[sdiy] AC Grounding

[klosmon]

Speaking of stupid questions, I have one...

Over the last 20 years, I've been dealing mostly with DC circuitry -- what 
happens after the step-down AC gets rectified & regulated (like,  for 
example, a synth power supply).

However, I was recently helping some friends finalize the installation of 
an intercom system in their beautiful Telegraph Hill home (near where the 
wild parrots hang out), and noticed that the instructions for the intercom 
amp specified that the two secondary legs from the 18v transformer should 
connect to terminals on said amp, and that one of the terminals should then 
be connected to earth ground.

I've never seen an AC leg going straight to ground before -- could someone 
who understands this explain it, so that even someone like me could understand?

Thanks.

~GMM