Emerson Process Management ControlWave EFM 3808 - Manual (Page 272)

S1400CW Page 3-4 Section 3 - Grounding & Isolation
Megger reads more than 10 ohms, the ground is considered ‘poor.’ If a poor ground is
indicated, one or more additional ground rods connected 10 feet from the main ground rod
should be driven into the soil and interconnected via bare AWG 0000 copper wire and 1” x
¼-20 cable clamps as illustrated in Figure 3-3). * Note: Megger is a Trademark of the
Biddle Instrument Co. (now owned by AVO International). Other devices that
may be used to test ground resistance are “Viboground”; Associated Research,
Inc., “Groundmeter”; Industrial Instruments, Inc., and “Ground-ohmer”; Herman
H. Sticht Co., Inc.
If the Megger still reads more than 10 ohms, mix a generous amount of cooking salt, ice
cream salt or rock salt with water and then pour about 2.5 to 5 gallons of this solution
around each rod (including the test rods). Wait 15 minutes and re-test the soil. If the test
fails, the soil is poor and a ‘Poor Soil Ground Bed’ will have to be constructed.
Figure 3-4 shows a typical Poor Soil Ground Bed Electrode. A Poor Soil Ground Bed will
typically consists of four or more 10-foot long electrodes stacked vertically and separated by
earth. Figure 3-5 shows the construction of a Poor Soil Ground Bed. For some poor soil
sites, the ground bed will be constructed of many layers of ‘Capacitive Couplings’ as
illustrated. In extremely poor soil sites one or more 3’ by 3’ copper plates (12 gauge or 1/16”
thick) will have to be buried in place of the electrodes.
Figure 3-4 - Ground Electrode Construction for Poor Soil Conditions
3.3.1.3 Dry, Sandy or Rocky Soil
Very dry soil will not provide enough free ions for good conductance and a single ground rod
will not be effective. A buried counterpoise or copper screen is recommended for these
situations. It will be necessary to keep the soil moist through regular applications of water.
Sandy soil, either wet or dry, may have had its soluble salts leached out by rain water,
thereby reducing conductivity of the ground. High currents from lightning strikes could also
melt sand and cause glass to form around the ground rod, rendering it ineffective. A buried
counterpoise or copper screen is preferred for these installations along with regular
applications of salt water.
Rocky soil can pose many grounding problems. A counterpoise or copper plate will probably
be required. Constructing a trench at the grounding site and mixing the fill with a
hygroscopic salt such as calcium chloride may help for a time. Soaking the trench with
water on a regular basis will maintain conductivity.
Units with phone modems require the use of a lightning arrester. The lightning arrester
must be situated at the point where the communication line enters the building.