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."
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