UNITED STATES OF DEPARTMENT OF AGRICULTURE

Rural Utilities Service

**BULLETIN 1751F-815**

**SUBJECT: [Electrical Protection of Outside Plant]{.underline}**

**TO:** All Telecommunications Borrowers

RUS Telecommunications Staff

**EFFECTIVE** **DATE:** Date of Approval

**EXPIRATION DATE:** Seven years from effective date

**OFFICE OF PRIMARY INTEREST:** Engineering Standards Branch,
Telecommunications Standards Division

**PREVIOUS INSTRUCTIONS:** This bulletin replaces Telecommunications
Engineering and Construction Manual (TE&CM) Section 815, Number 4,
issued April 1969; Section 816, Number 3, issued July 1969; Addendum
No. 1 for Section 816, issued January 1971; Addendum No. 2 for
Section 816, issued December 1972; Section 817, Number 1, issued
March 1980.

**FILING INSTRUCTIONS:** Discard TE&CM Sections 815, 816, Addenda 1 and
2 for Section 816, and 817 and replace them with this Bulletin. File
this bulletin with 7 CFR 1751 and RUSNET.

**PURPOSE:** To provide information concerning the electrical protection
principles for aerial and buried cable along with information on
effective grounding of cable shields. It describes the normal methods
for protecting aerial circuits from lightning and power contacts by
means of arresters, shield grounding, and shield bonding.

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_
\_\_\_\_\_\_\_\_\_\_\_

Administrator Date

**TABLE OF CONTENTS**

.Begin Table C.

1\. DISTURBING POTENTIALS 4

> 1.1 Lightning 4
>
> 1.2 Power Contacts 4

2\. PROTECTION PRINCIPLES 5

> 2.1 Lightning Protection 5
>
> 2.2 Power Contact Protection 5

3\. LIGHTNING PROTECTION FOR PLASTIC-INSULATED CABLE 5

> 3.1 Surge Dielectric Strength 5
>
> 3.2 Protection of PIC Cable 6

4\. BONDING AND GROUNDING FOR POWER CONTACT PROTECTION 8

> 4.1 Bonding at Power Crossings 8
>
> 4.2 Bonding in Joint Use or Joint Occupancy 9

5\. MISCELLANEOUS SITUATIONS 10

> 5.1 Underground/Buried Cable Dips in Aerial Cable Runs 10
>
> 5.2 Protection at Loading Points 10
>
> 5.3 Buffer Protection 10
>
> 5.4 Pole Lightning Protection Wires 11

6\. ELECTRICAL PROTECTION FOR BURIED/UNDERGROUND CABLE 11

> 6.1 Plastic-Insulated Conductor (PIC) Cable and Wire 11
>
> 6.2 Lightning Protection 11
>
> 6.3 Power Contact Protection 12

7\. LIGHTNING PROTECTION FOR BURIED/UNDERGROUND PLANT 13

> 7.1 Dielectric Protection of Buried/Underground Plant 13
>
> 7.2 Facilities Serving Severely Exposed Stations 13
>
> 7.3 Bonding/Grounding of Buried Wire and Cable Shields 13

8\. POWER CONTACT PROTECTION FOR BURIED/UNDERGROUND PLANT 14

> 8.1 Aerial Inserts 14
>
> 8.2 Aerial Line Extensions 15
>
> 8.3 Aerial Drop Wires Connected to Buried Plant 16

9\. GROUNDING OF BURIED WIRE AND CABLE SHIELDS 16

> 9.1 Grounding at Junctions 16

10\. METALLIC HOUSINGS ON VERTICAL POWER POLES 16

> 10.1 Bonding 16
>
> 10.2 When Carrier Equipment is not Grounded 16

11\. EFFECTIVE GROUNDING OF CABLE SHIELDS 17

> 11.1 General 17
>
> 11.2 The Purpose of Grounding Cable Shields 17
>
> 11.3 The Application of an Effective Grounding System 17
>
> 11.4 Isolating Damage Caused by a Lightning Stroke 17
>
> 11.5 Obtaining a 25 Ohm Ground 17

12\. EFFECTIVE GROUNDING THEORY 18

> 12.1 Effective Grounding 18
>
> 12.2 Spacing of Electrodes 18
>
> 12.3 Selection of Electrode 18

.End Table C.

