The guys just replaced a single phase air handler in a resturant with a new 3 phase AH.

The 3 phase panel doesn't have a seperate bond ground buss bar although the old single phase AH was grounded to the existing ground buss bar in the panel.

To look inside this old 3 phase panel you would think it's a regular single phase type because all the breakers are double pole. The only way to tell is by checking the voltage because it has no 120 volts, both hot legs are 240 to ground.
The new 3 phase Ah is supposed to have 4 wires but there is only 3 coming from the panel.

I haven't seen one of theses 3 wire - 3 phase panels since many years ago when one of my x-partners connected up a vacuum pump on the roof of a resturant with a cheater cord and found out 120 volt vacuum pump motors don't last very long on 240 volts.

Anyway, what would be the proper way to connect a case ground in this situation, just run another wire back to the panel and connect it to the ground buss?

Also, I know I'm showing my ignorance but it's been over 40 years since electrical school plus you just don't see these panels around here anymore (mostly we work with 208/120) but what is this type of 3 phase called?

What you have is a "corner grounded delta." That third leg- even though it hay read "O" volts to ground- is hot as hell!

I'm really surprised this type of three phase service was installed. I've only seen it for well pumps. I don't like it there- and I really object to it for above-ground uses!

How is the corner "hot as hell" if it is the only grounded conductor. "Hot" in reference to what? The only thing that makes this hotter than 3p wye is 2 phases are 240 above ground instead of 3 being 208 above ground.
(or one phase being 208 above ground and 2 being 120 in "red leg")

Reno - Don't see any indication in the thread that would point to a corner ground, could also be an open delta, or just a panel that a neutral wasn't run to. Seen that a few times.

Anyway, SparksNmore, you say "both hot legs are 240 to ground." Whats the third leg voltage? Is there a third leg? Your describtion sounds like there is still a single phase there.

The third leg is grounded, if I remember corretly it was "B" center leg because like I said everything looks just like any single phase residential panel.
The only voltage reading you can get is 240 volts.

Ah, then yes, you have a "corner ground."

See this thread... https://www.electrical-contractor.net/ubb/Forum15/HTML/000078.html

[q]"To look inside this old 3 phase panel you would think it's a regular single phase type because all the breakers are double pole. The only way to tell is by checking the voltage because it has no 120 volts, both hot legs are 240 to ground."[/q]

That sure sounds like corner delta to me

You really treat corner delta like it was 240/0/240 single phase. The neutral is white and isolated from the Equipment Ground. This is a 4 wire branch circuit and feeder.
If you did want to address the "hot" concern from Reno you would point out the neutral always carries the same current as the phases, not unbalanced current.
That is why it is not regrounded after the service disconnect.

240 volt Corner Grounded 3Ø 3 Wire Delta System.
Pretty interesting reading, thanks!

This is the only system that I know of still in use around here but I'm sure there are more, they're just not our customers.
Looks like we need to run an equipment ground back to the panel.

Thanks again, I've always enjoyed lurking around here, reading "the words of the experts".

Was there ever any accepted convention on phase colors for a corner-grounded delta such as this?

i.e. The grounded phase would have to be white, so which of the regular black/red/blue phase colors was normally dropped?

Guys, I hate to differ....

I would NOT want the grounded phase to be marked white! It is NOT a neutral, as we know it!

In a corner-grounded set-up, you have three transformers. One of these is shorted to ground; this is your third conductor.
Even if both other phases are disconnected, you open that "neutral" and you have all the power of that third transformer on one side. This is why, in this arrangement, the grounded phase get a fuse in it, and switches open it.

Disagree:

A corner grounded delta is exactly what the name says, a delta secondary where one of the terminals is grounded. Since each terminal of a delta has two transformers connected to it, this means that _two_ transformer secondary terminals are connected to ground. Any conductor connecting to that terminal is a _grounded conductor_ and should be coded white.

