[kostas1]

There is a process as outlined in my old GE MOV Suppresion manual, (I see it is a collectors item now!).
If we assume it is in fact a MOV, there is a way of doing a conservative estimate, first the peak voltage the device should ever see in normal operation, either AC or DC has to be known and then what would be the max energy it has to dissipate which is usually unknown without measuring, but the where a device is being replaced, the physical size is a good indication, go slightly larger if anything, on the energy value.
In most circuits, the device will still operate OK without the MOV, as it normally only comes into play when a spike occurs.
If the device does NOT operate, it could be there are other defective components also.
If you Google Transient Voltage Suppression, you should get some articles that may help in selection.
Al.

Yes, it does operate and sometimes I get the 'dl error' in VFD's screen...So if the spikes are the enemy, why not buying a UPS which could also stabilize the input voltage and protect it from lightings etc?
Thanks