Can someone explain why on a 3 phase delta transformer (460/208) on the load side that 1 of the phases if you check it to ground does not show 208V but the other 2 phases do. But if you take off the load you get 208 to ground and if you check phase to phase it shows 208v. I was told it was refered to as a "freak leg". Could someone please explain this.
Eng-Tips is the largest engineering community on the Internet
Intelligent Work Forums for Engineering Professionals
-
Congratulations waross on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!
Delta Transformer 3
- Thread starter deppler
- Start date
You need to do several things, before I and others start explaining everything under the sun:
1. Your data does not make sense. As it does not fit any standard "delta" or "wye" system in the USA.
2. Post the nameplate data of the transformer in question. It appears that you have not personnaly taken the readings or have not noted them correctly.
3. Use help of your electrical group, and verify how, if at all, the secondary of the transformer is grounded.
4. Look up a copy of NEC Handbook, articles 230 and 250 that one of your electrical collegue will have. All common tranformer configurations are explained in there. See which one of them fits your system readings.
1. Your data does not make sense. As it does not fit any standard "delta" or "wye" system in the USA.
2. Post the nameplate data of the transformer in question. It appears that you have not personnaly taken the readings or have not noted them correctly.
3. Use help of your electrical group, and verify how, if at all, the secondary of the transformer is grounded.
4. Look up a copy of NEC Handbook, articles 230 and 250 that one of your electrical collegue will have. All common tranformer configurations are explained in there. See which one of them fits your system readings.
Perhaps you actually have a three-phase 240V delta system with a "wild leg"? This will read 120V to ground on two phases and 208V on the third. These are the work of the devil and should always be shunned whenever possible.
Try a Google search on "wild leg delta".
Try a Google search on "wild leg delta".
- Thread starter
- #5
Ok let me elaborate a little further. I am a refrigeration tech not an electrician. Here is what I saw. On the phases A-B=208v A-C=208v C-B=208v. A to ground 208v, B to ground 208v, C to ground 0v. This is seen when there is a load. Take off the load, and have 208v to ground on all three legs. is this right?
-
1
- #6
davidbeach
Electrical
No, you can't have 208V to ground on all three phases while also measuring 208V phase to phase. 208V as a delta voltage is very uncommon; are you sure it is 208V delta? My guess would be that the 208V is from a wye but the neutral of the wye is not grounded. Either phase C is grounded or your load has a ground fault on phase C. Best to get an electrician to look at it.
- Thread starter
- #7
I did have the electrician look at it that is why I am not sure he is correct, but you will get those readings on corner grounded delta transformer, but even if you take off the load at the distribution panel the leg of the transformer should still be grounded and reading 0v to ground not 208v. Also doesn't make since that out of 10 other transformer with identical loads are wye grounded why would one of out 10 be delta corner grounded. Anyway everyone thanks for your help now I have some ammo to take to my boss.
-
1
- #8
If one leg is grounded with a load connected and not grounded with the load disconnected, the load must have a ground on it.
Also, the transformer is not grounded.
Also, 208 volts to ground on all three phases does not make sense.
You have at least two serious problems and possibly more.
1> Grounded load.
2> Ungrounded transformer.
3> The readings of 208 volts to ground on all three phases are probably not correct. If they are correct or even significantly different from the 120 volts that they should be, you may have a high resistance connection at the common wye and ground connection and possibly an interaction with the 460 volts. (Is this an autotransformer?)
I suspect that your transformer is 460V(or 480V)delta to 120/208V wye.
Just calling an electrician may not be enough. There are many industrial electricians that are familiar with these systems and are able to competently trouble shoot them.
There are many residential electricians who would be lost on this system. There are many more electricians in between.
BE AWARE THAT YOU MAY HAVE LETHAL VOLTAGES ON CIRCUITS OR WIRES THAT ARE NORMALLY AT GROUND POTENTIAL.
respectfully
Also, the transformer is not grounded.
Also, 208 volts to ground on all three phases does not make sense.
You have at least two serious problems and possibly more.
1> Grounded load.
2> Ungrounded transformer.
3> The readings of 208 volts to ground on all three phases are probably not correct. If they are correct or even significantly different from the 120 volts that they should be, you may have a high resistance connection at the common wye and ground connection and possibly an interaction with the 460 volts. (Is this an autotransformer?)
I suspect that your transformer is 460V(or 480V)delta to 120/208V wye.
Just calling an electrician may not be enough. There are many industrial electricians that are familiar with these systems and are able to competently trouble shoot them.
There are many residential electricians who would be lost on this system. There are many more electricians in between.
BE AWARE THAT YOU MAY HAVE LETHAL VOLTAGES ON CIRCUITS OR WIRES THAT ARE NORMALLY AT GROUND POTENTIAL.
respectfully
-
1
- #9
Be careful as you don't want to find a ground fault and remove the ground fault if it makes something live - until you've found the problem.
If you can completely remove the supply form the refer unit try checking from a known ground to each lead. See if one is common with the ground.
Again BE CAREFUL.
Keith Cress
Flamin Systems, Inc.-
If you can completely remove the supply form the refer unit try checking from a known ground to each lead. See if one is common with the ground.
Again BE CAREFUL.
Keith Cress
Flamin Systems, Inc.-
- Status
Similar threads
- Locked
- Helpful tip
- Locked
- Question
- Locked
- Question