MTTFd (Mean Time To Dangerous Failure)
It is the average time until a channel fails in the dangerous direction, and it is expressed in years. It is one of the three inputs to the performance level calculation; it is divided into three classes — low, medium and high — and is evaluated per channel in the calculation.
The critical point is that not every failure counts as dangerous. A broken contactor coil stops the machine, that is, it is a failure in the safe direction. A welded contact, on the other hand, prevents stopping and counts as being in the dangerous direction. MTTFd looks only at the second group.
- Classification: from 3 years to 10 years is grouped as low, from 10 years to 30 years as medium, and from 30 years to 100 years as high; 100 years is taken as the upper limit in the calculation.
- For mechanical components the value is most often derived from the number of switching operations; as the number of daily cycles rises, the equivalent in years falls quickly.
- The channel value is found from the contribution of all components in the chain, and a single weak component pulls the result down.
On a radio remote controlled crane this means that the frequency of starting and stopping enters the safety calculation directly. In an application with frequent jogging, choose the contactor by expected electrical life rather than by current rating.
The context in which this term is used: Wireless Crane Remote Control Selection Guide