Shock Hazard and Grounding
by Jack Sondermeyer
The power supply cord used on most modern electronic equipment has a three pin plug. This article will explain why the separate ground pin is used and why shock hazards will result if the ground system is defeated.
The power distribution system used in the United States is 120 volts "alternating current" at 60 cycles. For many years this standard AC (mains) system was a two prong plug and socket combination wired with one side "hot" (usually the black wire at 120 volts) and the other side neutral (usually the white wire at 0 volts or ground). However, in the early days there were no true standards. Often there was no color code and sometimes no hot or neutral wires (in this case, both wires were hot). Unfortunately, this two wire system is still found in many older buildings throughout the country.
There has always been a certain amount of "respect" for the chance of getting shocked by the power system, but most people are not sure how or why. Let's review the system. The "hot" wire is at 120 volts and the other wire is neutral or ground. If a person were to touch the neutral wire only, no shock would result, simply because there is no voltage on it. If he were to touch the hot wire only, again nothing would happen to him unless some other part of his body were to become grounded. A person is considered to be grounded if he comes in contact with a water pipe, metal conduit, the neutral or ground wire, or stands barefoot on a concrete floor. A person is usually insulated from electrical ground by rubber or leather shoes. In other words, neither wire is a shock hazard unless a person is grounded, and then only the hot is a potential shock hazard. Of course, if a person were to touch both wires at the same time, he would be shocked simply because his body is completing connection between "hot" and "ground" wires.
As the use of electricity became universal, it became apparent that the existing system had some serious problems, and various "standards" agencies began to review power distribution products and practices. Out of these efforts came Underwriters Laboratories (UL) and many others which were created to help design a system which would reduce the risk of shock hazard. Ultimately, dramatic changes were instituted which brought about the three prong plug, among other improvements in our system. However, these changes have caused many problems to manufacturers of equipment which plugs into this power system. These problems are due primarily to the fact that many older buildings still have the old two wire systems and two prong wall sockets. In order to use modern equipment in these older buildings, some people simply "break off" the ground prong from the plug. Although "ground adapters" are readily available, a few users simply "can't be bothered." Also, newer three prong sockets are often wired (or mis-wired) into older two wire systems with horrible results including shock hazards (which can cause destruction of electronic equipment) or, in the worst case, personal injury or a fire. Unfortunately the integrity of such installations is rarely questioned and the user often doesn't discover problems until it's too late.
Back in the early days, equipment and appliances fitted with the two wire power plug were readily accepted to be safe from shock hazard, because the metal housing was not connected to either wire of the line cord. In other words, the metal case is said to be "floating." Appliances such as toasters and irons are still supplied this way. Why not connect the neutral wire to the metal frame of such an appliance? This might seem like a good idea until you consider that the two-pronged plug or the receptacle might not be polarized. Now, you have a 50/50 chance of plugging the appliance in backwards, putting 120 volts on the metal frame of the appliance. A polarized plug has a larger prong on the neutral side so that it can only be plugged in one way. Many of today's appliances do have polarized plugs which will be discussed later in this article.
Whenever audio equipment is operated without a ground (floating chassis), strange things can happen. Under certain conditions the amplifier will be more susceptible to radio frequency interference (picking up radio stations or CB. radio). Also, without a suitable ground, amplifiers sometimes "hum" more when the musician picks up his instrument and provides a "pseudo" ground through himself. Both of these problems are, of course, very annoying. The only solution is to find a ground point to connect to the chassis, such as a water pipe. Sometimes this may just cause more problems, when what appears to be ground turns out not to be!
One of the problems with appliances and equipment which have a "floating metal case" is that a shock hazard exists if the case comes into contact with the hot wire. This so called "fault condition" may happen in many ways with some of the more common causes being a "pinched" line cord, failure of installation systems, or movement of components due to shock or vibration which will cause the "hot wire" terminal to touch the case. Naturally, if for any reason the case does become "live," then a person touching it may be shocked if he is grounded. If this "hot chassis" is connected to another chassis or instrument by a typical shielded cord, then that chassis or instrument will become hot also. The entire purpose of the present three wire system is to provide a separate ground path which will effectively eliminate any possibility of shock.
Today's modern (US.) mains cable consists of three separate wires: black, white, and green. The green wire is always connected to the large ground pin on the plug, and the other (green) end connected to the chassis of the equipment. The black wire is always considered to be the "hot wire," and as such, is always the leg which is connected to the switch and fuse. The white wire is always the neutral or common wire. The neutral wire is sometimes also wired to the power switch assembly but . . . it is rarely fused. . .
Vital Safety Note!!! This applies only to U. S. A. products. Other nations have different color coding.
