Single Fault Tolerance
This is the preservation of the safety function in the event that a single component in the circuit fails. A welded relay contact, a broken wire or a failed semiconductor must not, on its own, leave the machine in a dangerous state; the way to achieve this is redundant channels and fault detection.
Tolerance consists of two components: redundancy and diagnostics. Redundancy means a second path can do the same job. Diagnostics ensures the fault is noticed and rectified; otherwise the first fault stays hidden and the protection collapses when a second fault arrives.
The single-fault scenarios most frequently encountered in a crane panel:
- Contact welding: because of overcurrent or arcing, the relay or contactor contact does not stay open; motion cannot be stopped.
- Wire break: when a normally closed contact is used, a break produces a stop; a normally open arrangement hides the fault.
- Coil failure: the contactor does not pull in; this is a fault in the safe direction.
The cost trade-off is real: two channels, two contactors and a monitoring relay make the panel bigger. However, adding an auxiliary contact monitor that will detect welding is a low-cost, high-return step in most installations.
The context in which this term is used: Wireless Crane Remote Control Selection Guide