Calix 500 Series User manual

Calix ONT Grounding
Implementation
January, 2007
# 220-00178, Rev 10

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© 2001-2007 Calix. All Rights Reserved.
Contents
Introduction...............................................................................5
Grounding Purpose..................................................................5
NEC and Grounding.................................................................6
Testing the Earth Electrode.....................................................7
Full-of-Potential Method ..........................................................9

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© 2001-2007 Calix. All Rights Reserved.

5
Proprietary Information: Not for use or disclosure except by written agreement with Calix.
© 2001-2007 Calix. All Rights Reserved.
Introduction
This document details the National Electrical Code (NEC) requirements for properly
grounding an ONT during installation.
This document also provides specific information for installing the ground on the Calix 500-
Series or 700-Series ONTs, on the following models:
ONT Type Models Available
500-Series 410, 412, 422, 424, 502, 504, 522, 524, 560
700-Series 710, 711, 712, 714, 720, 721, 722, 724, 740, 761
Grounding Purpose
Providing proper grounding to the ONT has two different purposes:
yGrounding provides for the safety of the intended user and maintenance personnel. The
grounding requirements of the NEC provide a degree of safety in the event the outer
metallic case of the protected device becomes energized by a current-carrying conductor.
In this event, the grounded case would cause the over-current device to trip, thereby
removing the power to the device.
yThe protection and survivability of the electronics equipment. High voltage spikes that
occur during electrical storms, as well as static buildup on power utility lines, have been
known to cause a great deal of damage to electronic equipment. When the voltage
potentials reach sufficient amplitude, flashovers take place. In most cases, damage is
caused by the super heating present during the flashover. The high voltage potentials that
are induced into AC utility conductors and communications cables during electrical
storms are of a wavelength that approaches the RF spectrum. To be effective in the
discharging of induced RF energy, grounding conductors must provide a low impedance
path to earth. Keeping conductors as short as possible and using the most direct path,
free of sharp bends, will help to lower the impedance of the ground circuit.
Note: Grounding conductors must never be placed inside a metallic conduit unless bonded
at both ends of the conduit so that the conduit itself becomes the conductor.

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© 2001-2007 Calix. All Rights Reserved.
NEC and Grounding
A very important aspect of any ONT installation is to comply with the National Electric
Code (NEC), as well as all state and local codes. Article 800 of the NEC requires the ONT to
be grounded according to Article 250.56 of the code. To be properly grounded, the ONT
must be directly bonded to the building’s AC utilities earth ground electrode using a #6
AWG copper conductor. The earth electrode can be a metallic cold water pipe or a copper
clad steel rod that has been driven into the ground to a depth of at least 8 feet.
Metallic cold water pipes that run under ground for a distance of at least 10 feet are
considered the best ground electrodes. The NEC requires that if cold water pipes are used
they must be within 5 feet of the outside wall that the ONT is mounted on.
In the event that the ONT is installed more than 20 feet from the building AC utility earth
ground electrode, a suitable secondary ground electrode must be provided. The secondary
ground must be bonded to the building’s primary earth electrode. A bonding jumper of #6
AWG copper must be used between the two electrodes.

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Testing the Earth Electrode
The earth electrode is the bonding or connection point to the earth. The most common earth
electrodes are ground rods, metallic cold water pipes, underground grids and water wells that
use steel pipe. The ability of a grounding system to safely dissipate unwanted or harmful
potentials into the earth depends on the total resistance of the electrode, the resistance
between the conducting surface of the electrode, and the conducting surface of the soil that it
is in. The conductivity of the earth near and around the grounding system depends on the
chemical makeup and ambient moisture content of the soil.
The testing of an electrode measures the effectiveness of the earth electrode to provide
electrical contact with the earth. The lower the resistance to the earth is, the more effective
the ground system will be in dissipating energy safely.
To understand the testing procedure of an earth electrode, one must first understand some
of the principles of ground conductivity. The earth acts like it is made up of layers that bear a
resemblance to an onion. Like an onion, the earth is made up of expanding sections or peels
(layers). An electrode placed into the earth radiates current in all directions along the surface
of each of these layers, as depicted in the illustration below:
Ground Bond
#6 AWG
UL Listed
Clamp
Earth
Electrode
Earth
Layers
5229
© 2001-2007 Calix. All Rights Reserved.

