The Vanguard MCCB-250 Molded Case Circuit Breaker Tester is a programmable, high-current source designed specifically for testing molded-case circuit breakers, thermal magnetic and solid-state overload motor-protection relays. The MCCB-250’s built-in timer can test the time-delay characteristics of protection relays and molded case breakers. Once the test is initiated, the current source and the timer are automatically turned on at the next zero-crossing point of the AC. The timer stops when the MCCB-250 input detects a change in the dry contact or voltage input, or detects the removal of the test current. The test results are then displayed in milliseconds and fractions of cycle(s) on the unit’s back-lit LCD screen (20 characters by 4 lines). The MCCB-250 also provides an external current input (0-10A). Both internal and external current source readings can be viewed at the same time.