Application Note

12
Measuring ground resistance
At central offices
When conducting a grounding audit of a central
office there are three different measurements
required.
Before testing, locate the MGB (Master Ground
Bar) within the central office to determine the
type of grounding system that exists. As shown
on this page, the MGB will have ground leads
connecting to the:
• MGN (Multi-Grounded Neutral) or incoming
service,
• ground field,
• water pipe, and
• structural or building steel
First, perform the Stakeless test on all the
individual grounds coming off of the MGB. The
purpose is to ensure that all the grounds are
connected, especially the MGN. It is important to
note that you are not measuring the individual
resistance, rather the loop resistance of what
you are clamped around. As shown in Figure 1,
connect the Fluke 1625-2 or 1623-2 and both
the inducing and sensing clamps, which are
placed around each connection to measure the
loop resistance of the MGN, the ground field, the
water pipe, and the building steel.
Second, perform the 3-pole Fall-of-Potential
test of the entire ground system, connecting
to the MGB as illustrated in Figure 2. To get to
remote earth, many phone companies utilize
unused cable pairs going out as much as a mile.
Record the measurement and repeat this test at
least annually.
Third, measure the individual resistances of
the ground system using the Selective test of
the Fluke 1625-2 or 1623-2. Connect the Fluke
tester, as shown in Figure 3. Measure the resis-
tance of the MGN; the value is the resistance of
that particular leg of the MGB. Then measure
the ground field. This reading is the actual
resistance value of the central office ground
field. Now move on to the water pipe, and then
repeat for the resistance of the building steel.
You can easily verify the accuracy of these
measurements through Ohm’s Law. The resis-
tance of the individual legs, when calculated,
should equal the resistance of the entire system
given (allow for reasonable error since all
ground elements may not be measured).
These test methods provide the most accurate
measure of a central office, because it gives
you the individual resistances and their actual
behavior in a ground system. Although accu-
rate, the measurements would not show how
the system behaves as a network, because in
the event of a lightning strike or fault current,
everything is connected.
EI-162X
SENS
ING CUR
R
E
NT
TRANS
F
O
RM
ER
MGN
MGB
The layout of a typical central office.
Water pipe
Ground
field
Building
steel