Our friends at Fender share some info on the importance of grounding your gear:
Want to know what one of the most important safety features of your amplifier is?
More specifically, the ground prong of a standard three-prong U.S. power plug. The three-prong plug of a Fender guitar or bass amplifier—of any amplifier—that connects it to ordinary household AC current is an unassuming but key safety feature. It is a simple but crucial part that stands between you and, at the very least, a brief but unpleasant electrical shock and, at the very worst, oblivion.
The three-pronged plug is so important because it provides a protective measure called an “earth ground” electrical connection. An earth ground connection is important because in the undesirable event of electrical leakage current, it ensures that this potentially harmful current is either safely interrupted (by blowing a fuse) or safely sent through something other than you (the earth, usually).“Leakage current” is current that has “leaked” out of its intended circuit because of improper device operation, building wiring problems, faulty components, etc., and is flowing through some other path as it attempts to return to earth ground. That “other path” is undesirable because it can cause damage, signal noise, fire or electrocution.
Proper grounding is absolutely essential. Most musicians have been zapped at some point by improperly grounded or faulty gear and lived to tell the tale, but it’s worth noting that there are approximately 500 deaths by accidental electrocution in the United States each year.
Read that part again. Improperly grounded music gear can’t just hurt you—it can kill you. Very small amounts of voltage or, more accurately, very small amounts of electrical current can prove fatal if it passes directly through your heart or your head. Stone the Crows guitarist Les Harvey was killed onstage in 1972 at a Swansea, England, nightclub at age 27, electrocuted by touching a microphone with wet hands and having a lethal jolt sent through his body by ungrounded or malfunctioning gear. Longtime Shadows bassist John Rostill was found dead in his home recording studio in November 1973 at age 31, the victim of electrocution. Yardbirds singer and harmonica player Keith Relf died in his home at age 33 in May 1976, electrocuted by an improperly grounded electric guitar amp.
Kiss guitarist Ace Frehley was nearly electrocuted at a Dec. 12, 1976, concert at the Lakeland Civic Center in Lakeland, Fla. During the opening song, he touched an energized but ungrounded metal staircase railing; the resulting shock knocked him to the ground. The show was delayed for half an hour, and Frehley reported losing feeling in his hand for the rest of the show (Frehley subsequently wrote a song inspired by the incident, “Shock Me,” which appeared on 1977 Kiss album Love Gun).
On standard U.S. three-prong electrical plugs, the rounded U-shaped prong below the two flat parallel blade prongs (the “hot” and the “neutral”) is called the “earth ground” prong. It’s slightly longer than the hot and neutral prongs so that the device is grounded before the power is connected, and vice-versa.
The whole purpose of the grounding system is to protect people who use metal-encased appliances from electric shock should leakage current develop or should the metal case become “electrically hot.” This is accomplished by connecting the ground prong directly to the metal casing. If, for example, there’s a loose power wire inside an ungrounded metal case (like a guitar amp chassis) and the wire touches the metal case, the metal case will become energized or “hot,” and anyone who touches it will receive a potentially fatal shock. The electrical current tries to find the quickest path to ground, which may or may not be right be through you.
If the case is grounded, however, the current from the hot wire will flow straight to ground (and not through you), tripping the fuse or circuit breaker. The downside is that the device will then cease to work. The upside is that you won’t be dead.
For that exact reason, inside all Fender amps, the ground prong is wired to the chassis. It is also for that exact reason that never under any circumstances should you use an adaptor to plug a three-prong electric power cord into a two-prong wall outlet; nor should you cut the ground pin off a plug to make it fit a two-prong wall outlet. The protective earth ground system must be properly connected.
If you take either of these ill-advised steps, the device will still operate, but you will have disabled a crucial safety feature that protects you from electric shock. So just don’t do it. Never. Ever. Don’t do it even if it silences a noisy ground loop, because there are other safe means of eliminating hum caused by ground loops.
However, even if your gear is grounded, you’re still not necessarily safe from dangers posed by improper grounding conditions elsewhere in the immediate environment. Many musicians have experienced “getting zapped” by a vocal mic in an improperly grounded or faulty PA system, for example. What’s going on there is that you’re touching a grounded metal part of your guitar and your mouth touches the microphone used in a PA system that has leakage current somewhere else; your lips complete a circuit when your mouth touches the mic, and you get shocked. Leakage current in high-voltage systems can be fatal.
A simple safety precaution that can be taken, other than making sure you use three-prong plugs and receptacles, is to buy a simple and inexpensive circuit tester, which is available from any hardware store or DIY center. Used to verify proper and safe wiring in AC outlets, it’s a small device that looks like the plug end of a power cord, with a series of lights on it rather than an attached cord. You simply plug it into the outlet, and the lights tell you whether the outlet works right and whether it’s safe or not.
When in doubt, however, have a qualified electrician inspect the building power and earth ground system, or have your music gear checked out at an authorized service center.