120/208 3phase

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electricalist

electricalist

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Location
dallas tx
Where does the 208 to ground b phase our side c phase poco side. I assume part a and part c . Very interesting. If a phase c phase show 40 v each to ground . Why does a to b show 190v and b to c show 230?. Power outtage last night. When we checked thats what they had.
 
I'll bet I'm not alone in saying the first post ranks as the most confusing I've read in weeks :)
Are the two posts related to the same situation ?
 
Yes sir. We went to a call where customer says power is out. 120/208 3 phase b phase is hi leg 208 to ground. There is 1 400 a disc. 1 400a disc not in use 1 200 a disc and 1 200 amp single phase disc. Customer has sbout 5 offices with lights only working. Nothing else in building working that they know of. Voltage check shows a ph to gr is 40v b ph to gr 211 c ph to gr 40 v. A to B 190 and B to C 230 and I dont remember A to C . Poco said theres a power outage. I believe a and c are out. So how did I get v on a/c to b and how where 2 x 4 lights working
 
That would be my guess. POCO loss of a phase causing odd voltages throughout.
Voltages would vary depending on how much of their system was affected.
 
electricalist said:
Yes sir. We went to a call where customer says power is out. 120/208 3 phase b phase is hi leg 208 to ground. There is 1 400 a disc. 1 400a disc not in use 1 200 a disc and 1 200 amp single phase disc. Customer has sbout 5 offices with lights only working. Nothing else in building working that they know of. Voltage check shows a ph to gr is 40v b ph to gr 211 c ph to gr 40 v. A to B 190 and B to C 230 and I dont remember A to C . Poco said theres a power outage. I believe a and c are out. So how did I get v on a/c to b and how where 2 x 4 lights working
Click to expand...
In a 208Y/120 service there should not be a high leg. All phase lines should be 120V to neutral and 120V to ground.
But you should not be paying much attention to voltages to ground, just to neutral, when POCO has a grounded wye service.
The symptom of losing two phase wires from POCO would be that the voltages on those two wires to ground would be undetermined and only single phase loads from one phase line to neutral will be working.
All line to line loads and two out of the three sets of line to line loads (208V lighting or motor loads) will not work at all.

Are you sure that the service really is 208Y/120 rather than a 120/240 four wire high leg delta?
 
141003-2102 EDT

How many transformers on the pole, and comparison of sizes? If three transformers, then how are the secondaries wired?

.
 
Sorry I ve been workibg on a restaurant for 4 hrs. To answer the first question. Phase to ground / neutral lugs in 1 st disconnect where they are bonded. .maybe im confused but when the poco first came and hung the transformers. Their supervison looked and asked if we landed our riser b phases on the right where phase c is. I saud yes he said their high leg is always there and load side from meter we take c phase to b in the first disconnect. We payed close attention when feeding the single phase disc. To make sure it was a and b phase from the meter. 120/240
 
87808e9581d7c2d89c1d9967a3563202.jpg

Since I dont know honestly here I a diagram of 120/208 with b phase 208 to g/n
 
electricalist said:
87808e9581d7c2d89c1d9967a3563202.jpg

Since I dont know honestly here I a diagram of 120/208 with b phase 208 to g/n
Click to expand...
There is where we got disconnected from you.
Thus is called 120/240 three phase four with, with a 208V high leg.
You do not mention the 208 when naming it because there will normally never be a load from high leg to ground.
You can have 240V single phase loads between any two ungrounded lines, 240V three phase loads in delta only, and 120V lighting loads from A or C to Neutral.
If some of the lights are working and some are not, you have most likely lost B and either A or C.
To measure POCO feed, open all main and load breakers and use a low impedance meter or Wiggy to measure line to line and line to neutral.
If the voltages to ground are weird with a low impedance meter, then you may have lost the POCO neutral.
 
I think I learned 120/208 from reading panels. prints etc. All disconnects were off . A phase to b phase was 190 b phase to c phase was 230v. A phase to g/n 40 v b phase to g/n 211 c phase to g/n 40 v
 
141004-0931 EDT

electricalist:

A guess I have is the following:

You have not answered my previous question on how many transformers. So I will guess 3 based on the voltages.

I believe the power company primary is a wye supply, and that the leg supplying the primary of the transformer with A-CT-B terminals is open. Thus, no energy supply from the power company primary leg. This labeling is based on your description.

Energy is supplied to the transformers with secondaries A-C and B-C. B-C looks quite stiff, A-C looks not stiff (higher than expected source impedance), or A-C is very heavily loaded.

A-CT-B is funtioning as an auto-transformer and is deriving energy from transformers A-C and B-C. The total voltage for A-B is 80 V instead of about 240. There is some very major phase shift in this closed delta.

A vector diagram forming a closed triangle using your voltages produces a C to CT voltage of 208. Sufficiently close to your 211 to somewhat verify your voltages.

.
 
Sorry sir. 3 are 3 transformer s . I cant say if they are all the same size. I may have a picture. Xfmrs are fed from across the street where there are connected to top of poco pole line wires
 
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