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Grounding and bonding
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We’re going to take a look at the grounding and
bonding requirement for our TRs and entrance facilities
EFs.
Grounding and bonding is covered in the NEC in article
250 and is also covered in TIA/EIA 607.
Standard 607 essentially follows article 250 of the code
however you will be introduced to some new
terminology and methods taken from the 607 standard.
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Whenever we talk about grounding it is in reference to
earth, where as bonding is an electrical connection that
provides a path to earth.
Per NEC a ground is:
A conducting connection, whether intentional or
accidental, between an electrical circuit or equipment and
the earth or to some conducting body that serves in place
of the earth.
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Per NEC bonding is:
The permanent joining of metallic parts to form an
electrically conductive path that ensures electrical
continuity and the capacity to conduct safely any current
likely to be imposed.
The symbols for grounding are:
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Per TIA/EIA 607:
TMGB: Telecommunications main grounding bus bar
TGB:
Telecommunication grounding bus bar
TBB:
Telecommunications bonding backbone
TBBIBC: Telecommunications bonding backbone
interconnecting bonding conductor
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The ANSI/EIA/TIA-607 standard defines a
telecommunications grounding and bonding system and
the interconnections to the building electrical grounding
system.
The recommendations made in the standards do not
supersede the bonding and grounding requirements of
national and local electrical code.
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The system begins at the electrical service entrance,
travels to the TMGB and continues through to each
TGB located in individual telecommunications closets
on each floor of the building structure, finally looping
back around to the original TMGB.
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Here is the TMGB, TGB and TBB grounding system.
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The ANSI/EIA/TIA-607 standard requires that all
communications bonding conductors be listed for the
intended purpose and approved by a nationally
recognized testing laboratory such as UL or ETL.
Bonding conductors must always be insulated wires.
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The standard also requires that bonding conductors be
made of copper, other metal types are not supported for
use as a bonding conductor by the ANSI/EIA/TIA-607
standard.
In addition, the minimum size of all bonding
conductors must be at least a #6 AWG wire.
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The ANSI/EIA-TIA-607 standard prohibits placing
bonding conductors in a metallic conduit made of iron.
This standard requires that if the bonding conductor
must be placed in an iron conduit longer than 1m (3 ft.)
in length, then bonding conductor must be bonded at
each end of the conduit.
The wires used for bonding the bonding conductor
must be at least a #6 AWG wire.
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Connections to the TMGB or lugs should be
exothermic welds. Exothermic welds provide a
connection that helps ensure the long-term integrity of
the grounding system.
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The TBBIBC would also be used to bond two or more
TGBs installed in the same TR together.
The TBBIBC is also used to bond the TGBs installed in
different TRs that reside on the same floor of the
building.
This connection would follow the same requirements as
bonding multiple TBBs at the top floor and a minimum
of every third floor.
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The ANSI/EIA/TIA-607 standard requires that when two or
more TBBs are installed vertically in the intra-building backbone
pathway, the TBBs must be bonded together.
The telecommunications bonding backbone interconnecting
bonding conductor (TBBIBC) is the component used for this
function (see the figure below).
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The ANSI/EIA/TIA-607 standard requires that the
TBBIBC be installed at the top floor and a minimum of
every third floor.
The minimum size of the TBBIBC must be no smaller
than the TBB conductor size.
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Telecommunications grounding and bonding is
additional grounding and bonding installed specifically
for telecommunications.
This is not a replacement for grounding and bonding
specified by the National Electrical Code (NEC) but
typically is additional to address telecommunications
system performance.
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The images below are grounding/bonding bus bars,
notice that they’re made of ¼” thick copper plates and
are pre drilled with a variety of different holes and also
have insulated mounting brackets.
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The TGMB located in the entrance facility or MDF is
normally a much larger universal grounding bus bar
than the TGB.
All connections between the TGMB and the TGB must
be installed with what size ground wire?
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How do we identify a ground wires of sizes 6 gauge or
smaller?
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An insulated grounded conductor of 6 AWG or smaller
shall be identified by a continuous white or gray outer
finish or…
By three continuous white stripes along its entire length
on other than green insulation.
This means that a green insulated conductor is also
acceptable as a grounding conductor.
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NEC article 250.8: Connection devices or fittings that
depend solely on solder shall not be used.
Why should we not solder a ground connection?
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Notice that grounding and bonding connections made
to the TGB are done with electrical wire connectors and
a bolt and nut are used to make the connection.
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All ground connections per the NEC shall be as short
and straight as practicable, this means soft bends and no
sharp 90 degree bends.
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The next slide will show a fully compliant grounding
system in a TR.
Take a few minutes to see all of the connections and
gauge sizes used for bonding.
You will notice the term future growth used we
mentioned future growth in a previous lecture.
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Here are some other listed grounding/bonding clamps.
Pipe clamp
Cable tray bonding
strap
Acorn clamp for
ground rods
CATV ground block
Telecom pipe
strap
Small grounding bus bar
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All ground clamps are listed for a specific conductor
size and whether or not they can be used for a single
connection or multiple connections.
Ground clamps are also listed for direct burial use,
direct buried clamps are coated for protection against
corrosion.
The best possible ground connection is an exothermic
weld.
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The grounding electrode system shall have a resistance
of 25 ohms or less.
If the grounding electrode system is not 25 ohms or less
than it shall be augmented with an additional grounding
electrode.
If two grounding electrodes are used they shall be no
less than 6’ apart and shall be bonded together with a 6
AWG ground wire.
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The grounding conductor can be stranded or solid.
The minimum allowable ground wire size is a 14 gauge
and is typically used only in residential dwellings and
in one and two family dwellings shall not be longer
than 20 feet.
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¾” conduit can be used as a ground rod, article 250.52
(5).
Rod and pipe electrodes shall be installed such that at
least 8’ of length is in direct contact with the earth.
If rock bottom is encountered than the rod shall be
driven at an angle up to 45 degrees, if it can’t be driven
in at 45 degrees it shall permitted to be buried in a
trench no less than 30 inches deep.
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The AHJ will inspect the grounding electrode system
when he is on site and will want to see that all racks are
bonded.
The grounding electrode system on construction
projects is typically installed by the electricians and the
low voltage technician bonds all of his equipment in all
of the TRs on the site to the installed grounding
electrode system.