As stipulated in AS 4024.1501/1502, the use of basic and well-tried safety principles must be applied to safety categories 1-4. The design and construction of safety contactors implement these safety principles and there shall be used when designing safety systems in accordance to the standards. Some of the features that define safety contactors are:
Positively guided contacts:
The auxiliary contacts used for feedback to the safety system are mechanically linked to the power poles. This feature means the auxiliary contact will not change state if the power poles weld. In AS 4024.1502 the use of mechanically linked contacts for monitoring is defined as a well-tried safety principle, required for Category 1-4.
No manual operation:
Unlike standard contactors that can be manually operated from the front of the unit, the safety contactors do not allow manual operation. This feature avoids the possibility of an operator creating an unsafe state due to unexpected start up. In AS 4024.1502 the prevention of unexpected start-up is defined as a basic safety principle, required or category B-4. In addition, one of the safety design objectives (AS 4024.1501) is to take into account foreseeable human mistake, thus remove the opportunity for the operator to start the operation on an unsafe state.
Permanent fixed contact block:
The contact block for the safety contactors is permanently mounted; this avoids the possibility of the auxiliary block becoming separated from the contactor due to environment causes (eg.vibration) or human intervention. If the auxiliary block was to be removed from the contactor this would disable the safety systems ability to monitor the function of the contactor. In AS 4024.1502 the secure fixing of inputs to the safety system is defined as a basic safety principle, required for category B-4. In addition one of the safety design objectives (AS 4024.1501) is to take into account foreseeable misuse, thus having a permanently fixed contact block protects against an operator removing the contact block to bypass a welded contactor.
For use in category 1 the safety contactor also must meet the requirements of a well-tried component A well-tried component is to be widely used in similar applications in the past with success or is proven to use the appropriate safety principles. For a contactor to be considered a “well-tried component” it must use basic and well-tried safety principles, as discussed above. Also the following should be considered:
- Influences such as vibration
- Designed using safety principles such as over dimensioning (AS 4024.1502)
- Current to the load is controlled using thermal protection
- The circuits are protected by a protection device.