• closure of the barrier where this is necessary for the safety of the hinterland
• timely closure of the barrier when it needs to be shut
• protection of the barrier itself; keeping the barrier "preserved".

If the barrier fails to close or to do so on time this can have disastrous consequences for the hinterland. Exceptional reliability is therefore the most important demand made of the system. In order to ensure such reliability the BOS hardware contains two redundant branches. The script interpreter explores its way and starts with the first branch. If an error arises in that branch, the computer switches to the second hardware branch, which is identical to the first. If an error later arises in the second branch, the computer then continues with the sound element of the former branch. This is a continuous process: at any one point in the course of a procedure the BOS can - if necessary - decide to switch between the two branches. This makes BOS highly reliable.