Section 3 - Grounding & Isolation Page 3-3 S1400T-2808B
the soil may be to dry and/or salt free to a good conductor. If salt is lacking add rock salt
(NaCl); if the soil is dry add calcium chloride (CaCl
2
).
3.3.1.2 Soil Types: Good
Poor
Damp Loam Back Fill
Salty Soil or Sand Dry Soil
Farm Land Sand Washed by a Lot of Rain
Dry Sand (Desert)
Rocky Soil
Ground Beds must always be tested for conductivity prior to being placed into service. A
brief description of ground bed testing in ‘Good Soil’ and ‘Poor Soil’ is provided herein.
Details on this test are described in the National Electrical Code Handbook
. Once a reliable
ground has been established, it should be tested on a regular basis to preserve system
integrity.
Figure 3-2 - Basic Ground Bed Soil Test Setup
Figure 3-2 shows the test setup for ‘Good Soil’ conditions. If the Megger* reads less than 5
ohms, the ground is good. The lower the resistance, the better the earth ground. If the
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.
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