A testing process for an electrical system, the electrical system comprising: a power converter including: two or more direct current DC converter terminals (4a, 4b, 4c), and two or more converter legs connected to the DC converter terminals, each converter leg including a plurality of semiconductor switches and an alternating current AC terminal that defines an AC phase of the electrical system and is connectable to a respective AC terminal of an electrical load, wherein the plurality of semiconductor switches are controllable to put each converter leg in an off-state, or in one of a plurality of on-states that defines a respective voltage level of the converter leg; and a DC bus including: two or more DC bus terminals (14a, 14b, 14c), each DC bus terminal (14a, 14b, 14c) being connected to a respective DC converter terminal (4a, 4b, 4c), and at least one DC bus capacitor (16a, 16b); the testing process being a fully automated testing process where a sequence of different diagnostic tests are carried out on the electrical system, each diagnostic test testing one of the power converter and the DC bus to determine if it is responding as expected or operating within normal parameters; wherein the testing process further comprises: a plurality of first diagnostic tests carried out sequentially with the converter legs in a different state for each first diagnostic test, wherein the plurality of first diagnostic tests comprises a first diagnostic test wherein each converter leg is in the off-state and a first diagnostic test wherein each converter leg is in a same first one of the plurality of on-states, each first diagnostic test comprising: measuring a DC bus voltage while a DC voltage is supplied to the DC bus for a period of time, and generating a fault signal if the measured DC bus voltage does not exceed a first expected DC bus voltage. a power converter including: two or more direct current DC converter terminals (4a, 4b, 4c), and two or more converter legs connected to the DC converter terminals, each converter leg including a plurality of semiconductor switches and an alternating current AC terminal that defines an AC phase of the electrical system and is connectable to a respective AC terminal of an electrical load, wherein the plurality of semiconductor switches are controllable to put each converter leg in an off-state, or in one of a plurality of on-states that defines a respective voltage level of the converter leg; and two or more direct current DC converter terminals (4a, 4b, 4c), and two or more converter legs connected to the DC converter terminals, each converter leg including a plurality of semiconductor switches and an alternating current AC terminal that defines an AC phase of the electrical system and is connectable to a respective AC terminal of an electrical load, wherein the plurality of semiconductor switches are controllable to put each converter leg in an off-state, or in one of a plurality of on-states that defines a respective voltage level of the converter leg; and a DC bus including: two or more DC bus terminals (14a, 14b, 14c), each DC bus terminal (14a, 14b, 14c) being connected to a respective DC converter terminal (4a, 4b, 4c), and at least one DC bus capacitor (16a, 16b); two or more DC bus terminals (14a, 14b, 14c), each DC bus terminal (14a, 14b, 14c) being connected to a respective DC converter terminal (4a, 4b, 4c), and at least one DC bus capacitor (16a, 16b); the testing process being a fully automated testing process where a sequence of different diagnostic tests are carried out on the electrical system, each diagnostic test testing one of the power converter and the DC bus to determine if it is responding as expected or operating within normal parameters; wherein the testing process further comprises: a plurality of first diagnostic tests carried out sequentially with the converter legs in a different state for each first diagnostic test, wherein the plurality of first diagnostic tests comprises a first diagnostic test wherein each converter leg is in the off-state and a first diagnostic test wherein each converter leg is in a same first one of the plurality of on-states, each first diagnostic test comprising: measuring a DC bus voltage while a DC voltage is supplied to the DC bus for a period of time, and generating a fault signal if the measured DC bus voltage does not exceed a first expected DC bus voltage.