This improves security by allowing the phase with the low harmonic ratio to be cross-blocked by a phase with a higher ratio,preventing possible false trips. However, the inrush current is generally several times the rated current, and the insulation bears the most severe mechanical stress during energization. If there is an internal fault in one or two phases, the high second harmonic ratio in a healthy phase may block the percent differential protection until the fault spreads to all three phases. The differential protection with cross-blocking harmonic method is very secure but least dependable.Two-out-of-three restraint method is a slight variation of the cross-blocking method. The blocking of differential operation
will need at least two phases to detect sufficient harmonic level. This method still suffers the same basic disadvantage of the
simpler cross-blocking method.