The grounded conductor carries full current, and if opened presents the same sort of risk as when opening any neutral carrying current. I guess in ideal operating conditions a neutral would carry zero current, while a corner grounded conductor would carry full phase current, so the risks are different, but IMHO this is no more of a risk than a three wire feed (including the 'neutral') from a 'wye' secondary, which also carries full current if the two phase conductors are balanced.

The grounded conductor in general should not be opened, although it is permissible to open the grounded conductor with suitable switches/breakers that open both grounded and ungrounded conductors at the same time.

I suppose that one could have a 'corner' grounded open delta, or even a 'corner' grounded wye secondary (the latter being an explicit code violation since the selection of the grounding terminal is supposed to be one that minimizes the voltage to ground of the other terminals). I don't see these are any riskier or safer than any other grounded conductor.

-Jon

John,
It is a grounded conductor and Article 200 applies. It is not a neutral, but the word neutral does not appear in Article 200. It must be white or gray.
Again it is a grounded conductor and a fuse is not permitted unless the fuse is providing motor overload protection. Look at 240.22.
Don

Winnie, you are corect- it is that one of the connections between two transformers is grounded.

As for marking this conductor white: Don, I agree that the wording of the code supports your position. However, the code does only requires this where the conductor is insulated- and I see no requirement that it be insulated at all! For example, 200.2 states "the grounded conductor, where insulated...."
I think the idea of having this particular conductor uninsulated would never occur to an electrician. Nor, for that matter, connecting this conductor directly to the case of the equipment.
Perhaps the code needs to re-consider this %$# power set-up.

I will admit to my ignorance regarding corner-grounded deltas. The only place I've seen then is at wells- where the only single-phase load is the control circuit, whose neutral is created at a transformer in the panel.
I fail to see the advantage of this system. Heck, you've already got three transformers- why aren't they connected in a "wye" to begin with?
With each leg being 240 to ground, there is also the issue of whether the breakers are suitably rated.

This is a "three phase" system. Yet, when using two pole breakers, you may set them up as follows:
- 240 leg to 240 leg; in essence, similar to our usual 240 circuit; or,
- 240 leg to 0 leg; in essence, similar to our usual 120 circuit, but with double the volts.
Now, these two circuits cannot be equivalent, unless we are also somehow drawing current from the "0" leg. That would make for an interesting "neutral!"

I just don't like this arrangement. Does anyone know what the advantages are, as compared to a "wye"?

More opinions on the matter.... http://www.iaei.org/magazine/02_a/johnston.htm

John,
It is a grounded conductor and it can only be connected to "ground" at the one point, the main bonding jumper, just like any other grounded conductor. If the conductor is bare, it would be connected to ground at multiple points. All of the rules that apply to the grounded (neutral) conductor of a 120/240 volt system also apply to this system.
Three transformers are not needed for this connection. It can be an open delta with only two transformers. One advantage is that you can use cheaper single phase equipment for this three phase system. You must pay close attention, as you stated, to the voltage rating of the breakers and the listing information on the equipment. While a some single phase equipment is listed for this application, other single phase equipment is not.
Don

1) Of course the _grounded_ conductor in this case is _not_ the neutral. With respect to the circuit and loads, it is the equivalent of the other phases. Think 'grounded phase'.

2) Even with a true neutral (say the mid-point of a wye connection) you _must_ be able to get current out of the neutral...the current flow at the neutral has to balance whatever is coming out of the other transformer terminals.

3) The grounding has nothing to do with the current flow from the transformer terminals. Any of these circuits would work just fine even if totally ungrounded. Corner grounded wye would work just fine; you could even come up with a bastard system where you connect a random transformer secondary to the neutral point on a wye, and ground the other side of that secondary, so that the 'neutral' is at 10KV to ground, with all of the 120V 'phases' at roughly 10KV to ground as well. It would be hell on the insulation system, but your standard 120V lamp would work just fine

-Jon

Err... nevermind.

[This message has been edited by SolarPowered (edited 08-17-2005).]