PCB Copper Layer Spacing & Voltage Rating
One standard used for spacing conductors based on voltage is the IPC-2221. This standard differentiates spacing requirements for inner and outer conductors. Does the IPC minimum separation between conductors on an inner copper layer apply to the minimum dielectric thickness between copper layers? In other words, when sizing the dielectric thickness between a supply and return plane does the IPC standard for minimum distance between conductors on an inner layer apply? If it does not apply, what differentiates the two scenarios (because the same dielectric material is between the conductors)? If it does apply, why is the IPC requirement so severe compared to the dielectric voltage ratings provided by the manufacturers?