Pole mounted metering units

[Hebrew]

Hi,guys.We install two types of metering units on poles;all have three bushings on the line side and some have three or two bushings on the load side of the unit.I was asked when do we use one as opposed to the other and honestly could not say as I have seen them installed as they are available.I was told thet the 3/3 bushing types are for Y-Y systems and the 3/2 are for wye-delta installations.Reason being,if the three phases are balanced,the third one goes from line to load without being metered as it is assumed that is equal to the two that are metered.Can I have an perspective on this?Much appreciated!