Issues with designing ground plane for switching relays in high noise environment
I once assisted a consultant who had ICs burning out, because his PCB was located only 40 milliMeters from a 5,000 volt power buss that switched 2,000 amps in 1 microsecond.

The upset in the Ground plane was computed, assuming 4" by 4" loops in the Ground, as

V = [MU0*MUr*Area/(2*pi*Distance)] * dI/dT

which simplifies to 

Vinduce = 2e-7 * Area/Distance * dI/dT

and inserting numbers, we have

Vinduce = [2e-7 * 0.1meter*0.1meter / 0.04 meters] * 2e+9 amp/sec

Vinduce (that is, voltage induced into loops in the Ground plane)
= (2e-7 * 1/4) * 2e+9

Vinduce (from this  10MegaWatt bus) is

100 volts

So you need to identify your rate-of-change of currents, and compute how much GROUND UPSET can be generated.

Oh.....how much credibility does this math have? 100 volts around 4"by4" loop. I had the "consultant" make a 1"by1" loop, attached to coax cable; note this is 1/16 of the area I used in the math, so we could expect 100/16 = 7 volts.

What did the consultant measure? 3 volts, or 4 volts in some locations on the PCB. Versus 7 volts predicted. For a 2,000 amp flat buss (air cooled, I recall) located 4cm away.