Fault Voltage

Would I be correct in saying that you are attempting to diagnose whether you have an earth fault in one phase of the secondary winding of a 415 volt supply transformer?

If this is correct I'm not sure that measuring a voltage rise in the other two phases would be the way to go about it. If this is incorrect perhaps a diagram would help to illustrate the question.

Taking the question on face value, grounding one leg of a floating three phase transformer would not have any effect on the other two phases. If it's like most three phase supplies where there is a neutral/earth involved, grounding one phase will result in a flash and a bang and all that dratted smoke will escape.

Dear friend Yr hunch is correct.When one phase is solidly grounded,the voltage in the other two phases rises to L-L voltage ie sqrt 3 times. This is also known as arcing ground. Normally 440V systems are star connected and the neutral is solidly grounded ,to ensure human and equipment safety. Single earth fault cannot be detected in ungrounded systems.U may have to wait for second earth fault(which is likely to be severe) to detect the approx zone of fault. This is only for yr information. regards Ravipra01

Sounds like you are getting confused about what 3 phase voltages are all about. What you need to understand is that when we talk about AC voltages they are not just a voltage but have a direction or angle associated with them. In other words they are vectors and you can't just add up the voltages. Look at the diagram below.

The diagram represents a 3 phase voltages and how they add together. As you can see there are three phases A B and C and each of these has a voltage and direction with respect to N or neutral in the center. Phase A is the red line phase B is the yellow line and phase C is the blue line.

If you now measured the voltage between phase A and B you would get the orange line and this is where the square root of three 3^½ comes in. The length of the orange line is 3^½ times the length of the red line. You need to keep in mind though that this line also has an angle.

What the above diagram represents is both a star and delta configuration, The red, yellow and blue lines represent the voltages when you have something connected in star format while the orange green and purple represent a delta connection.

Now what happens with the star connection is that the neutral is tied to ground so that you have a reference to measure everything against and it keeps everything safe. If you have just a delta configuration like you have no way to reference it to ground and this can be a very dangerous configuration. The usual format is to have one end of a transmission line connected in delta format and the other in star format with the center of the star or neutral tied to ground. This has the effect of keeping the whole system referenced to ground and since there is only one connection to earth stops any earth loops form happening.

If you don't have any load on the secondary side of your transformer then it's not referenced to ground. While the phase to phase voltage will be 440V the phase to neutral voltage could be anything and can even be as high as the phase to neutral voltage on the primary side.

One of the things you could do is to connect a balanced three phase load in star format and connect the center of the star to ground. This would have the effect of generating a neutral point and would keep the whole thing referenced to ground.

This doesn't really answer you question directly but I hope it gives you an understanding that enables you to answer the question yourself.

If possible re-connect both of your windings to STAR/STAR- then earth the star point of the secondary.

Assuming 440v is the output, you will then get 440/1.732 = 254 volts with respect to earth from each phase.

If you simply earth one point of the delta output, you will get zero volts on that phase with respect to earth, and you will get 400v from the other two.

I am not sure about voltage rise. Logically the volts will drop under fault conditions.