FIGURES SECTION (NOT AVAILABLE ON RUSNET)

Figure 1: Lightning Damage Probability Map 19

INDEX:

Protection

Protection, Electrical

Protection, Outside Plant

**ABBREVIATIONS**

**ANSI** American National Standards Institute

**AWG** American Wire Gauge

**CEGB** Cable Entrance Ground Bar

**GPR** Ground Potential Rise

**MGB** Master Ground Bar

**MGN** Multigrounded Neutral

**NEC** National Electrical Code

**NESC** National Electrical Safety Code

**NEXT** Near-end Crosstalk

**NFPA** National Fire Protection Association

**PCM** Pulse Code Modulation

**PIC** Plastic-Insulated Conductor

**PM2A** Ground Wire Assembly Unit

**TE&CM** Telecommunications Engineering and Construction Manual

**DEFINITIONS**

**Cable** A group of insulated conductors formed into a compact core and
covered with a protective sheath.

**Non-current carrying conductor**s Non-telecommunications conductive
materials such as metallic cable shields and strength members, metallic
cable lashing wires, guy wires, and cable support wires, etc.

**Plastic-insulated conductor (PIC) cable** Cable consisting of
conductors covered with an extruded coating of plastic.

**Sheath** The nonmetallic outer covering over the shield of certain
types of cables.

**Shield** The metallic components of a cable sheath that help to
minimize outside electrical interference in cable conductors and that
help to direct lightning and power cross discharge currents to ground.

**1. DISTURBING POTENTIALS**

**1.1 Lightning:** Lightning is a transient discharge between a charged
cloud and the earth or another cloud, involving high peak currents
(several tens of thousands to more than a 100,000 amperes) usually
lasting a few hundred microseconds. Lightning surges in cable plant may
occasionally arise because of direct strokes to pole tops or to the
cable itself. Successful protective measures against damage are usually
not practicable for direct lightning strokes to elements of telephone
cable plant. Fortunately, however, lightning disturbances more commonly
appear in cable plant in lower energy levels by conduction from
connecting distribution wire, by induction from nearby strokes-to-earth,
by conduction from the earth at or near the stroke point to the cable
through guys or pole grounding wires, or because of a rise in ground
potential at nearby grounded points such as station protectors. If
adequate protective measures are not taken, lightning discharges may
result in the breakdown of the insulating materials between cable
conductors and the grounded metallic cable shields or between the
conductors themselves. The effect of dielectric failure on service
outage will depend on the magnitude and duration of the surge and the
susceptibility of the materials involved to permanent damage, such as
melting of the conductor and conductor insulation or carbonization of
the conductor insulation.

**1.1.1** Excessive lightning surge currents can cause
telecommunications conductors to fuse open. The use of improved plastic
insulation, however, has increased the dielectric strength of cable
conductors to the point at which dielectric failure seldom occurs. The
dielectric strength of cable causes the current surges to flow through
the conductors rather than being bypassed through conductor insulation.
As a result, lightning damage to plastic cable plant is more likely to
be caused by conductor fusing rather than from dielectric failure.

**1.2 [Power Contacts]{.underline}:** Mutual association of telephone
cable (or wire) facilities with power distribution circuits, as a result
of joint use or joint occupancy of poles or at crossings, involves the
possibility of incidental electrical contacts between these systems.
Such electrical contacts usually are caused by severe mechanical
stresses produced by high wind, heavy ice and snow loads, or
combinations of these factors. Although peak currents in cable plant as
a result of such contacts are likely to be in the order of hundreds
rather than thousands of amperes, their duration is in the order of
seconds rather than microseconds. Consequently, power contacts are
likely to subject telephone plant to heating and burning, as well as
dielectric stress. Power contacts with metal shielded cable and strand
may occasionally burn down cable because of resulting arcing and current
flow, or more likely may only generate sufficient enough current flow in
telecommunications conductors to result in fusing of conductors.
Fortunately, however, the frequency of power contacts is much lower than
that of lightning interference.

