Window type ground fault sensors are usually used to sense Zero Sequence Ground Fault Currents. This requires that all three phases and the neutral (in the case of a four wire system) be placed through the window of a single current transformer. Under normal conditions the current transformer will not see any current as the sum of a balanced three phase system is zero thus the name “Zero Sequence”.
In as much as all three phases and neutral (if there is one) must pass through the same window. This requires a current transformer with a large window to accommodate all of the conductors.