The integrity of the separate ground path is directly related to the quality of the chassis/green wire/ground pin combination. When the ground pin is removed, the separate ground path is destroyed and then fault conditions may cause shock hazards. Any modification of the 3 wire mains system completely eliminates the protection given by the three wire configuration. The integrity of the separate ground path is also directly related to the quality of the receptacle and the wiring system in the building itself. Today's three wire 120 volt receptacle has some very important features which should be understood. First, the ground pin socket hole always has a green-colored screw for the ground wire attachment, and nothing but the "ground wire" should be attached to this. A correctly installed receptacle should always be vertical with the ground pin beneath the two parallel blade slots. In this position the right slot has a brass or copper screw for "hot wire" attachment, and the left slot has a silver or chrome screw for "neutral" wire attachment. The left slot is also larger than the right slot. This is the polarizing feature previously mentioned. Many television and stereo components, as well as appliances, are being fitted with polarized plugs which have a wider spade for the neutral line cord wire. Since the receptacle has a larger neutral slot, the manufacturer can connect the neutral side of the line cord to the chassis and be assured that the 50/50 chance of plugging the line cord in backwards is eliminated. . . that is if the receptacle is wired correctly!! The standard wiring for the power receptacle is as follows: The black (hot ) wire goes to the brass or copper screw which is connected to the right (smaller) slot. The white (neutral) wire goes to the silver or chrome screw which is connected to the left (larger) slot. The bare wire(ground) goes to the green screw for the separate ground path. The wiring system for the receptacle must be three separate wires (black, white, and bare or green, usually found in 2 wire + ground Romex and ~X) and must be connected correctly to the respective "hot," "neutral," and "ground" connections at the power source.
These connections should always be made by a qualified electrician! When the three wire line cord is plugged into a correctly wired receptacle, the wires in the line cord are: black-hot, white-neutral, and green-ground. The key word here is "correctly" wired. The whole system is no good if the receptacle is not wired according to accepted wiring standards of codes. The other key point is the use of a third, separate wire for ground. Often, older two-wire systems are "updated" to use the three wire receptacles. Since there is no third bare ground wire in the system and might be very costly to provide, many "electricians" simply connect the white wire to the neutral and ground connections at the receptacle. This is marginally better than no ground at all, but again defeats the purpose of the three wire system, which is to have a separate wire to ground for shock hazard protection. Also, at times the green "ground" screw is left open (unwired), which is the same as breaking off the ground pin on the line cord. Even in most older buildings, one of the wires in a two wire system is "cold," but since the receptacles are not polarized the plug can be inserted either way. If you have a piece of equipment with one side of the line cord connected to the chassis, you have a shock hazard if the plug is inserted backwards. The usual "solution" to this dilemma is the "bypass capacitor" which is usually wired between the white (neutral) wire and the equipment chassis ground. A capacitor is a device which offers a relatively low impedance to high frequencies (such as generated by radio stations and CB. radios) thereby providing them with a "short path" to ground in order to eliminate this type of interference.
On the other hand, a capacitor offers a high impedance (AC Resistance) to low frequencies. Since, in the US., the frequency of the 120 volt supply is 60 hertz (a relatively low value) the capacitor will not offer much of a signal path for this frequency. Thus, the bypass capacitor will minimize the shock hazard problem. To understand this, let's assume we are using equipment with a three prong plug and a three wire line cord on an older two wire system with a suitable ground adapter that is not grounded. Plugging into this two wire system, the 50~50 chance could result in the line cord white wire being "neutral." In this case the bypass capacitor, which is connected to that white wire inside the equipment, will help minimize outside interference due to the grounding action of the neutral wire. Now let's take the opposite (and worst) case . . . the line cord white wire is "hot." In this case the bypass capacitor will be connected to 120 vac. . . and the chassis will be "hot". . .but because of the capacitor characteristic, the current flow will be very small. There will be enough current to be felt, but not enough to cause bodily injury. If the user determines that he has a hot chassis with a system connected like this, the solution is to simply reverse the plug (if he can). A more modern solution to this problem is the use of an on-off-on switch like that found on many Peavey amplifiers. This switch flips the bypass capacitor from one side of the line to the other, so the user can keep the bypass capacitor on the neutral leg. A newer version of this is the three position ground reversal/lift switch found on many of the newer Peavey amplifiers. This switch not only reverses the polarity of the bypass capacitor, but the center position removes the capacitor from the circuit completely for use in most situations where the three wire power system is intact . . . In this case its presence may actually cause more hum and noise than if it were not there. This 3 way switch is a convenient way to remove it.
In order to assure that the bypass capacitor is the correct value and will protect the user from a serious shock hazard, a few audio manufacturers (including Peavey) perform what is referred to as a "high-potential" (hi-pot) test. During this individual product test, the integrity of the third wire ground is tested and then a very high voltage (1500 Volts) is applied between both line cord wires and the third wire ground. This test is mandatory to pass the requirements of CSA and other testing and approval agencies mentioned earlier and must be performed on each and every Peavey unit before it is shipped. This test also checks for faulty insulation and pinched wires inside the amplifier. If the equipment has passed this test, the user can usually be confident that the manufacturer has done all that can be done to assure a safe and shock-free product. It is then up to the user to operate the equipment safely and rely on the integrity of the power (mains) system being used. Fortunately, there are new devices available which can effectively check the integrity of the power plug. Peavey offers such a device called a 'ground monitor" in our Accessory Program. We highly recommend its use each time a musician plugs into a strange power plug.
The following is a list of problem areas which should be avoided with suggestions to prevent a serious shock hazard:
1. Never use two wire "extension" cords.
2. Never use extension cords with non-polarized plugs or ones with broken off ground pins.
3. Never break off the ground pin on electric equipment.
4. If necessary, always use a suitable ground adapter... and if possible, ground that extra wire on the ground adapter.
5. If no ground exists... find one... but make sure it is ground.
6. Always check the integrity of a "strange" power plug with a ground monitor device... if it checks "bad" don't plug into it!!!
7. Always use a qualified electrician to do all your "wiring."