Hi, 

Typically unidirectional, but a bidirectional TVS would work similarly for our purpose. In this case, the chosen bidirectional TVS would just operate as if its unidirectional, and it won't operate in the other region due to diode blocking. 

I don't knw why PI Expert is assigning  a bidirectional TVS for the primary clamp - I'll bring it up with the web team. A unidirectional device is the usual option for the primary clamp, and likely less expensive as well.

 When selecting TVS diodes for RCD circuits, whether they should be uni or bi-directional depends on the specific requirements of the application. sling billing issues Uni-directional TVS diodes protect against voltage spikes in one direction, while bi-directional TVS diodes safeguard against spikes in both directions. Consider the polarity of potential transients and select TVS diodes accordingly to ensure effective protection against overvoltage events in the RCD circuit.    
 

If you are needing to clamp  negative-going spikes with a bidirectional TVS, then the transformer design is seriously compromised, likely due to excessive  intrawinding capacitance on the primary.  Adding a damping resistor of a few tens of ohms in series with the clap helps to damp  excessive ringing due to stray capacitance.