I am wondering about ground cuts and signal breaks. If I have traces on the top layer with the 12V plane under it and a GND plane on the bottom, would a cut in gnd still effect a trace that goes over the cut? I was thinking that the 12V plane would block it, but am I wrong?

 

It depends upon the frequency/rise time of the signal on any trace transversing the cut. For low frequency signals there likely will be no problem.

 

One of the amazing facts that I picked up on this site is that high frequency circuit traces interact locally with the ground plane. As crutschow said, it all depends on what frequency. High enough frequency and the return current will have to find a more convoluted path around the cut(s) than it would normally prefer. Of course, this will have some degrading effect on the rise time of digital signals.

 

Not only can this ground cut degrade the rise-time of fast digital signals, but the return signal is now forced to take a different path which could affect sensitive signals in other areas of the board.

 

What is the definition of low frequency?

 

What is the definition of low frequency?

Not high frequency.

More seriously, in this case, it's where the trace distance is less than about 1/10th wavelength of the frequency (at the propagation signal delay of the trace on the circuit board).