How to detect zero crossing in 3 phase without neutral

 

Use three detectors.

 

How to detect zero crossing in 3 phase without neutral

With no neutral, then you detect the zero crossing of the voltage measured across each phase.
If you want to measure all three phases, then that requires three isolated differential detectors.
An opto isolator can be used for that purpose (as long as an approximate 1.3V offset in detecting the zero crossing point is not a problem).
If that's too much offset than a comparator could be added at the input of the opto.

 

please can you help with the below circuit.

 

I would use the Earth as a reference point and detect between each phase and earth, if you do it between phases you need three circuits.

 

Thanks for the reply

 

I would use the Earth as a reference point and detect between each phase and earth, if you do it between phases you need three circuits.

Um what? Say that again?

 

I would use the Earth as a reference point and detect between each phase and earth, if you do it between phases you need three circuits.

If there's no neutral, then the earth reference is likely to be rather noisy as compared to the phase voltages.

And how does that reduce the number of required circuits?

 

Same number of circuits required, for full zero crossing detection either between phases or PME.