Survey & Ground Measurements
by Lightning Mike
ITD of Destin, Inc.
 When we are asked to conduct our basic survey, we also visually inspect each site for
National Electrical Code (NEC) violations along with ground resistance measurement of the available ground cables. We feel that
effective measurement of ground resistance should be taken when electrical equipment operation is off and the ground connection
is isolated or removed from the electrical connection. However, most often electrical equipment cannot be turned off or the connection
of the earth electrode system cannot be disconnected. Therefore, we choose to use the AEMC Model 3710 Clamp-on Ground Resistance
Tester, National Stock Number NSN 6625-01-377-8030. The testing device measures ground rod and small grid resistance without the use
of auxiliary ground rods. It can be used in multi-grounded systems without disconnecting the ground system under test. The 3710 tester
simply clamps around the ground conductor or rod and measures the resistance to ground with measurement parameters of 1 ohm to 1200 ohms.
Any recording under 1 ohm is considered a loop measurement. That means it is on top of a ground ring and not at a ground rod. We are also
able to measure existence of any current at the rod or conductor. The tester will measure current parameters of 1mA to 30 Amps. One
important reason we like to use this tester is to verify the quality of grounding connections and bonding. Today's microprocessor
electronics rely on good electrical bonds between equipment, racks, and associated systems. We feel that the military has issued standards
toward accomplishing a safe and effective earth electrode ground system. In as much as the work has been determined "all inclusive"
by content, there are geographical site differences and atmospheric activities that will effect each grounding system accordingly. For
instance, a ground system in southern California will not be subjected to the same lightning activity as a site located in central Florida.
The two sites are geographically different as well as atmospherically different. The same earth electrode ground system may be installed
at both locations. While the one in California will be safe and effective, the one in Florida may sustain equipment damage from surge
events created by lightning. Another reason for using the 3710 tester is for locating ground rods or conductors that may not be bonded
to the entire system or have become disconnected. When this tester indicates a ground resistance measurement, it means we have located
a ground rod source that appears not to be connected to a ground ring and a ground resistance measurement can be achieved. According to
the NEC standards, ground resistance should be 25 ohms or less for a safe system. According to military standards for communication and
electronic facilities, the ground resistance should be 10 ohms or less. (MIL-STD-188-124b Section 5.1.1.1.3.1) However, the military
standards are interpreted by different thoughts. The military standards of 10 ohms state that if 10 ohms are not met with one ground
rod "see MIL-HNBK-419A for alternate ground methods." The handbook offers two popular solutions. Solution 1) Drive a second rod,
twice the length of the rod away, and connect the to driven rods with a minimum gauge 2/0 bare copper cable. (No additional measurements
are required once the second rod has been installed according to standards. Achievement of 10 ohms has not been met, only additional rod
and cable contact with the soil.) Solution 2) Install a ground ring around the facility with rods spaced twice their length apart and connected
with a minimum of 2/0 bare copper ground wire. (No additional measurements are required once the ground ring is installed.) The NEC standard
of 25 ohms is not a requirement, only a suggestion. If an eight-foot ground rod is driven in the ground and a #6 solid copper wire is
connected to the rod and the electrical service entrance power panel, NEC code has been met without meeting 25 ohms. (No ground resistance
measurement is required by NEC.) Next during our visual survey is to look for applications of protection devices and their effectiveness. For
instance, an installed protection device of hybrid multi-component design with high capacitance versus a single small component like a
Metal Oxide Varistor (MOV). During the survey, we will note the need for effective protection at site electronics and record equipment
voltage & signal configurations to design the best application required to achieve effective long term protection of the electronics.
What determines a good ground system? ITD has learned that the soil conditions and other factors at each site determine ground systems.
However, effective grounding electrode systems for sensitive electronics must bond all site equipment to the ground system in such a way
that all of the equipment samples ground potential at only one point commonly known as a "single-point ground". Otherwise a
difference in potential across or within a piece of equipment could cause damage. If there is a difference in potential between the two
pieces of equipment, that potential attempts to equalize using the conductive path of equipment. The single-point ground is defined as the
point at which the unified ground passes through this plane. A single-point ground system has a proven track record in lightning protection
when coupled with effective protection components. The term "single-point ground" has also been misinterpreted as only "one"
ground rod which is a false deduction.
Standards: These standards will be referenced.
- MIL-HNBK-419A Grounding, Bonding, and Shielding for Electronic Equipment and Facilities
- MIL-STD-188-124B: Grounding, Bonding and Shield for Long Haul Tactical Communication Systems Including Ground Based Communications-Electronics Facilities and Equipment
- AFI 32-1065: Grounding Systems
- NFPA 70 (NEC) National Electrical Code Article 250 Grounding
- NFPA 780 Lightning Protection Code
- FAA-STD-019B Lightning Protection, Grounding, Bonding and Shielding Requirements for Facilities
- FAA-STD-020B Transient Protection, Grounding, Bonding and Shielding Requirements for Electronic Equipment
ITD of Destin, Inc.
800-766-0486
Additional information and site survey information can be obtained by contacting
POC: Mike Helms mike@lightningmike.com
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