A good electrical ground serves a couple of purposes. As I pointed out earlier, all of the shields in your audio cables are connected to ground. As the name implies, it “shields” the ‘hot’ conductor (in the case of unbalanced) or + and – conductors (in a balanced cable) from external electrical noise and shorts that noise to ground. As long as the ground connection is a good one, and the current flow is not excessive, the noise goes away, and no problems are caused.
There is one aspect of grounding which you MUST take into account, and personally check. This is the ground integrity of everything on stage. Here’s why: Let’s say that there is a guitar player who defeats the ground on his guitar amplifier and manages to plug the AC cord in so that his chassis is connected to the “hot” side of the AC line instead of the neutral side. Now, let’s say that he also sings, so you place a mic on stage in front of him, and your mic is properly grounded through its mic cable. So, if the guitar player touches his guitar strings, which are at the same electrical ‘potential’ as the chassis of his amp (which is SUPPOSED to be at ground potential, but which in this care, is NOT!), his body is also at the same potential as his amp chassis (i.e.-‘hot’). If he touches the mic at the same time, with either his hand or his lips, he can receive a fatal shock, or at the very least feel like he has been bitten on the lip by an angry badger. Even if it’s not fatal, it’s not very conducive to his continuing to put on a good show, as he will spend several minutes trying to breathe.
This is very serious stuff. There have been guitars players KILLED by this, and thousands of others shocked. Check out all of the amps that the guitar players will use. If you see any vintage amps, make sure that they have a 3-prong AC plug. In all cases, I strongly suggest taking your AC Voltmeter and checking every guitar amp. Plug in a guitar and, with your meter, measure from the guitar strings (or bridge) to something you know has a good ground (like the metal casing of a mic, or the metal case of an XLR connector, or, if you are close enough, a chassis ground lug on something in one of the PA or monitor racks. It may take a few minutes, but it’s an important issue. Safety is about the only thing I consider more important than sound. I doubt that sound quality is very high on the priority list of someone who is riding in an ambulance to the hospital after getting shocked. And since few musicians have health insurance, it can be a costly experience on top of everything else.
Just remember: BE SAFE!! MAKE IT SAFE FOR OTHERS!!!