BK Precision 4011A Betriebsanleitung PDF herunterladen (Seite 48)

Appendix B: Equipment and Instrument Circuits

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wire is colored white. The load is contained in a metal case. Now, consider what could happen if

the ungrounded black wire came loose and made contact with the metal case. It then becomes

possible that a person in contact with the ground, by contact with a metal pipe or wet concrete

floor, could complete an electrical circuit, being electrocuted in the process. To prevent this from

occurring, we use a third wire, often colored green, that is connected directly to ground. This is

the green wire we see on some three-to-two prong adapters. By connecting this green wire to the

metal case and to the building ground, we force the circuit breaker to blow if the black wire

should make contact with the case, completing the circuit through the wires, instead of through

people. Grounding equipment enclosures through the use of the ground wire is an important safe-

guard when working with high voltages.

Figure B.2: Possible electrocution scenario.

Grounding and Electrical Noise

Another function of proper grounding is to eliminate or control electrical “noise”. Noise in elec-

tric systems arises from many sources, such as thermal electron movement in hot cathodes, cur-

rents induced by nearby conductors, and high-frequency electromagnetic emissions from radio or

television transmitters. For most of the laboratory exercises, operation at levels of 10 or 20 volts,

and at frequencies from DC to a few hundred Hertz offer few noise problems; however, as volt-

age or current levels are decreased, and as frequency levels are increased, extraneous signals may

interfere with the desired signals and must be eliminated or reduced. This is done by grounding

all items of equipment with a chassis ground and by using a coaxial conductor between equip-

ment, as well as by eliminating the source of noise when practical.

The coaxial conductor is one in which the active or “hot” lead is enclosed within an outer metal-

lic shield conductor that is separated by a layer of insulating material. Another layer of insulation

usually surrounds the shield. The outer shield conductor is usually connected to ground. The

standard laboratory connection between equipment and coaxial conductors is the “BNC” connec-

tor, a twist-lock connector that is easy to use and makes a good connection. Reasonable care

must be taken to avoid breaking the connection between the BNC connector and the wires of the

active conductor or the ground shield in what is commonly called a “coax cable”.

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BK Precision 4011A Betriebsanleitung Seite 48