ECN Forum Forums General Topics of the Trade General Discussion Area No ground buss

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).]