A clamping failure detector assembly (30; 50; 70) comprises a frangible electromagnetic radiation conduit (32) that extends between first and second portions (34, 36) of at least one thyristor clamped assembly (10). The detector assembly (30; 50; 70) also includes a an electromagnetic radiation source (31) that is arranged in communication with a first end of the frangible conduit (32) and an electromagnetic radiation detector (33) that is arranged in communication with a second end of the frangible conduit (32). The frangible conduit (32) ruptures on movement of the first and second portions (34, 36) of the or each thyristor clamped assembly (10) away from one another to interrupt the transmission of electromagnetic radiation from the electromagnetic radiation source (31) to the electromagnetic radiation detector. The electromagnetic radiation detector (33) indicates a clamping failure when the electromagnetic radiation transmitted from the electromagnetic radiation source (31) is interrupted.