The concept is this:
If anything in the system is not working properly then no parts will be moved to the “good” box and shipped to the customer.
On the eDART this means that you assign the “Sorting Output” to mean “Good”. This is a relay (OR2-D) output which is like a switch. When it is ON at the end of the cycle (contacts closed) then the sorting equipment moves the part to a “Good” location (down a conveyor, into a box and so-on).
Thus, in order to ship a good part the eDART must be running, have the alarms set (set correctly), the wires connected to the part sorting equipment and so on. This is the eDART’s part of “Fail-Safe”
There is also a mechanical component to fail-safe sorting. For example, the
customer must find ways to make sure parts can’t get hung up from one shot and the accidentally drop into the box on the next shot. And they have to hook up power to their part sorting equipment in such a way that if it fails (e.g. flipper chute loses its air power) the parts will not get shipped.
You can do fail-safe sorting without doing 3-way sorting. Some customers have gotten the idea that if they don’t adopt the 3-way sorting approach they cannot do fail-safe. Just using the “Good” output is fail-safe all by itself. Adding “Reject” makes it 3-way.
If you use the “Reject” output from the eDART and do not use the “Good” output then you are not doing fail-safe sorting.