Electrical equipment is grounded for safety reasons. Grounding can also help prevent RF interference
First… let’s get some agreement on nomenclature… since I’m writing, I get to pick. [:neener:] There are many different ways to refer to “ground” and much depends on the context. As with everything else we measure, we need a reference point, and in electronics we call the reference point either “common” or “ground.”
A) Earth Ground: this refers to the fact that your house wiring is grounded by the Earth (the planet we live on) and we use and we use the terms “earth” and “ground” interchangeably. Your house wiring is grounded by attaching a large gauge wire from the Neutral wire at your breaker panel to a buried water pipe or a long stake driven into the ground. If your house were not grounded the voltage on the neutral conductor of your house could be very high compared to the piece of ground you are standing on. Fortunately with transformers, the primary side of the transformer doesn’t necessarily care what “ground” is on the secondary side.
As shown in last weeks sketch, the neutral coming into your house is earth grounded, this “earth” is distributed around your house along with the “line” and “neutral” to the outlets around your house.
“So if the neutral is grounded at the breaker panel, why do I need another ground wire?” you ask. A very good question indeed.
Let’s take the chandelier above your dining room table. You have three wires that come to that round hole in your ceiling: L, N, and E (Line, Neutral, and Earth). In the US the Line is BLACK, the Neutral is WHITE and the Earth Ground is bare wire. Now you attach those three wires to corresponding wires in the chandelier. There is a two wire cable that passes through a metal tube or decorative chain that goes to the light bulbs. The ground wire is connected to the metal parts or can pass through the tube or chain of the fixture. (In the sketch I showed the Line and Neutral running along side the chain for clarity.)
Now let’s say the person that connected the light decided not to attach the ground wire, everything works and nobody knows any different. Now imagine that after time the insulator on the LINE connection wears through or has a small cut and is now in contact with the metal parts of the chandelier. If only the LINE connection is cut, then the light will still work, but the metal parts of the light are now “hot” (energized). Now if you touch the light fixture you can get zapped.
If the ground wire had been connected correctly, and the insulation on the LINE connection failed, the current would go through the ground wire back to the breaker panel and the circuit breaker would simply open, removing the voltage from that circuit, and no one would be shocked.
B) Chassis Ground: chassis ground plays the same role as the ground wire in the chandelier in the above example. A grounded chassis will have the Earth wire entering the chassis and then securely attached to the metal of the chassis. In case of insulation failure or component failure inside the chassis, the voltage on the chassis will not change; it will remain firmly “grounded.” If insulation failure occurs inside the chassis, the fault current will be routed to earth ground, and either a fuse or circuit breaker inside the chassis will open, or in extreme cases, the breaker at the house panel will open, eliminating the risk of electric shock.
WARNING: If you are thinking about using a “cheater” plug on your audio equipment to eliminate a ground loop, re-read the above paragraph, and then don’t do it, or fault current could be routed through YOU.
The chassis ground wire according to UL, CSA, and EU standards must be able to carry the full fault current without fusing (melting the wire). This is required since if the chassis ground wire fuses before the internal fuse or panel breaker opens, the chassis will remain “hot” and you can still be shocked.
Electrical equipment that does not utilize a three-prong AC cord does not have an earth grounded chassis. In this case UL, CSA, and EU standards require that hazardous live voltages must have double or reinforced insulation (two layers of adequate insulation). This is also referred to as Class II insulation. Transformers of Class II products also have to meet the insulation requirements. Parts that are not insulated must meet specific “creepage and clearance” requirements (which is to say there must be adequate space between conductive parts and hazardous live voltages – the air is the insulation).
Some equipment will also have a polarized (wide blade) AC plug. This ensures that the fuse or switch inside the equipment is always on the “hot” side of the AC line. If you defeat the polarized plug by replacing it or cutting it, it is possible to flip the plug over and you are putting the fuse on the neutral. Now if you have an insulation failure the chassis could still be “hot” even if the fuse blows!
WARNING: If you can’t plug in a polarized AC plug, get an electrician to change your obsolete AC outlet. Defeating the polarized plug can be very dangerous.
C) Signal Ground: This is the reference voltage for the audio (or video) signal in your system. Each component in your audio rack will have a signal ground. The signal ground connection must be “clean,” in other words, any voltage on this ground will be treated just like any other voltage on the signal line; it will be amplified and become part of the sound coming out of your speakers, or if in the video, can cause a noisy picture. Sometimes this is called “common” as in a “common emitter” amplifier.
D) Digital Ground: This is the ground reference for digital signals. As with analog signals, poor ground on digital signals will have an equally degrading effect. One common problem is “ground bounce” this occurs when there is noise on the digital ground and the digital circuit misbehaves because the voltage of the digital bits is no longer valid. Ground bounce can also cause many digital circuits to reset without explanation, as it would if there were a power loss or a “brownout” (a partial loss of voltage).
E) Power ground: The power supply in your audio equipment needs a return path too. We call this the “power ground,” and sometimes this is also called “common.” Electrical current needed to power the circuit components returns to the power source on the power ground.
Next Week: Ground Loops 101