**2. PROTECTION PRINCIPLES**

**2.1** **[Lightning Protection]{.underline}:**

**2.1.1** Direct lightning strokes to cable plant are likely to cause
extensive damage because of the magnitude of the currents involved.
However, because the cost for total protection, if even possible, would
be immense and because such strokes occur so infrequently, protection
against direct strokes is impracticable.

**2.1.2** Lightning surges may also reach the conductors and/or shields
of aerial cable by currents conducted from non-cable plant, by induction
from nearby strokes-to-ground, and by currents developed because of rise
in ground potential at stroke points near station protector
installations. (The term \"non‑cable plant\" as used here refers to wire
circuit facilities not enclosed in a metal shield, such as overhead drop
wires.) Cables having plastic-insulated conductors do not require
protective measures except in unusual circumstances as outlined in
subsequent paragraphs.

**2.1.3** To prevent dielectric failure of the cable conductor
insulation (by cable design), all conducting elements of a cable
installation have to be at the same potential. The shields of all aerial
cable sections should be bonded together and to connecting underground
or buried cable shields and to the central office ground. Cable support
strands (messengers) should be electrically continuous and cable shields
should be bonded to the support strands (messengers) at appropriate
intervals.

**2.2** **[Power Contact Protection]{.underline}:** Important protective
construction practices also consist of providing low impedance paths to
ground which will aid in rapid de-energization of a power line in the
event of a power contact. Such grounding is also required by the
National Electrical Safety Code[^1] (NESC1).

**3. LIGHTNING PROTECTION FOR PLASTIC-INSULATED CABLE**

**3.1** **[Surge Dielectric Strength]{.underline}:** The surge
dielectric strength of cable utilizing plastic-insulated conductors is

conservatively considered for engineering design purposes to be as
follows:

**[SURGE DIELECTRIC STRENGTH]{.underline}**

**INSULATOR TYPE** **CONDUCTOR-TO-CONDUCTOR CORE-TO-SHIELD**

**SOLID GEL FILLED** **20 kV 35 kV**

**GEL FILLED,\
EXPANDED** **15 kV 25 kV**

Cables having dielectric strengths of these magnitudes will in most
cases be free from damage by lightning. The relative immunity of PIC
cable without arresters (except in special circumstances) to lightning
damage has been well established by experience in the Bell Systems and
RUS borrower systems over a period of many years.

**3.2** **[Protection of PIC Cable]{.underline}:** The protection of PIC
cable in all areas normally should consist of: (1) bonding and grounding
cable shields at the central office, (2) maintaining electrical
continuity of the shield, (3) bonding cable shields to support strands
(messengers), (4) grounding of cable shields (in aerial circuits via
grounding messengers, etc.,) (5) providing gas tubes or the equivalent
at junctions with facilities serving severely exposed stations,
(6) protecting against fusing of cable conductors, (7) complying with
the NEC[^2] at service drops, and (8) providing supplementary protection
measures at known severe exposure locations.

