I have heard a lot recently about ground wires and bridges, but I don’t understand why a bridge needs a wire attached to it. This is probably really obvious, I apologise in advance.
It's mainly so that when you're playing, you can ground your body via the bridge and strings to stop it acting like an aerial for RFI. It also means that your body is now capturing and grounding a lot of the RFI that would pass through you and get to the guitar pickups.
If you've ever noticed that a lot of guitar hum stops when you've touched the strings, then that's the reason why. With the strings ungrounded, you can't ground your body properly, so you can't reduce the hum.
The downside of grounding the bridge is that it does increase the risk of electrocution if you are both playing and singing into a mic (wired, not wireless) and there's a big electrical fault in the PA system putting mains voltage onto the mic. The circuit to ground with you in the middle is obviously more conductive (and dangerous) with a good ground path than a bad one. Modern earth leakage circuit breakers (if fitted) should stop power before any lethal currents are applied but if you are playing in a venue with unknown electrics, then it's always best to use one of your own.
For this reason, EMG advise against grounding the bridge when using their active pickups as they are so noise-free that grounding the bridge makes no difference, so you are better off having the extra safety benefit of not having your body well grounded.
Since the guitar gets noisy without a ground and 99,99% of guitars have a ground wire (even my passive EMGs) this is a pretty good advertisement for cordless mics!
PAs have become a lot safer. The early packaged ones were the most dangerous, being valve powered with some really nasty voltages inside. One loose internal wire and anything could have several hundred volts DC on it.
With active PA speakers and small mixers with external power supplies, there is very little that can go dangerously wrong these days. Also, venue electrics have normally been improved greatly since the '60s and '70s. Still, having some form of earth-current sensing circuit breaker is always a very good idea.
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