A robot arm (202 fig. 1) for a robot (200 fig. 1) such as a surgical robot comprises a joint (205 fig. 1) permitting a second link (204 fig. 1) to move relative to a first link (204 fig. 1), a motor (208 fig. 1) for driving the joint (205 fig. 1) and a controller (209 fig. 1) for the motor (208 fig. 1) configured to electrically brake the motor (208 fig. 1) in response to detection of a fault in the robot arm (202 fig. 1) by applying a braking current to the motor (208 fig. 1) so as to maintain the position of the joint (205 fig. 1) against gravity. The motor (208 fig. 1) may be a multiple-phase motor and in response to detection of the fault the controller (209 fig. 1) may apply a braking current to all phases 301, 303, 305 of the motor (208 fig. 1) or between a first pair of phases of the motor (208 fig. 1) only. When the fault is a loss of power the braking current may be connected from a primary power supply external to the robot arm (202 fig. 1) or from a back-up power supply external to the robot arm (202 fig. 1). A fault may be indicated when a current limit is exceeded between a load switch 307, 308, 309 and a motor drive circuit 310, 311, 312 or if a Pulse Width Modulation signal is not generated in a motor drive circuit 310, 311, 312.
