Modification of Pull Rope Switches in a Cement Plant

The simplest way to do this would be to break-out the auxiliary contacts on each of the safety relays onto individual inputs to the control system. Once that has been done, the rest is configuration programming.

No downtime, with a bit of care.

We had similar issues. We even used the spare contacts to turn on little light on top or near the base of the pull cord. It was very unsatisfactory. The problem was that each set of contacts are independent. That frequently resulted in the safety circuit being tripped but the indicator set of contacts needed a little more travel to work.

Now, we always know exactly which one is tripped. Each safety device has its own input channel. There no "shared" conductors, and the system meets all the new safety standards.

If the parallel sets of contacts don't cause you problems, then the light bulbs will burn out without letting you know about it. You will still be in that sinking boat.

Not sure which DCS or CGS system you are using however most of the newer programming software packages have "first out sequencing" function blocks for this very reason.

Because the processor program is scanned in sequence on a set timing interval the logic is fast enough to detect the time difference in cascaded input events.

If you are working with ladder logic and there is not a "first out sequencing" function block available you can simple create a ladder with the first switch in the physical first position electrically in the first executable rung, then the second switch in the physical second position electrically in the second executable rung, and so on. Once this routine is created and proved, simply assign it as a subroutine that executes continously with a "call" instruction and label with appropriate name.

The logic feedback circuit to accomplish this requires an input for each and every switch being monitored installed at/on the downstream side of the contact and returning to the I/O as a digital input.

Be sure to assign the switch identification tags in the program to match the physical location in the electrical circuit.

Once you set up the "first out sequencing" table, you can create the HMI graphics on screen with the switch identity and location for ease of troubleshooting.

If you need help feel free to contact me via email at: james_morris@fmi.com