Power Supply Tripping

Are you sure it is intended to have the neutral connected to the line side? Do they provide a terminal for it that specifically identifies it as a neutral connection, not PE? Assuming this is a solid state voltage controller, I've never seen one for 3 phase output where the incoming neutral would be of any value. Usually if you want a 4 wire output, you use a transformer on the output side that will recreate a new neutral with reference to the new variable voltage 3 phase output. Carrying a common neutral through the unit with different voltages on either side would be difficult to imagine.

I give up, it's the path of least resistance!

The input cable is a 4 wire cable of red,yellow,blue and black wires.so it means the black wire wil be neutral.same color output terminals are available at output side.it is the old type transformer type power supply not a solid state supply

Maybe the color code was different where the power supply was manufactured? For instance, what if blue = neutral?

Why Connect Neutral at all?

3 phases take care of any/all unbalance .

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Do you have loads that go to neutral?

When the neutral is not connected, is there a voltage seen on the terminal where the neutral should be connected? If so, you have a fault on one of your circuits after the incomer!

Most 3 phase loads do not require neutral. Neutral is basically only needed when single phase loads are needed.

You need to use a voltmeter and to identify 100% the phases and neutral/ground from each other. Measuring phase to neutral will show half the voltage as that from phase to phase.....do be very careful, these are killing levels......

On thinking about it, the black wire will most probably will be ground........R Y & B are generally the phases in my limited experience of AC NATO warships.....

Connecting ground to the wrong place will cause a ground fault condition that will trip any proper working breaker that senses ground faults (a good thing to have!).....exactly the problem you seem to have....

GE picked up where I was going with my question about loads to neutral. If all loads are 'phase to phase' (single phase), or three phase, this becomes very easy to trouble shoot. Help us to help you.:)

Do you only have 3 phase stuff where you are? How about lighting and single phase sockets? Do these have there own breakers? If so, are they single pole or double pole? With only the 3 phase breakers on and all the other breakers off, do you still have the breaker tripping with the neutral connected?Do you have a photograph of your Board opened up so we can get a better idea as to what is going on inside?

I have a 3 phase 400vac power supply whose input is a 4 wire three phase 400ac while output is 3 phase 4 wire variable 400vac-------------------

We all missed this Prime Suspect VARIABLE

The last reply spells out what is essential for continuity of discussions .

looking for help;

You can Help with a Circuit diagram and photos as suggested.

"We all missed this Prime Suspect VARIABLE"

Speak for yourself, I caught it the first time, but asssumed it meant it was an SCR type voltage controller, which is apparently incorrect.

So coolyar if "it is the old type transformer type power supply" does that mean it is a 3 phase variac, kind of like this?

If so, are you sure it was originally designed to have a 4 wire input?

If so, I'm kind of leaning towards Tornado's suggestion that your color scheme may be off. All these are is a series of autotransformers with a wiper. Any neutral connection should go from one side to the other.