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As the layers extend outward from the electrode, their surface area becomes larger compared
to the layers nearest the electrode. The resistance of each layer decreases in relation to the
amount of surface area. From a starting point next to the earth electrode, the resistance
measurement to earth will increase with distance. The amount of resistance increase due to
distance will taper off at some point. As the distance from the electrode increases, the
resistance to earth does not increase. As more measurements are taken beyond this point, the
amount of resistance to earth will start to increase again.
The test setup, as illustrated below, uses an Earth Resistance Megger to determine the
resistance of an earth electrode. Grounding system tests are commonly made with a “fall of
potential” test set. Within the test set is an AC current source, a voltage meter, and a number
of test terminal binding posts. The AC voltmeter inside the test set reads out directly in ohms
of resistance. By reading the voltage drop across a current path and applying Ohms Law, the
total resistance of the ground, plus the surface resistance of the earth electrode to the
surrounding ground, can be measured.
AC Voltage
Source
Volt/Ohm
Meter
Test Rod “P” Test Rod “C”
Earth
5225
© 2001-2007 Calix. All Rights Reserved.

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© 2001-2007 Calix. All Rights Reserved.
Full-of-Potential Method
The “Full-of-Potential Method” (sometimes referred to as the “Three Terminal Method”) is
the most commonly accepted way to measure the total resistance of a grounded earth
electrode. Providing that the measured resistance at the conclusion of the test meets the
specified requirements, no additional testing is required. The test procedure measures the
total resistance of the earth ground electrode, the contact resistance of the electrode to the
earth, and the earth near the grounding system.
An earth resistance megger is a test set used to make resistance measurements of earth
electrodes. An AC voltage is applied across two electrodes that have been placed into the
ground at a certain distance apart from each other. As current passes through the ground
between the electrodes, a voltage can be measured at a point between the electrodes. The test
set provides an internal voltage meter to accommodate this measurement. The voltage
reading taken by the test set is converted to ohms of resistance by using Ohms Law (R=E/I).
You start the test by connecting the megger to the earth electrode that is being tested. A
second test electrode is placed into the ground at a distance out and away from the outer
current electrode that is under test. The distance between the electrode being tested and the
outer current electrode may be limited by the physical characteristic of the surrounding area,
or the length of the test conductor being used. It is not uncommon for the outer electrodes
to be spaced at 100 feet. A third test electrode is used as a probe and placed into the ground
at a number of points that are along a line between the outer electrodes. Each time the test
probe is placed into the ground, a resistance reading is taken and recorded. A simple
resistance curve is then plotted, as illustrated below. At about 60% of the distance between
the earth electrode under test and the outer current electrode, the resistance over distance
does not continue to increase. It is at this point that the resistance measurement taken is
most accurate.
In cases where the outer current electrode is placed too close to the earth electrode that is
under test, the earth shells that surround each electrode will overlap each other. Resistance
readings taken when earth shells overlap will be very inconsistent and, for the most part,
erroneous. As resistance measurements are taken and plotted, a curve of increasing resistance
over distance will accumulate. The resistance curve will not flatten out.
Most earth megger test sets will indicate the presence of stray electrical currents in the
ground. Resistance measurements taken under such a condition should not be relied on as
accurate. The source of the electrical currents should be identified and, if possible,
eliminated. The presence of electrical currents in or near a ground system will compromise
the overall integrity of the grounding system.

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Calix recommends that the testing of grounds be conducted on a preventative maintenance
schedule to insure the maximum safety possible. Maintenance records should be kept for
each ONT installation. In some locations, extremely high earth ground resistance is present.
In these locations, chemicals that lower earth resistance can be added to the earth that
surrounds the electrode. This type of treatment must be done on a scheduled maintenance
routine complete with resistance records.
10 20 30 40 50 60 70 80 90 1000
10 20 30 40 50 60
Resistance (Ohms)
Distance (Feet) 5226
© 2001-2007 Calix. All Rights Reserved.