**3.2.1** **[Bonding and Grounding Cable Shields at Central
Office]{.underline}s:** The shields and other metallic members of plant
entering a central office should be bonded to each other and to the
central office ground. This bonding helps to minimize harmful
differences of potential between the various cables entering the central
office. RUS requires that special provisions be undertaken for bonding
and grounding of outside plant cable shields, metallic armor, etc., with
terminating cables in a central office. Basically, non-current carrying
metallic outside plant items (shields, armor, strength members, etc.,)
are all bonded together in a entering cable vault and they are in turn
bonded to a Cable Entrance Ground Bar (CEGB) installed within the vault.
The CEGB is, in turn, bonded to the office\'s Master Ground Bar and the
elaborate grounding provisioning established at the office. The shields
of the office\'s terminating cables are (deliberately) electrically
isolated in the cable vault and connected to the office ground
[only]{.underline} at their other end, at the office mainframe ground
bar. The purpose of this grounding and bonding arrangement is to divert
any incoming surges that may be on the outside plant cable shields,
armor, etc., directly to ground and not provide a path for these surges
to make it directly to other parts of the mainframe. For more detail on
these special central office procedures, readers should refer to TE&CM
Section 810 (proposed conversion to RUS Bulletin 1751F-810).

**3.2.2** **[Maintaining Electrical Continuity of
Shields]{.underline}:** It is important that electrical continuity of
aerial cable shields be maintained and that such shields be bonded to
any connecting buried or underground cable shields in order to provide a
path to ground for lightning and power currents, and to provide an
effective noise shield. The installation of ready-access enclosures and
the application of cable splicing procedures as covered in RUS Standard
PC-2, Bulletin 345-6 (proposed conversion to 7 CFR 1755.200), will
usually ensure adequate bonding of shields from a protection standpoint.

**3.2.3** **[Bonding Cable Shields to Support Strands
(Messengers)]{.underline}:** Cable shields should be bonded to support
strands (messengers) at frequent intervals to prevent arcing and to
provide a low impedance path to ground for power contact or lightning
related surge currents. Plastic-jacketed cable should be bonded between
the shield and strand at all splices, terminals, and loading points. The
methods of bonding the shield to the strand depend on the types of
enclosures used and are described in detail in PC-2.

**3.2.3.1** Four or more bonds per mile (Two and one-half or more bonds
per kilometer \[km\]) should be provided if possible without opening the
plastic jacket solely for this purpose. Where long runs without splices,
terminals, or load points are involved, at least one bond per mile
(1.6 km) should be provided even if the cable sheath has to be opened
solely for this purpose. If more than one cable is attached to the same
pole, the shields of the various cables should be bonded together:
(1) at crossing poles; (2) at the beginning and ending of multi cable
runs; and (3) at approximately 1500 foot (460 meter) intervals in long
multi-cable runs.

**3.2.4** **[Grounding of Cable Shields/Support Strands
(Messengers)]{.underline}:** Normal construction practices and NESC
provisions require that cable shields, messengers, and other non-current
carrying metallic hardware be effectively grounded. It is especially
important to effectively ground cable shields, messengers and
non-current-carrying metallic hardware at dead ends and other junction
points for noise mitigation, personnel protection, and/or power contact
protection. Such grounds are also beneficial in reducing lightning
potentials between the core and the shield if voltage limiting gaps
(such as terminal studs or arrester gaps) are applied to the conductors.
Grounds are also beneficial in reducing the probability of fusing of
cable conductors from lightning surges by diverting a portion of the
surge to ground before it reaches the cable conductors. (See Section 4
of this bulletin for additional detail on grounding provisions.)

**3.2.5** **[Gas Tubes or the Equivalent at Junctions with Facilities
Serving Severely Exposed Stations]{.underline}:** At junctions with
facilities of any type or length serving stations that are severely
exposed to lightning surges (such as fire towers and radio towers), it
is recommended that 800 volt or greater breakdown gas tube arresters be
installed on the exposed pairs between the conductors and the shield. An
accepted alternative to gas tubes would be to install yellow-coded,
10 mil (0.3 mm) gap carbon arresters.

**3.2.6** **[Protection Against Fusing of Cable
Conductors]{.underline}:** The probability of fusing cable conductors
can be minimized by providing conducting paths for surge currents, which
divert the incoming surges to the shield and ground before they reach
the cable conductors. The measures described in Paragraphs 3.2 through
3.2.5 usually accomplish this.

