The present invention relates to a method for determining the existence of an earth-fault on-the-fly and thereby protecting a motor controller having a high- and a low-side DC-link, and having high- and low-side switching elements, the high- and low-side switching elements being operatively connected to the high- and low-side DC-link buses, respectively, the method comprising the steps of generating a fault signal, generating at least one test vector in response to the fault signal by switching at least one of the switching elements on, and measuring, while the at least one switching element is on, the magnitude of a current flowing in that DC-link operatively connected to the conducting switching element in order to detect an earth-fault. The present invention further relates to additional methods for on-the-fly determination of earth-faults.