It is possible when in engineering mode to move the telescope hardware without needing the computer. It is used by technical staff to move the telescope for such jobs as top end changes or by the astronomer for the initial setting up at the start of the night. The telescope is protected from being driven too far by the use of limit switches. See Section 9.2 for the procedure to use in the event that these limit switches are activated. The relays that protect the telescope are set so that it is only possible to drive away from trouble. Note however that hitting limits is regarded as an unusual case. In these circumstances, the computer loses control over the hardware and so when a limit has been reached it is impossible for the computer to drive out. It is then necessary to use Engineering Mode and no guarantee of the accuracy of the telescope position is made until the encoders have been freshly zeroset. Engineering Mode is the default mode on powering up the system.