Due to the way the low voltage limit switch wires are run in the same cable as the 115VAC motor control for 3-4feet (~1metre) even a PLC input would be hard pressed to withstand that abuse for an amount of time. You end up spending more time and resources trying to cope with a problem that really just needs to be re-designed to avoid the problem in the first place. The circuit as is needs to be able to withstand a significant transient happening regularly - not the occasional once off. The poor transzorb has a hard time and it needs to have some circuitry to 'soften' the transients presented to it otherwise it will fail. As I've mentioned previously, run the limit switch circuit at 115VAC and either use a relay or design the input circuit to expect 115VAC signals.