120/240 V, 3 phase NOT Delta CT????

[master1]

We are designing a machine for a customer who said they have:

240 vac / 120 vac

We need 3 phase for our machine.

I want to be sure they did not mean 208/120 wye so I went over to the plant.

3 Pole main / Nuetral bus / ground bus in panel OK

2nd pole had org tap OK probably center tap of delta but know high leg (208v)

239 V A-B
239 V B-C
239 V A-C

120 VAC A-N
120 VAC B-N
120 VAC C-N

Does anyone have experience with this type of system.

I left thinking we should be fine (not sure of how they were doing this) had 3 phases and 120 vac. I just had my Fluke meter and did not verify this was a true 3 phase system???

First post Thanks

 

[augie47]

Welcome to the Forum.

Most unusual (to me). Did you measure from the line side lugs ?
Some 240/120 hi-leg panels have a breaker arrangement where Phase B drops out after the 1st few breakers so if you measured off load breakers B phase may not have been present..
With 120 to ground on all three one would think 208Y-120 but obviously the 239 is high especially in relation to the 120.
Can you request info from POCO ?

 

[master1]

Revising post

Revising post

Just read my post

1. know means no
2. want means wanted
3.
These are the voltages a read with my fluke
239 V A-B
239 V B-C
239 V A-C

120 VAC A-N
120 VAC B-N
120 VAC C-N

 

[master1]

Most unusual (to me). Did you measure from the line side lugs ?

Yes, great point

I do not remember seeing any three phase breaker other than the main

I should have check the number of bus bars

Some 240/120 hi-leg panels have a breaker arrangement where Phase B drops out after the 1st few breakers so if you measured off load breakers B phase may not have been present..


With 120 to ground on all three one would think 208Y-120 but obviously the 239 is high especially in relation to the 120.

Yes and the 120 was 120 not higher?

Can you request info from POCO ?

 

[jim dungar]

Your readings do not follow the laws of physics for a solidly grounded system.

A system where all three phase voltages have the same voltage to neutral, means you must have a wye connection from the utility. The L-L and L-N voltages in a wye system must be different by the square root of 3 (~1.732) yielding either 208Y/120 or 240Y/136 as the nominal voltages.

In a center tapped delta system one L-N voltage must be 1.732 times the other two L-N voltages.

Did you measure the N-G voltage?
Did you measure the L-G voltages?

 

[master1]

Moderator,

Just started digging into this problem.

Is the reason they drop the b phase (have no B bus at all in panel even though 3 phase breaker) to be sure that the panel does not have the high leg voltage available?

This way a electrician (or ignorant engineer) would not have the ability to wire (and outlet / branch circuit) into the high leg bus (obvious now / I think)

Thanks

 

[hillbilly1]

I would say the panel was not three phase, but single phase, I have done this before using a three phase main disconnect feeding a wireway tapping off for a single phase panel for general power, then tapping off with three phase disconnects for the few three phase loads that they had.

 

[master1]

Jim,

I measured (from the main supply lug / not load side):

239 V A term - B term
239 V B term - C term
239 V A term -C term

120 VAC A-N bus (white / many neutral wire connected) right side of panel
120 VAC B-N
120 VAC C-N

Green - ground bus was on the left side of the panel many green wires connected.

I should have but did not check the voltage between ground bar and neutral bar. Also did not check for bonding between the two.

It was a clean (well wired / prof. job) panel in an industrial plant

I am about ready to go back and look at the transformer if I can find it

I am with you on the calculations / that is why I am looking for answers

 

[augie47]

It's a first for me. Let us know what you find, ,please.

 

[jim dungar]

Is the reason they drop the b phase (have no B bus at all in panel even though 3 phase breaker) to be sure that the panel does not have the high leg voltage available?

This way a electrician (or ignorant engineer) would not have the ability to wire (and outlet / branch circuit) into the high leg bus (obvious now / I think)

Yes it is.

 

[jim dungar]

I should have but did not check the voltage between ground bar and neutral bar. Also did not check for bonding between the two.

A lack of appropiate neutral-ground bonding and/or an open neutral can create all sorts of wierd voltage readings.

 

[LarryFine]

239 V A-B
239 V B-C
239 V A-C

120 VAC A-N
120 VAC B-N
120 VAC C-N

Is it possible your meter isn't accurate?

First post Thanks

Welcome to the forum!

 

[david luchini]

Is it possible that this was an ungrounded 240V delta system that had a grounded neutral point created by a zig-zag transformer or other method?

I've never seen a zig-zag transformer installation, and haven't been able to find any good information on what the "created" line-to-neutral voltage would be.

Maybe someone here is familiar with them.

 

[jim dungar]

Is it possible that this was an ungrounded 240V delta system that had a grounded neutral point created by a zig-zag transformer or other method?

A zig-zag effectively creates a wye transformer, therefore the L-N voltage is Vll/1.732 = 240/1.732 = 138V.

 

[david luchini]

A zig-zag effectively creates a wye transformer, therefore the L-N voltage is Vll/1.732 = 240/1.732 = 138V.

I guess that's true. I got thinking about it after reading that the windings of the zig-zag transformer should be "rated" for 1/3 of the system voltage, and wondered what would happen if the rating was more or less than 1/3. But after looking at a zig-zag connection diagram, you can see that there will be a voltage drop across each winding of 1/3 the system voltage, regardless of what the winding is "rated for."