**3.2.7** **[Service Drops]{.underline}:** Arresters for lightning
protection are not normally required at points where service drops are
connected along aerial cable runs. Connections of drop wires to aerial
cable conductors normally should be made so as to meet National
Electrical Code (NEC), formally identified as ANSI/NFPA 70, fuse
coordinating requirements for station protection. Connection details are
shown on the Construction Drawings in the \"Telephone System
Construction Contract,\" RUS Form 515.

**3.2.8** **[Supplementary Measures]{.underline}:** In addition to the
above items, under severe exposure conditions, supplementary protection
measures may be needed, e.g., plant protection near electric power
generating stations and substations. Details of such supplementary
protection should be determined by a borrower after a careful study.

**4. BONDING AND GROUNDING FOR POWER CONTACT PROTECTION**

**4.1** **[Bonding at Power Crossings]{.underline}:** Where practicable,
crossings between aerial telephone cables and electric distribution
lines of any type should be made on jointly used or jointly occupied
poles. At joint pole crossings with Multi Grounded Neutral (MGN) type
power lines, the cable support strand (messenger) should be
interconnected to the MGN via a vertical pole ground wire. Where it is
not practicable to obtain joint pole crossings with electric
distribution lines and for all aerial crossings with electric
transmission lines, in span crossings may be used. For all in span
crossings, protection of the telephone plant depends primarily on
adequate structural strength and clearances, which in some cases may
require putting the telephone plant in underground conduit or using
buried cable.

**4.2** **[Bonding in Joint Use or Joint Occupancy]{.underline}:**

**4.2.1** Where a telephone cable is supported by the same poles used
for electric supply circuits of the MGN type, the cable shield and
suspension strand (messenger) should be grounded by bonding the strand
to the MGN as described in Paragraph 4.2.4. These bonding connections
should be made at the following locations:

**4.2.1.1** Where the joint use or joint occupancy arrangement begins
and ends;

**4.2.1.2** On every electric supply pole that carries a vertical pole
ground wire to which are connected transformers, capacitors or other
types of power equipment that draw load current under normal conditions;
and

**4.2.1.3** If the joint use or joint occupancy section is longer than
1/2 mile (0.8 km), bonds should be made to the MGN every 1/4 mile
(0.4 km). The NESC requires additional grounding considerations for
certain messenger sizes where the messengers are exposed to possible
power contacts, power induction, or lightning. If the ampacity of the
messengers are not adequate for system grounding conductors, grounding
of messengers has to be increased to intervals of eight per mile
(1.6 km).

**4.2.2** Where telephone cables are supported by the same poles used
for electric supply circuits of the non-MGN type, cable shields should
be grounded by means of their connections to the central office ground
and by such other additional grounds as necessary to satisfy the
frequency of occurrence described in Paragraph 4.2.1.3. Cable suspension
strand should be bonded to the vertical pole ground wire on poles
carrying vertical pole ground wires to which are connected transformer,
capacitors, or other types of equipment that draw load currents.

**4.2.3** Vertical pole ground wires on electric supply poles
interconnected to transformers or capacitor banks should be connected
directly to the power system neutral. The transformers or capacitor
banks should also have direct connections to the power system neutral.
At such locations visual inspection from the ground should be made,
before climbing, to ascertain that the vertical pole ground wire is
actually connected to the neutral. [If the vertical pole ground wire is
not connected, this fact should be reported to the power company; and
the wire should be regarded as energized. The pole should not be touched
or climbed by the telephone line workers until the condition has been
corrected by the power company.]{.underline}

**4.2.4** Where interconnection of the support strand (messenger) to the
MGN is recommended in 4.1 and 4.2.1, the interconnection should be
accomplished by the appropriate method for the conditions prevailing at
the pole in question as listed below:

**4.2.4.1** If the pole is already equipped with vertical pole ground
wire connected to the MGN, then a ground wire assembly unit (PM2A)
should be installed. A bonding conductor should be attached to vertical
ground wire by telephone construction personnel if it is satisfactory to
the power company.

