In the field of power transmission networks, particularly high voltage direct current (HVDC) power transmission networks, there is provided an electrical assembly (10). The electrical assembly (10) comprises a converter (12) that includes at least one AC terminal (14A, 14B, 14C) for connection to an AC network (16) and at least one DC terminal (18, 22) which is operatively connected to a DC power transmission medium (20, 24). The electrical assembly (10) also includes a signal injection circuit (32) which is operatively coupled with at least one DC power transmission medium (20, 24). The signal injection circuit (32) includes a signal generator (38) to selectively inject a plurality of different frequency signals into the said at least one transmission medium (20, 24). In addition the signal injection circuit (32) includes a signal analyser (42) to establish a response signature of the said at least one transmission medium (20, 24) to the plurality of different frequency injected signals. The signal injection circuit (32) also includes a control unit (46) that is programmed to detect a dielectric breakdown in the said at least one transmission medium (20, 24). More particularly the control unit (46) is programmed to control the signal generator (38) to inject a plurality of different frequency signals into the said transmission medium (20, 24), retrieve the corresponding response signature of the said transmission medium (20, 24) from the signal analyser (42), and compare the response signature to a plurality of reference signatures corresponding to dielectric breakdown and non-breakdown conditions in the said transmission medium (20, 24).