how+guard+rings+help+in+esd
usually for ESD device, you need guard ring, and some times even double guardring for two reason:
1. guard ring with enough contact and metal connection can help distribute ESD zapping current, sometimes current flows more through the guardring than from the substrate.
2. isolate the noise introduced by the diode under normal operation, since IO signal may easily have negative glitch, and will introduce noise into substrate without guardring the collect the carrier current.
but the distance of double guardring need to be controlled well since it can easily form a collateral npn transistor. and without enough conatct or strong metal power bus on top of metal ring, it will be less effective.