Application Note

What is a good ground
resistance value?
There is a good deal of confusion as to what
constitutes a good ground and what the ground
resistance value needs to be. Ideally a ground
should be of zero ohms resistance.
There is not one standard ground resistance
threshold that is recognized by all agencies.
However, the NFPA and IEEE have recom-
mended a ground resistance value of 5.0 ohms
or less.
The NEC has stated to “Make sure that system
impedance to ground is less than 25 ohms
specified in NEC 250.56. In facilities with sensi-
tive equipment it should be 5.0 ohms or less.”
The Telecommunications industry has
often used 5.0 ohms or less as their value for
grounding and bonding.
The goal in ground resistance is to achieve
the lowest ground resistance value possible
that makes sense economically and physically.
Why ground?
Why test?
2
Grounding basics
4
Methods of earth
ground testing
6
Measuring
ground resistance
12
Table of
contents
3
Why test? Corrosive soils.
Why ground? Lightning strikes.
Use the Fluke 1625-2 to determine the
health of your earth ground systems.