**4.2.4.2** If the pole is not equipped with vertical ground wire, a
ground wire assembly unit (PM2A) should be installed and sufficient
slack left to permit the bonding wire to be extended to and connected to
the MGN if the pole in question is at the beginning or at the end of the
joint use section. Connection of the bonding wire to the MGN should be
made only by the power company. For intermediate bonds recommended by
4.2.1, a pole already equipped with a pole ground wire should be
selected and a ground wire assembly unit (PM2A) should be installed.

**4.2.5** In most instances, interconnection of the cable shield to the
MGN will result in a decrease in noise levels on the telephone system
because of the additional shielding effect provided by the neutral
conductor. In a few instances noise levels could increase if excessive
residual power currents flow in the shield as a result of bonding. This
situation is most likely to occur if the resistance of the neutral to
ground is relatively high. In such instances removal of a number of
bonds to the MGN to reduce the shield current usually will be
beneficial.

**5. MISCELLANEOUS SITUATIONS**

**5.1** **[Underground/Buried Cable Dips in Aerial Cable
Runs]{.underline}:** No special protection is required at the junctions
of aerial cable and short underground or buried plastic-sheathed cable
dips in aerial cable runs.

**5.2** **[Protection at Loading Points]{.underline}:** Loading coils
meeting RUS specification PE-26 are designed such that no supplementary
protection is required in addition to the measures recommended herein
for protection of the associated cable.

**5.3** **[Buffer Protection]{.underline}:** Where a good shield ground
such as a MGN of approximately 25 ohms or less cannot be obtained at or
within 200 feet (60 meters) of a cable-noncable junction, the beneficial
effect of such a ground may be achieved by placing buffer protection in
the form of yellow coded arresters between the non-cable pairs and
ground at a point about 1500 feet ± 1000 feet (460 meters ± 300 meters)
from the junction, [provided a ground of approximately 25 ohms or less
can be obtained at that point]{.underline}.

**5.4** **[Pole Lightning Protection Wires]{.underline}:** Lightning
protection wires may be necessary to prevent the splitting of wood poles
used for cable supports in certain areas of high lightning incidence and
severe exposures. RUS borrowers should make a study of local conditions
to determine to what extent this protection is required. Normally,
extensive use of lightning protection wires is necessary in the shaded
areas of the map on Figure 1, [Lightning Damage Probability
Map]{.underline}, and in unshaded areas which have more than
60 thunderstorm days per year. In systems within areas affected by high
levels of lightning damage and where local experience clearly indicates
the need, lightning protection wires should be installed on poles which
are severely exposed because of being on or near the top of a hill with
little or no shielding such as buildings, trees, or a higher foreign
pole line. In hilly areas, installation of protection wires on a number
of consecutive poles is desirable. With flat terrain where the exposure
is more uniform and less severe, protection wires should be installed on
every third or fourth pole. In the unshaded areas of the map in
Figure 1, which have less than 30 thunderstorm days per year, pole
lightning wires are not necessary. (Note: RUS Borrower areas not shown
on Figure 1, such as Alaska, Hawaii, and the Federated States of
Micronesia, Guam, Puerto Rico, Republic of Marshall Islands, Republic of
Palau, and the Virgin Islands are considered low probability lightning
damage areas.)

**6. ELECTRICAL PROTECTION FOR BURIED/UNDERGROUND CABLE**

**6.1 [Plastic-Insulated Conductor (PIC) Cable and]{.underline}**
**[Wire]{.underline}:** Polyethylene insulated conductor and jacketed
cable and wire require a minimal of electrical protection because of
their inherent high dielectric strength and quality. Difficulties caused
by dielectric failure have been reduced to a minimal amount through the
extensive application of PIC cable and wire. Power cross, lightning
surge, and other sources of unwanted high currents on the telephone
plant can, at times, cause cable or wire conductors to fuse. While finer
gauge conductors are more susceptible to this type of damage, the
economic advantage in their use will usually outweigh the higher rate of
fusing incidence as compared with coarser gauge facilities.

