The test of current injection method is initiated by closing the making switch (MS), which initiates the flow of the current i1, from the high current source through the Insulation Breaker(IB) and the Test Breaker (TB). When the current approaches its zero crossing the spark gap is triggered and at time t_(2 ) as Figure 3 the injected current i_(2 )begins to flow. The current i_(1 ) + i_(2 )flows through the test breaker until the time t_(2 )is reached. This is the time when the main current, i_(1 ) near to zero and when the insulation breaker separates into two power sources. When at time t_(3 ) the injected current is interrupted and the high voltage supplied by the high voltage source provides the desired TRV which subsequently appears across the terminals of the circuit breaker that is being tested.