Voltage Loss on Delta Service

[bytebyte]

We have a monitoring device installed at a 3-ph, 480V 3-wire delta service to monitor our disturbances. When a fuse on b-ph blows, the voltage on b-phase drops to 320V and the voltage on C-ph drops to 250V, but A-phase remains at ~480V. I would think that if the fuse blows on a certain phase, then that phase voltage would be close to zero and the voltage on the other two phases remain the same.

Can someone explain the theory behind this? Thanks.

 

[itsmoked]

Yes.
The rotating machinery and transformers served by your service will try to recreate that missing phase in a hard to predict way.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jghrist]

What do you mean by "voltage on b-phase"? On a delta system you would be monitoring voltages between phases, not the voltage of single phases.

 

[bytebyte]

You're right, jghrist.

The graph from the monitoring device has 3 channels which would typically represent the 3 phases. Channel 1 measures A-B voltage, Channel 2 measures B-C and Channel 3 measures C-A.

Therefore, Channel 2 is measuring 320V, Channel 3 is 250V and Channel 1 is 480V. Thanks.

 

[wareagle]

If you draw the delta configuration and designate the phases as you have shown in your last post you can see what happens when a fuse blows. If you were to have no load and the B phase blows, coil AC has full 480 volts. Coil AB and BC are now in series and parallel to AC and you would get 480 volts across both coils or 240 across each. That would answer your question about AB and CA. I would think BC should be the same unless, as Smoke indicated, you have some motors trying to maintain voltage and this adds to the 240 volts. I would think that this would occur on the other phases also. I hope you know that this is a single phase condition and you can damage the motors.