When measuring voltage from ground to each leg on a 240 volt 3 phase circuit what ahould be your volt readings on each leg?

To answer some of your questions, it is a 3phase3wire setup, we have a central bus running across the plant that supplies the presses. But I dont know which leg to come off to get my 120 volts?

[This message has been edited by Mustang (edited 06-14-2002).]

If it is 240 V phase to phase it should read 140 V phase to neutral.

The voltage phase to ground depends on if the service is a corner grounded 240v delta or center tapped 240v delta or if the service is true three phase three wire delta (ungrounded). If the service is the last, the voltage phase to ground is probably power less, meaning the voltage drops to zero if a load is applied. (The load can be as small as a wiggie.) This phanton voltage is usually detected with a DMM or DVM, which have high impedence.

What type of service do you have? & what are the voltage readings you are getting?

Well it does depend on your Xfmr configuration, but your most common set up is a delta 240/120 which gives you a 208V to ground on your high leg which should be your B phase in your panel (and tagged orange). The C xfmr is center tapped and carries all of your 120V loads. In an industrial application where no 120V leg is needed the set up will be different.

[This message has been edited by arseegee (edited 06-07-2002).]

Not being there it’s a bit hard to speculate, but in the US it is characteristic for utilities to ground the midpoint of one transformer on a delta or open-delta service, even of the grounded conductor is not carried in to the building. If it is corner grounded, then one phase {typically Bø} must be marked white as a grounded circuit conductor. Grounding the midpoint of one transformer allows serving additional 1ø3w loads, even if only three conductors are brought to the 3ø servce.

Ungrounded 240V ∆3ø3w is more commonly found in larger induistrial buildings served from 480V-primary dry-type transformers for servce to localized machinery, and with the exception on drive-isolation transformers, a wye 240V source is not to prevalent, for it requires 139V coils on the transformer. In most cases locally served 240V 3ø ungrounded servce is immaterial if ∆ or Y.

To some extent it depends on the age of the installation, with 208Y and 480Y outpacing ∆ service for standardization and flexibility. It also depends on who owns the serving transformer.


[This message has been edited by Bjarney (edited 06-07-2002).]

Mustang,
Simple answer, and I see you are an industrial type guy.

240 volt, 3 phase, 3 wire, 120 volts to ground.
That said, that's IF it is a grounded case.

240 volt, 3 phase, 4 wire, A & C phase 120 volts to ground, 208 to ground B phase. 384-16 (e)(f)

Ungrounded, wierd readings to ground based upon load at the time, if you have single phase type loads on it, voltage can vary widely.

George,

Wouldn't the 120 to ground only appear on the 2 ungrounded legs? The 3rd (grounded) phase being 0 volts to ground?

Redsy,
There's at least 4 ways to connect a Delta. That's why it's REAL difficult to answer his question.

Reading his profile, he's an industrial engineer, so he could have nearly anything under the sun.

1 - 3 wire ungrounded. Very sensitive industrial equipment, hospital type situations. Voltage fluctuates without ground reference. Rare

2 - 3 wire grounded. Don't use the neutral, same equipment as above but with stable ground reference. You would render a high leg with this, but with no neutral, it would not matter, but you would read 208 to ground from B phase ( or 198 if it is a 230 volt system)

3 - 4 wire center tap ground. Use the neutral, this is the one, of course, that creates the high leg.

4 - 4 wire corner ground (there can also be 3 wire corner grounds) the wierdest of the bunch and danged rare, usually industrial, never seen one around Wash. DC (no industry) but worked 'em a lot in Baltimore and Richmond.

Then you've got open or closed delta.

Only on one of those would your meter register 0 to ground on one phase. The center tap would be A to G= 120, C to G=120, B to G= 208 (or phase to ground voltage from A or C phase times 1.73)

I have not hit my books since reading his question, with the voltage reference he gave, it is also possible he's dealing with an old Scott type connection, or 2 phase. He would have 4 hot wires on this system normally. Or to put that another way, typically he would have 4, there are 2 wire Scotts as well.

[This message has been edited by George Corron (edited 06-08-2002).]

