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When testing high voltage circuit breakers synthetically the question arises as to how the test results can be compared with test results in direct tests. In the paper the thermal equivalence between direct testing and the method of voltage injection is investigated. The circuit breaker is modelled by the Urbanek equation. It is shown that the prospective voltage alone cannot define the test circuit to ascertain thermal equivalence. To lay out the synthetic test circuit to obtain results equivalent to direct testing requires the exact knowledge of all circuit elements including the auxiliary breaker and the test breaker. There is no problem, however, if this method is applied for tests in which the breaker is dielectrically stressed.