A fire or smoke barrier 1 has a curtain 2 deployed from a motorized roller 3 mounted in a head box 4. Remote from the head box and connected to the motor via a cable 5 is a motor controller circuit 6. A short circuit protection circuit 9 is connected to the motor within the roller at the motor end of the cable 5. On the motor side of the circuit 9 is a retarder circuit 10 connected across the motor terminals.The short circuit circuit comprises a pair of relays 11, 12, arranged with their normally open contacts 111,121 arranged in parallel. A drive voltage one 11 of the relays has its winding 112 across the input lines 14 of the circuit and the emf one 12 of the relays has its winding 122 across the output lines 15 of the circuit. The input lines 14 are connected to the cable lines 51,52 and the output lines 15 are connected to the motor terminals 16, via the retarder 10.