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Old 03-16-2007, 12:09 AM   #1
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Default Someone explain 480 delta 3 phase?

We're working in an old mine machine factory, recently converted to a gear shop. They have 3 phase delta 480. Not sure if that's the technical name or not. So on A phase to ground you have straight 480V. B phase to ground you have 0V. C phase to ground you have 480V. No neutral comming in. A to B 480v, A to C 480v and B to C 480V.

I've been told that the B phase is called the grounding leg. What gets me is that all 3 phases are connected to transformers outside. I suppose my questions are.

What purpose does the "Dead Leg" serve? Is it acting like a neutral?

Is it possible to transform this down to 277v with a neutral?

That's the only two I can think of off the top of my head.

And here's something for the saftey file, this 1200A service has no ground, just sitting on the concrete!

Thanks in advance.
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Old 03-16-2007, 01:44 AM   #2
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to order decribsing what you have what we call " corner grounded delta " service

this was pretty common some timeback but nowdays it getting rare due the safety reason and some of the electricians lack the knowage on old corner grounded delta system

unforetally you can not get 277 out of this one because there is no netural at all

I will suggest that just replace the electrical panel and get the 480/277 wye system it far more common nowdays.

please note that i am sure that most POCO will not upgrade old delta service anymore,.

my POCO regulation book qouted say it will only increase the size to match the exsting main breaker of that service it feed anymore bigger will have to convert to wye connection
and it say delta service both 480 and 240/120 4 wire delta service will be only exsting no new comustmer will get this set up at all.

I will try to find the wiring digram of corner grounded service look like so you get the idea

this is the photo of delta service i am not sure how big it will show up on your computer

This is from Mike Holt website few members expain about the corner grounded delta service as well

if more question i will look for more info

Merci , Marc
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Old 03-17-2007, 12:12 AM   #3
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What Marc said is correct. This is a corner tapped Delta system, also known as an ungrounded system.

The phase you measured zero volts on is actually the grounded conductor where it is tied to everything metal in the structure.
It could be called a neutral because it is common with everything in the building even you while standing in there though energized at 480 volts.

It's the same principal as the neutral you're used to whereas the neutral is actually the 'hot' half the time(60 times per second on a 2 wire circuit). The 'difference of potential' or lack there of is the reason why it's 'common' or 'neutral'.

This type of installation is ONLY allowed in supervised locations. Since this place has changed ownership I'd say it's highly illegal for them to operate with an ungrounded system.

It is possible to have a 1to1 ratio xfmr where the primary would be delta and secondary wye to give a 480/277 system.
But if I were you I'd be very hesitant about what you do in this building seeing as at any point you could recieve a 480 volt electrocution.
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Old 03-17-2007, 02:56 AM   #4
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An ungrounded system would technically READ nothing to ground, but depending on the size of the distribution system and load on the system the phases would be capacitive grounded, so there would be a reading.

A corner grounded is just that one phase is bonded to ground.

There is also a resisitve grounded system, where the neutral of a wye system is bonded to ground through a resistor, this limits the fault current to some know value. In the systems I worked on this was 25 amps.

The purpose of the ungrounded system and resistance grounded systems is minimize fault damage on the first fault, and the facilty can remain in operation, while techs search for the fault. Ungrounded systems and resistance grounded systems have fault alarms.

These systems are normally utilized in industrial applications where a fault would disrupt operation processing that in some instances destroy the factory or shut downs are extremly costly. A real problem with these systems is the second fault does shut you down and there are issues with extreme voltages (impulses? I NEED TO CHECK THIS) with the ungrounded systems.
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Old 07-16-2010, 01:12 PM   #5
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I ran saw this and have worked on a lot of delta unground and grounded. Be careful when you find 480 to ground on two phases and one phase at zero to ground. Unless you see the ground connection between the transformer secondaries and ground it may be an ungrounded system with a fault in it. If you see fuses on only two phases it should be a grounded system.

On grounded delta systems I consider the phase a grounded current carring conductor, just like a netural. It should be insulated like a neutral, it should be connected to ground at one point like a neutral. It carries current like a neutral and if you open it up on a live circuit it hurts like openning a neutral.

On the ungrounded systems the problem becomes that the refrence to ground changes as the item that is grounded is turned on and off. If the ground is in a location where the equipment is not always energized it can sometimes be tough to find.
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Old 07-16-2010, 01:22 PM   #6
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Andy welcome to the forum but this thread is more than 3 years old.
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Old 07-16-2010, 06:32 PM   #7
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Originally Posted by Bob Badger View Post
Andy welcome to the forum but this thread is more than 3 years old.
Bob, I liked the other avatar. I don't even know what show this new one is from. "Americas Most Wanted"
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Old 07-16-2010, 09:24 PM   #8
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The first time I ever saw one installed I refused to be in the same room when they turned it on, from what I new at the time I thought that grounded b phase was going to be an explosion.
When ls lunch?
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