3 phase with high leg?????

[hockeyoligist2]

Some of you may remember I was asking about ground faults on a 3 phase delta with a a high leg? And I'm probably asking a dumb question. But I have never worked with it. OK, on to the subject.........

It is a new service and I went this morning to wire some motors that finally got installed, I checked the voltage from A to B I got 208v, B to C 208v, A to C 240v. It does not have a neutral only a ground. A to ground was 743 volts B to ground was 915 volts C was 762 volts. Called POCO and they said this is OK!!!!!!!!!!!!!!!!! Whats going on? Asked them to explain and they just said thats acceptible........ Help a Dummy Please?

 

[bob]

Some of you may remember I was asking about ground faults on a 3 phase delta with a a high leg? And I'm probably asking a dumb question. But I have never worked with it. OK, on to the subject.........

It is a new service and I went this morning to wire some motors that finally got installed, I checked the voltage from A to B I got 208v, B to C 208v, A to C 240v. It does not have a neutral only a ground. A to ground was 743 volts B to ground was 915 volts C was 762 volts. Called POCO and they said this is OK!!!!!!!!!!!!!!!!! Whats going on? Asked them to explain and they just said thats acceptible........ Help a Dummy Please?

What is the voltage supposed to be? delta or wye? These reading are wierd.
A 240 volt delta is 240 volt between phases, 120 v phase 1 and 2 to ground and 208 v phase 3 to ground.

 

[catchtwentytwo]

Did you confirm the readings with another meter?

 

[hockeyoligist2]

yes we checked it with 3 differant meters and got the same reading. it is a delta 120/208

 

[bob]

yes we checked it with 3 differant meters and got the same reading. it is a delta 120/208

If its delta it should be 240 v across the phases and 120 v from the grounded coil to neutral.

 

[bob]

Voltage Readings

Voltage Readings

It is a new service and I went this morning to wire some motors that finally got installed, I checked the voltage from A to B I got 208v, B to C 208v, A to C 240v. It does not have a neutral only a ground.

The voltage you are showing are for a wye connected bank. The 240 volt
reading is abnormal or the 208 volt is abnormal. If you have a delta connected
secondary and you do not have a ground, then the reading from phase to ground are meaningless.

 

[scott thompson]

Sounds like you maybe have an Ungrounded 3 Phase 3 Wire 208V Wye system, since the voltage to ground is so much higher than the L-L voltage.

BUT....

This all depends on what type of Voltage Meter being used to test the system's Voltage, and if the power system is _SUPPOSED TO BE_ Grounded or Ungrounded.

If the Meter is a High input Impedance type (typical of the DVMs commonly used), then the L-G Voltage displayed would be "normal" of an Ungrounded AC Power System (results of Capacitive Coupling effect). Also, the L-L readings will be displayed as normal (within the voltage rating of the system).

If the Meter is a Low input Impedance type (typical for a "Wiggy" or scalable Meters which load the tested circuit), then on an Ungrounded AC System, you will see near zero readings between L-G, but normal readings between L-L.

If the AC System is _INTENTIONALLY_ Grounded, and the Equipment Grounding Conductor (or metallic enclosure) that you are using for L-G voltage testing is _NOT BONDED_ to the System's Grounded Conductor, then you will see these high L-G voltage readings.

You may have an Ungrounded 3 Phase 3 Wire 240 Volt Delta., or the previously mentioned 208V 3 Phase 3 Wire Ungrounded Wye.
The best way to determine what type of system you have is to examine the Transformer setup - or to verify L-L + L-G voltages as close to the Transformer as possible.
Verify if the system has any one of it's conductors physically bonded to the metallic enclosure of the service equipment, plus to a local Grounding Electrode System.


Readings for L-L loads appear to be higher between "A" and "B" + "B" and "C"; with a lower load level between "A" and "C".
Lines "A" and "C" to Ground look equal, which may be due to the connected loads on these two Lines.
Line "B" to Ground may be higher from either connected load characteristics, or a slight voltage difference on that winding at the Transformer.

Scott

edited to fix spelling air-ors (errors)

 

[scott thompson]

Bump-a-dee-bump

Bump-a-dee-bump

*** El- BUMP-O***



To the "OP": - Whazzup so far???-

Scott

 

[beanland]

Voltage to Ground

Voltage to Ground

I think you have an ungrounded service. The voltage to ground will float depending on the leakage capacitance in the transformers. Get the service grounded ASAP. You want either 240/120V 4-wire delta, 208Y/120V 4-wire wye or 240V 3-wire corner gounded delta.

I am suspicious that the POCO installed a 3-pot bank using 120/240V transformers and connected it in an odd manner. I have seen 208Y/120V 3-phase and 120/240V single-phase services fed from the same transformer bank.

 

[jtester]

I am suspicious that the POCO installed a 3-pot bank using 120/240V transformers and connected it in an odd manner. I have seen 208Y/120V 3-phase and 120/240V single-phase services fed from the same transformer bank.

Can you show us a connection that provides 208/120 3 phase and 120/240 single phase from the same bank? I've seen 240 3 phase and 120/240 single phase from one bank, but never 120/208 and 120/240.

Jim T

 

[beanland]

120/208v & 120/240v

120/208v & 120/240v

Can you show us a connection that provides 208/120 3 phase and 120/240 single phase from the same bank? I've seen 240 3 phase and 120/240 single phase from one bank, but never 120/208 and 120/240.

Jim T

Let me describe it: take three single-phase 7200-120/240V transformers. Connect their primaries in wye to 3-phase source. Connect their center-taps all to ground. You now have a 6-phase system, six voltages 60 degrees apart. You can get 120/240V and 120/208V. In fact, there are two 120/208V systems 60 degrees apart. Need a picture?