How's the leg George? I'm with George on this one guys. The most common set up I've seen is 3 ungrounded conductors at each corner of the delta, and one coil center tapped. This will give you the "B" phase high leg marked orange. I've come across some delta systems that were grounded on one corner, but they were few, and far between.
George are you an industrial "Guru"? Sound like it.
Mustang, I wouldn't worry about "high leg" as much I would "dirty leg", or those two "legged deer", or in George's case, a broken leg, depending on your lifestyle. Unless of course your job depends on the answers given on this forum, which may leave you on your "last leg", and never forget "three legged Willie Williams" (only his wife knew for sure).

One leg up,
Doc

"Unless of course your job depends on the answers given on this forum, which may leave you on your 'last leg'..."

Oh, man... You're talking about me again...

Don't know where I'd be without y'all taking me by the hand...

(As always, many, many thanks...)

Yo Watt,
Howz it hangin' dude? In all seriousness, after the purple left the leg, it started getting better, been working about a month (part time at first) and it seems to be getting better, but I'll always have to wear the heavy duty (30-40mm) compression hose.

Frankly, I think it was that special medicine you prescribed, but I still don't understand the worm at the bottom of the bottle of medicine, but I gotsta admit, sho' took the pain away.

I realize I got the voltage from sparkys thread, geeez it's awful to get old.

Industrial guru? Nah, probably not, but I've worked at Bethlehem Steel in Baltimore, RJ Reynolds in Richmond, every press around Wash. DC, coal mines in SW VA, so every cockeyed, wierd, oddball transformer has been mine to hook up. Add to that a stint in the Metro tunnels of DC when they were building them - Delta 3 phase UNGROUNDED for the continuous miners, oh and when we started the actual construction instead of changing the transformers, we had to connect a zig-zag to create a neutral, I've probably seen some very odd combos. Then throw 17 years of teaching transformers to apprentices and the subject becomes clear as mud.

Good to hear from you again Watt, how'd you make out from your little debacle?

George,
What does the 4th wire connect to in a 4 wire corner ground system? I've never heard of that one.
Don

If it's a 240V corner-grounded delta, my guess would be that the 4th wire is a center tap on one of the winding connected to ground to provide 120V for light loads.

Am I anywhere near the mark???

Paul,
How can you ground both a corner and the center of one winding. Looks like that would produce smoke, not power.
Don

This may be way off the mark, but corner-grounded 3ø4W∆ doesn't sound right. If one coil center tap is not grounded, then one 120V winding is properly grounded, but the other 120V winding is 120V to ground on one side, and the other is 240V to ground. That could make for a deadly screwshell in a 120V lampholder. That seems to go against a lot of electrical custom in the US, particularly when the mid-coil ground would provide two grounded 120V connections.

There might be some old systems found that way, but to do it intentionally doesn't seem likely. There would be no point or advantage to such an arrangement, and it would take a more complex metering arrangement to measure power flow.


[This message has been edited by Bjarney (edited 06-08-2002).]

Sorry -- I didn't mean to ground the center tap. It was late & I didn't phrase my message very clearly.

Here's what I meant:
Take a 240V delta with, let's say, phase B grounded. Now take a center-tap from either the A-B winding or the B-C winding and bring this hot line to a separate 120V busbar.

Now you have a 240V corner-grounded delta for 3-ph (or 240V 1-ph) loads, but it's possible to get 120V from the "tap" wire to phase B (ground). I would imagine such a strange set-up being used only where a very small 120V load was to be powered, perhaps a couple of lights in a pumping-house full of 3-ph 240V motors or something like that.

Just a guess, mind you.

P.S. I can't think how this would have any advantage over your more conventional 3-ph 4-w delta, but it would be a way to get a small amount of 120V power if one phase of the delta had to be grounded for some obscure reason.



[This message has been edited by pauluk (edited 06-09-2002).]

A center tapped transformer with a 240 3Ø corner Grounded would not be per 250-20(b)(1)

as one of the legs of the center tap would be 240V to ground, thus exceeding the 150V to ground.

I wasn't thinking of having two 120V circuits run from opposite ends of the center-tapped winding.

Just a single 120V busbar for very light loads fed from the center-tap. This would only be 120V to ground.

If the 120V terminal to ground (the center tap) is available, and it would be to be able to use the 120V tap, then someone will certainly use the remaining 120V coil ( center tap to the matching ungrounded leg of the center tapped transformer. Just because the terminals are available.

Codes are for overall safety, I hope.

[This message has been edited by Gwz (edited 06-11-2002).]