**6.2 [Lightning Protection]{.underline}:** Buried cable and wire are
not normally as susceptible to damage from lightning surges as other
categories of outside plant. However, buried plant may be subjected to
lightning damage in one of the following ways: (1) by direct strokes to
the shield or strokes-to-ground which are eventually directed to the
shield; (2) by surges conducted to the buried plant from connecting
facilities which are struck; (3) by induction from nearby
strokes-to-ground; and (4) by currents developed because of a rise in
ground potential at stroke points near grounded station protectors or
other grounded facilities. While the dielectric strength of PIC cable
will withstand most of the surges occurring from items 2, 3, and 4
above; direct lightning strokes will normally cause damage to the cable.
The damage is caused by the magnitude of currents involved. However,
direct strokes to buried cable occur infrequently.

**6.2.1** Current surges which reach buried and underground cables from
conductors of connecting plant such as buried line wires, buried
services, or aerial non-cable type plant, are usually confined to the
conductors by the high dielectric strength of the plastic insulation on
the conductors and the inner jacket. Frequently the exposed connecting
conductors have a coarser gauge than the buried cable conductors to
which they connect. Both the coarser gauge of connecting conductors and
the high dielectric strength of the plastic insulations tend to increase
the probability of fusing the cable conductors.

**6.2.2** The use of buried fine gauge cable conductors in telephone
systems has resulted in conductor fusing becoming more prevalent than
dielectric failure.

**6.2.3** As noted previously, loading coils meeting RUS specification
PE-26, are designed to withstand substantial lightning surges without
damage. Present designs will withstand current surges which approach the
fusing current of 28 AWG wire. While this fusing current is less than
that of 24 AWG cable, it is large enough that only a small percentage of
surges reaching cables will cause loading coil damage. Therefore, gap
protection of loading coils in buried plant is not normally required. In
areas susceptible to very severe lightning damage, protection for
loading coils can be added at a later date if needed. Where protection
of loading coils is necessary, standard or heavy duty gas tubes with a
slowly rising dc breakdown voltage of approximately 350 Volts should be
used. It is recommended that these gas tubes be connected in a
longitudinal non-grounded bypass tube configuration, as discussed in
TE&CM Section 822, \"Electrical Protection of Carrier Equipment\",
(proposed conversion to Bulletin 1751F-822).

**6.3 [Power Contact Protection]{.underline}:** Cable and wire
facilities which are actually buried in the earth are not considered as
being exposed to power line contacts except those buried in the same
trench with power distribution cables. Protective measures are,
therefore, not required for buried plant except for those which may be
required for the protection of connecting facilities, subscriber
stations, and exposed aerial inserts as prescribed in practices covering
such plant and as supplemented herein. Aerial inserts in buried plant
are considered as being exposed when the possibility exists that they
could be contacted by power line conductors operating in excess of
300 volts to ground. Where joint buried construction is involved,
protection measures should be applied as described in TE&CM Section 640,
\"Design of Buried Plant-Physical Considerations\", (proposed conversion
to Bulletin 1751F-640).

**7. LIGHTNING PROTECTION FOR BURIED/UNDERGROUND PLANT**

**7.1** **[Dielectric Protection of Buried/Underground
Plant]{.underline}** is limited to the following measures: (1) gas tube
or equivalent arresters at junctions with facilities serving severely
exposed stations and (2) bonding and grounding of buried wire and cable
shields.

**7.2** **[Facilities Serving Severely Exposed Stations]{.underline}:**
Terminal blocks equipped with standard or heavy duty gas tube arresters
having dc breakdown voltages of 800 volts or greater and installed in
suitable mountings and enclosures are recommended at junctions between
buried wire or cable and facilities serving stations that are severely
exposed to lightning (such as fire towers and radio towers) regardless
of the length or type of connecting facilities. Yellow coded (10 mil)
(0.3 mm) carbon blocks may be used as an alternative to gas tubes.

