Zero Crossing Detection

How to make zero crossing detection in 3 phase without using neutral point.

There are only zero crossings on the individual phases in a three phase system, there is no point in time where the voltage across all three phases is zero.

If you sum the three voltages you can generate a virtual neutral that you can compare each phase against. You can do this using current sensors and an op-amp adder if you are trying to detect the zero crossing under load. If you need to detect the zero crossing while the load is disconnected, you'll have to divide the voltages down and add them up. If you are trying to disconnect a reactive load, use the current sensors.

Once you have a virtual neutral you can use a simple comparitor to detect the zero crossings.

Alternatively, you could use a 3 phase power monitoring ADC (available from TI and others) and do everything digitally.

Thanks for the reply.
if you dont mind can you give any idea about the circuit.

What voltage is the three phase power you are trying to measure?

440v

In a single phase like this, use a step-down transformer to reduce the voltage to a safe level (ideally under 10V). You can then use a resistor in series with a zener and an led + resistor with the zener and led_resistor in parallel. The led can be an opto-coupler.

Alternatively you can drive a window comparator with the transformer output (possibly through another resistive divider and with a protection zener across the output of the divider).

I am hoping you are not going to build this, if you are you should work in person with someone with experience in working at these voltage levels. Simple mistakes can result in death or serious injury and these types of mistakes are not always obvious.