circuits, noise is reduced. Lab experiments and tests in actual hubs confirmed that
if an unbalance of 0.8 ohms in the ground circuits can be reduced to 0.3 ohms, the
noise floor in the 5 MHz to 50 MHz spectrum can be reduced by 8 dBmV.
Figure 1 represents a real-life scenario in a headend environment. There were
noise problems affecting a DOCSIS platform in the headend. It was thought that
grounding and bonding problems were causing the noise. The resistance of the building ground was measured and found to be acceptable. The continuity of the grounds
was looked into for bad bonds. It was discovered that the bonds were good enough
and did not adversely affect the protection ability of the ground.
However, the resistance of these daisy-chained grounds varied by 1.0 ohm from
rack No. 1 on the left to rack No. 5 on the right. In other words, those grounds were
unbalanced. The ground resistance of a shelf in the bottom of the left-most rack,
while meeting the specification for ground resistance, even in a best-case scenario, is higher than the shelf at the top of the right-most rack, resulting in unbalanced grounds.
So, what can be done about the imbalance? How can we see if reducing the
imbalance will reduce noise?
Figure 1 illustrates a “daisy-chained” ground. Ground circuits can also be “home
runs” as shown in Figure 2. This is where each rack has a ground connected in parallel with the other rack grounds connected to the ground bus.
As shown in Figure 1, several grounds that are daisy-chained, because of the
different number of bonds and greater difference in length, will have more variation
in their resistance (be unbalanced) than would several home run grounds with their
similar number of bonds and similar lengths. Changing these daisy-chained grounds
to home run grounds helps to reduce noise.
How to determine if grounds are balanced
We cannot measure the resistance value of a ground at a shelf or similar place in a
hub or headend. We can, however, easily measure and compare the continuity and
balance of the various ground circuits of a hub or headend.
To measure ground balance in a hub or headend, a clamp-around ground tes-
ter that measures to a tenth of an ohm is required. All clamp-around testers use
Ohm’s Law to calculate resistance.
If we open the jaws of such a tes-
ter, we will see the ends of two coils
(Figure 3). One coil of the set, the
“transmit coil,” induces current of
a known voltage onto the circuit
under test. This current seeks the
path of least resistance back to the
other coil, the “receive coil.” If the
circuit under test is a complete cir-
cuit, the receive coil measures the
voltage. The test set now knows
voltage and current and can calcu-
If the path of least resistance
is all metal (does not involve any
soil), the resistance measurement
is not the resistance of a ground,
but rather of the continuity of the
metal circuit. When looking for
unbalanced ground circuits in hubs
and headends, that is what is done.
If testing shows that the headend and hub grounds are not balanced, what
can be done?
Fixing the balance
Daisy-chained ground circuits can be changed to home runs if the headend and hub
grounds are not balanced. Bonds need to be double-checked that they are installed
to the manufacturer’s specifications, and the resistance of individual bonds may
be measured with a digital low resistance ohmmeter. It is also important to review
grounding and ground testing practices, as well as to ensure that the ground was
reconnected if it was disconnected during building renovations. Grounds also need
to be checked for continuity.
Other environmental factors also need to be considered. All kinds of noise generators, like air-conditioners and impulse motors, are being installed. Are they grounded
and are the grounds tested? These noise sources make it more urgent that grounds
be balanced to reduce potential points of ingress for noise.
As you can gather, it is no longer sufficient to have grounds that only protect
products and people. We must make sure that they also protect the performance of
equipment and products being used. We also must remember that the balance of
the elements of a hub or headend ground is important for noise mitigation in modern digital communications circuits. And although it seems as if visually checking
these elements is important, it is also important to test them to determine if they
are truly balanced.
ED ROUSSELOT is national communications sales manager for Megger.
FIGURE 2: “Home run” ground circuits
FIGURE 3: The coils in a claw tester
that is open
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