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Audel electrical course for apprentices and journeymen - Rosenberg P.

Rosenberg P. Audel electrical course for apprentices and journeymen - Wiley & sons , 2004. - 424 p.
ISBN: 0-764-54200-1
Download (direct link): audelelectricalcourseforapprentices2004.pdf
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The results of additional depths of driving the electrode may be seen in Figure 34-6. It becomes evident from the graph that any increase in length beyond 10 ft doesn’t have much effect in reducing the resistance.
1601----n---------------------------------------------------------------
140
0 1 2 3 4 5 6 7 8 9 10 11 12
DEPTH OF ROD IN FEET -------------â–ş
Figure 34-6 Effect of depth on electrode resistance.
There are methods of reducing the resistance of the earth around a made electrode. They are not recommended as a general practice, as they are not permanent, and it is far better to go the permanent route in securing good grounds, unless the earth is under a controlled system of periodic inspections. Figure 34-7 shows a typical layout.
Of the three methods of reducing the grounding electrode resistance discussed above, the driving of additional rods is preferred.
Grounding and Ground Testing 361
Figure 34-7 Chemical application to ground electrodes.
The NEC states that connections to encased rods or rebar shall be made by metal fusing if the connection is encased. Another common expression to indicate metal fusing is exothermic welding. In this, metal fusing the iron and copper conductor shall be clean and dry to ensure proper results.
To check a metal-fused connection, the following proves very satisfactory: A source of DC, such as from a DC welding machine, is applied across the weld. See Figure 34-8. The amount of amperage used depends upon the ampacity of the grounding conductor. Satisfactory results can be obtained by using 100 amperes DC with 4/0 copper conductors. The weld resistance shouldn’t exceed 0.01 ohm, and good welds will be much less than this. Take the ampere reading and divide it into the voltage drop across the weld and this will give the resistance reading in ohms.
The above method of reading resistance on connections is very satisfactory, but it is cumbersome and time-consuming.
The same results may be obtained by means of a Ducter®, which is a registered trademark of Biddle Instruments. This also uses the fall-of-potential method of testing, but instead of using separate voltmeter and ammeter, the results are indicated on a meter directly, by a cross-coil true ohmmeter.
The instrument consists of two coils mounted rigidly together on a common axis in the field of a permanent magnet. The source of
362 Chapter 34
GROUNDING CONDUCTOR
z
0
â–  lilt
DC GENERATOR
Figure 34-8 Method of checking resistance of metal fusing.
current and voltage may be batteries, chargeable or not, or a rectifier plugged into 120 volts AC.
The Ducter® operates independently of voltage, as the voltage and current vary in direct proportion; thus, no rheostats or other balancing devices are required.
Ducters® have 0- to 10,000- to 1,000,000-microhm readings and operate at 1 to 100 amperes, depending upon the model picked and the range setting. The resistances of the leads supplied are compensated for in the reading obtained, and so don’t enter into the readings.
Since the writing of the first edition, Biddle Instruments has come out with a very compact and light low-resistance ohmmeter, DLRO®, which is self-contained and will read down to a half-millionth of an ohm. Biddle Instruments sent me one to evaluate. I took readings previously done with a Ducter® and compared them with the readings on the new instrument, and the results were the same on cadwelds.
System Grounding
The most common industrial distribution systems, as far as grounding is concerned, are
1. Ungrounded (see Figure 34-9)
2. Resistance Grounded (see Figure 34-10)
3. Reactance Grounded (see Figure 34-11)
4. Solid Grounded (see Figure 34-12)
Grounding and Ground Testing 363
Figure 34-9 Ungrounded systems.
Figure 34-11 Reactance-Grounded system.
364 Chapter 34
The ungrounded system is actually high-reactance capacitance grounded as a result of the capacitive coupling to ground of every energized cable, conductor, bus bar, or machine coil. A single-phase ground won’t trip an overcurrent device, but the other two phases are subjected to about 73% overvoltage on a sustained basis and this system may be subjected to voltage as high as five times normal with an arcing fault.
In resistance grounding, the circuit acts more as a resistance than a capacitor and doesn’t subject the other phases to overvoltage with a ground fault on one phase. The current, when a ground fault occurs on just one phase, won’t trip the circuit. Two phase grounds would cause circuit interruption. This type of system effectively bleeds off disturbing influences. These may be surges by lightning, switching, ferroresonance, etc.
Reactance-grounded circuits are not ordinarily encountered in industrial power systems. Transitory overvoltage will occur on repetitive restriking in an arc on a ground fault.
The solid-grounded system is used extensively in systems of 600 volts or less. It is very effective in giving the greatest control of overvoltages.
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