Mitigating noise pickup from contactor turn off

Question

I am working on an industrial control system that uses a microprocessor to control a bank of 6 contactors. The contactors have 120V coils. When all of the contactors turn OFF at the same time, they generate a huge burst of noise which occasionally gets picked up at some GPIO inputs to the MCU. The inputs are normally high and are set to generate negative edge interrupts.

The inputs are driven by LM339 comparators and it looks like the noise is getting induced into the net that connects the comparator output to the MCU input. I say it looks like this because lifting the pins on the comparator and MCU and running the net on a twisted pair gets rid of the false triggers. Also, there are several comparator to MCU connections and the longer nets have more noise pickup issues. The longest net is about 6" long and the PCB is about 2 feet from the contactors, but about 6" from the mains input. Adding suppressors to the contactor coils reduced but did not eliminate the noise issues.

While I could just use RC filtering or firmware filtering, I would like to reduce the noise pickup as much as possible first.

My question is what are the best practices for PCB routing that would be similar to a twisted pair as far as reducing pickup from the noise generated when the contactors turn off?

Answer 1

If you have four layers (or more) route the trace between ground planes and stitch the planes with vias around the traces.

If you only have two layers, add ground traces close to the trace to create same effect as the twisted pair. Though I think that doesn't help much, and the best bet is to add a shield above the traces.