can be mitigated by increasing the bank size as the unbalance is a function of load/capacity. But better to properly size the motor.

This becomes an issue when the customer does not perform proper motor derating for normal system voltage unbalance. Some irrigation contractors are bad about cheaping out and pushing everything to the limit without proper derating. Nameplates aren't the whole story.

Buy a bigger motor or pay for tighter than standard voltage regulation from the supplier.

 

[mivey]

What is wrong with calling it an open delta if it is an open delta?

Nothing. It is standard industry terminology.

 

[oldsparky52]

I would chase the possibility of a deteriorating neutral just as if it was a single-phase service, then move on from there.

 

[retirede]

I would chase the possibility of a deteriorating neutral just as if it was a single-phase service, then move on from there.

+1

 

[ptonsparky]

I would chase the possibility of a deteriorating neutral just as if it was a single-phase service, then move on from there.

That would be my approach. It does not mater to him if it is three phase open, or closed Delta with a high leg at the pole. The property is being feed single phase.

 

[romex jockey]

Am i mistaken in that 'open delta' is terminology associated with two can configuration, cuz the third boint up.....?:? ~RJ~

 

[ptonsparky]

Am i mistaken in that 'open delta' is terminology associated with two can configuration, cuz the third boint up.....?:? ~RJ~

.

I believe it is economics. Light three phase loads get by with the open. Heavier loads, they close it.

 

[jim dungar]

What is wrong with calling it an open delta if it is an open delta?

An open delta can supply at least 6 different systems:
240/120V 3PH 4W center-tapped
240V 3PH 3W corner grounded
240V 3PH 3W ungrounded
480/240V 3Ph 4W center-tapped
480V corner grounded
480V 3PH 3W ungrounded

A center-tapped open delta can only supply 2.

In my experience, POCOs often do not follow the NEC naming conventions as found in the NEC Informational Note#2 for the definition of Voltage, Nominal. NEC 110.15 uses 4W delta-connected to describe the center-tapped system.

 

[electrofelon]

An open delta can supply at least 6 different systems:
240/120V 3PH 4W center-tapped
240V 3PH 3W corner grounded
240V 3PH 3W ungrounded
480/240V 3Ph 4W center-tapped
480V corner grounded
480V 3PH 3W ungrounded

A center-tapped open delta can only supply 2.

In my experience, POCOs often do not follow the NEC naming conventions as found in the NEC Informational Note#2 for the definition of Voltage, Nominal. NEC 110.15 uses 4W delta-connected to describe the center-tapped system.

I agree it was technically ambiguous for the OP to describe the system as "open delta" but it was apparent in the body that it was the 120/240 variety.

 

[mshields]

you guys are awesome!

you guys are awesome!

Thank you!

 

[mshields]

One more thing I just learned

One more thing I just learned

I'm told that the engineer, not being familiar with Open Delta systems, opted to not even bring the high leg out to the panel. i.e. they only brought the two legs that would give you 120V, Line to neutral. Could that cause problems. I don't see how? But....

 

[mivey]

I'm told that the engineer, not being familiar with Open Delta systems, opted to not even bring the high leg out to the panel. i.e. they only brought the two legs that would give you 120V, Line to neutral. Could that cause problems. I don't see how? But....

No problem, as long as you don't overload the winding. Single-phase loads don't care about the high leg.

 

[LarryFine]

I'm told that the engineer, not being familiar with Open Delta systems, opted to not even bring the high leg out to the panel. i.e. they only brought the two legs that would give you 120V, Line to neutral. Could that cause problems. I don't see how? But....

That's good for you, just like any typical residential service. Now you need to confirm that the voltages are what they should be.