The mains ground must be symmetrical to the phases in the mains net. As the sensor is only using one phase and ground, this is necessary to detect the actual zero crossing according to the three phases in the mains net. If ground is not symmetrical it is not possible to know when the crossing is using only one phase. This can only be a problem in delta shaped mains net. Most countries, except Norway and Albania, use star shaped mains system and will not have this problem.