GROUNDING PRINCIPLES, SHIELDED VS. UNSHIELDED CABLES

With any electrical device, proper grounding is essential, both for safety and for reliable operation. The manufacturers of computer devices have already taken care of most grounding considerations during the design and manufacturing stages. However a few very important tasks must be left in the hands of the user.

AC GROUNDING

Electrical equipment which is not properly grounded can give hazardous shocks under certain circumstances. Modern electrical equipment is much better designed in this respect than equipment from several decades ago. The primary reason for this is the standardization of the use of 3-pin AC cables and outlets. At one time, only two pins were used for all AC wiring, and since many buildings still exist where the AC sockets have not been upgraded to the current 3-pin standard, it can be both annoying and unsafe to use modern 3-pin cabled devices when only 2-pin sockets are available. Not all current devices are sold with 3-pin cables. Those that are not are generally considered safe except in wet environments, since they use alternative grounding techniques suitable for limited situations. If a device is sold equipped with a 3-pin AC cable, it must be assumed that the 3rd pin is required for safe operation. If a so-called "3-pin adapter" is used, the unattached wire coming out of it must be attached to a proper "absolute" ground. No general rules for locating and connecting to an absolute ground can be stated here; an electrician should be consulted in such cases. The 3rd pin of such a device must never be sawed off or removed, nor should a 3-pin adapter be used unless the ground wire is properly connected to an absolute ground.

Besides protecting the safety of the user, this 3rd pin has important functions for the device itself. In some cases the device will not operate correctly unless it is grounded. In some equipment it is necessary to protect the device from being damaged by other equipment to which it may be connected. This is particularly true for computer equipment. When two pieces of equipment are connected to each other, and each of the devices has its own 3-pin AC cord, potentially serious problems can occur unless the two devices have identical AC grounds. The way to ensure that they do is to make sure that both devices are plugged into the same AC circuit. The clearest way to do this is to plug them both into the same outlet box. If the outlet box does not have enough sockets for all the devices being connected together, a 3-pin "UL approved" extension box should be used, rather than running extension cords to other sockets in the room. Be certain that the total AC requirements of the devices do not exceed the current ratings for the extension box or the wall circuit. It is recommended that the extension box have its own fuse or circuit breaker, in addition to the building's protection for the wall circuit. If any of the devices have 2-pin AC cords, the manufacturer should be consulted before connecting the devices to other devices with interface cables. One exception to this should be noted, however. Devices which have "battery-eliminator" type AC cords where a transformer is mounted in the AC plug can generally be connected as is without problems. These are commonly found only on small modems.

SIGNAL GROUNDING

In order to ensure reliable operation in a world where electrical devices are everywhere, the circuits of sensitive devices must be shielded against outside interference. Radio Frequency (RF) waves that travel through the air are intentionally and unintentionally generated by a wide variety of electrical devices. These include computers, radios and televisions, flourescent lights, microwave ovens, photocopiers, elevators, electric motors, and many others. RF Interference occurs when RF signals unintentionally generated by one device are unintentionally received by another device in such a way that they interfere with the correct operation of the receiving device.

The most common way to reduce a device's sensitivity to external RFI is to shield it with a conducting material which has been electrically grounded. Manufacturers shield the circuits inside their equipment during manufacturing, but external cables that connect computer devices together must be provided by the user. These cables should therefore be shielded to reduce their sensitivity to RFI. In an environment where significant amounts of RFI are present, it will be essential to use ground shielded cables to avoid interference. In addition to protecting the device itself, properly shielded cables also prevent computers from interfering with other sensitive devices such as TV sets and radios. In fact, government FCC regulations have been established to set limits in residential locations. No matter how carefully a manufacturer designs equipment, if it is used with unshielded cables, it can emit unacceptable amounts of RFI.

To be considered fully shielded, all wires in a cable must be encircled by a conducting shield, usually foil or braided wire, which is connected to the metal housing shield of the plugs at each end of the cable. Note that many computer plugs on the market today are not designed for shielded use, since they are not made of metal or constructed with integral metal shields. In order to make a fully shielded cable it is necessary to use shielded plugs.



FULL SHIELDING

The most desirable form of shielding is full shielding. This provides the most protection against RF interference, both to and from the devices. However, this form of shielding cannot be used unless the following conditions are met:

. Both devices must have 3-prong AC cords and plugs, and be plugged into properly grounded   three-prong outlets. . Both devices must be plugged into the same AC circuit box.

To implement full shielding, use cable that has an outer conductive shield enclosing all the other conductors. You should also use shielded cable plugs that are made of metal, or contain integral metal shielding. Shielded plugs were not commonly used until recently. Do not assume a plug is shielded unless the packaging specifically states that it is.

PARTIAL SHIELDING

This method provides some control of RFI, though not as much as full shielding. This form of shielding may be used if the conditions listed above for full shielding cannot be met. To implement this form of shielding, again use shielded cable, but with any type of plugs. The shield is connected to the ground pin at only ONE end of the cable.

NO SHIELDING

Used when no ground shielding is supported by the device manufacturers. Shielded cable and plugs, which are more expensive, need not be used. If they are, it is important that no connection be made between the shield and the #1 pins or the plug shields. Although such connections will not cause problems in most cases, they can cause operating or safety problems for a few devices on the market.

To minimize RFI, keep these cable lengths as short as possible, and place the cables as far away from radios and TVs as practical. It is not as likely that RFI will cause problems for the computer devices as it is likely that the computer equipment will interfere with any radios or TVs nearby.


SIGNAL GROUND

The grounding principles described above refer to the safety and shielding aspects of grounds. In addition to these, grounds have one other function, that of providing an electrical reference point for the interface signal lines. In serial interfaces (25-pin connectors), this function is usually provided by the #7 pins. Although these pins are often referred to as "signal grounds," they are quite different from the #1 pin grounds discussed in the sections above.