OkiePC
Lifetime Supporting Member
JesperMP said:Forgive me for being a little bit anal here.
But thats the wrong medicine you are taking !
You experienced a local shortcircuit phase-to-phase, and your shortcircuit protection failed to contain it to the locality where it occurred.
Adding mechanical interlocks will not remedy that for the next time it happens. Mechanical interlocks can help but not guarantee against shortcircuits.
In stead, you should check your overload/shortcircuit protection system with regards to selectivity and shortcircuit breaking capacity.
We checked and had adequate local short circuit protection. I've seen a few other cases where, for unknown reasons, the fuses blow in a bus duct even though lower rated fuses in a local panel may or may not survive. Our plant already had a standard for reversing motor starter wiring, and after the incident I described it was reinforced.
Lancie's logic looks fine. If both start buttons (FWD & REV) are pressed at exactly the same time, the forward command will latch, and the reverse will be inhibited. I can't see any way to reverese directions without resetting the timer, which will provide the alarm and reversing delay required. I would recommend separate timers. What if the plant decides it wants a five second startup alarm and a 12 second reverse delay? Just my opinion...