FFXI Haven - Ваш Источник #1.

The term, ground, comes from the fact that most electrical systems find a local voltage level by placing some metal in the earth (ground). This is then connected to all of the electrical outlets in the building. If there is an electrical fault, the current will be drawn off to the ground. The term, common, refers to a reference voltage that components of a system will use as common zero voltage. Therefore the function of the ground is for safety, and the common is for voltage reference. Sometimes the common and ground are connected.

The most important reason for grounding is human safety. Electrical current run ning through the human body can have devastating effects, especially near the heart. Fig ure 31.7 shows some of the different current levels, and the probable physiological effects. The current is dependant upon the resistance of the body, and the contacts. A typical scenario is, a hand touches a high voltage source, and current travels through the body and out a foot to ground. If the person is wearing rubber gloves and boots, the resistance is high and very little current will flow. But, if the person has a sweaty hand (salty water is a good conductor), and is standing barefoot in a pool of water their resistance will be much lower. The voltages in the table are suggested as reasonable for a healthy adult in normal circumstances. But, during design, you should assume that no voltage is safe.

current in body (mA)

0-1 1-5 10-20 20-50 50-100 100-300 300+

effect

negligible (normal circumstances, 5VDC) uncomfortable (normal circumstances, 24VDC) possibility for harm (normal circumstances, 120VAC) muscles contract (normal circumstances, 220VAC) pain, fainting, physical injuries heart fibrillates burns, breathing stops, etc.

Figure 31.7 Current Levels

plc electrical -31.10

Aside: Step potential is another problem.

BACK | NEXT

Easy Access To All Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106