**7.3** **[Bonding/Grounding of Buried Wire and Cable
Shields]{.underline}:**

**7.3.1** Complete bonding of shields of buried cable and wire to
maintain electrical continuity of shields throughout the buried plant
and the connections from buried plant to aerial plant, central offices,
and subscriber station protectors is recommended. Grounding of buried
cable and wire shields is advantageous from lightning protection
considerations and is a requirement of the NESC (to effectively ground
non-current carrying metallic plant).

**7.3.2** The shields of all buried wires and cables entering a central
office should be bonded to each other and interconnected with the
central office ground bus or grounding conductor. This bonding
eliminates harmful differences of potential between the various cables
entering the central office. RUS requires special bonding and grounding
at central offices. See Paragraph 3.2.1 of this bulletin for some of the
basic considerations of the special grounding and bonding that RUS
requires at central offices. For more specific details, see TE&CM
Section 810 (proposed conversion to RUS Bulletin 1751F-810).

**7.3.3** At junction points the shields and support wires of
interconnecting facilities should be bonded to the metallic shield of
the main buried cable or wire. The grounding bracket in terminal
housings should be used to facilitate making these connections. The
grounding bracket should be connected to a ground rod with at least a
#6 AWG grounding conductor. The shield of buried service wires should
likewise be bonded to the metallic shield of the main cable and
connected at the subscriber end to the station protector grounding
terminal.

**7.3.4** The use of pulse code modulation (PCM) carrier systems and the
concern for near end crosstalk (NEXT) has led to the development of a
cable in which the pairs are divided into two bundles which are isolated
from each other by a \"screen\" designed to reduce the crosstalk from
one bundle to the other primarily by reducing the capacitance unbalance
between pairs. Because this screen does not have the same insulation as
the cable shield, the screen should not be treated as a shield where
electrical protection is concerned. [The screen should not be connected
to ground at any point and it should deliberately be made electrically
discontinuous at pedestals.]{.underline}

**7.4** Protection of connecting facilities having appreciably lower
dielectric strength than plastic-insulated conductor (PIC) cable and
wire is required as follows: (1) arresters at the central office, and
(2) arresters at carrier repeaters and terminals.

**7.4.1** Arresters at the central office provide main frame arrester
protection on all cable pairs entering the central office.

**7.4.2** Carrier repeaters and terminals and most other electronic
equipment used on cable pairs have low dielectric strength compared with
the dielectric strength of cable pairs. Such equipment should,
therefore, be protected in accordance with RUS TE&CM Section 822
(proposed conversion to RUS Bulletin 1751F‑822).

**8. POWER CONTACT PROTECTION FOR BURIED/UNDERGROUND PLANT**

**8.1 [Aerial Inserts]{.underline}:** Aerial inserts may consist of
strand supported buried cable, underground cable, aerial cable, or wire
supported buried wire. When an aerial insert of any type is exposed to
power contacts by being joint with, crossing under, or otherwise subject
to possible contact with power conductors operating at voltages in
excess of 300 volts, the aerial insert should be considered as exposed.
Support strands (messengers) or wires of exposed aerial inserts should
be effectively grounded by connecting to an MGN. If an MGN is not
available, effective grounding by driven electrodes or other means is
necessary. The support strand (messenger) of aerial inserts should be
grounded in accordance with the NESC at four or eight times a mile
(1.6 km), depending on the conductivity of the support strand
(messenger). (See Rule 215C3 of the NESC and Paragraph 4.2.1.3 for
details). Because RUS has standardized on use of #6 AWG copper grounding
conductors, seven wire steel strand messengers of 1/4\" (6.4 mm) and
5/16\" (7.9 mm) should be grounded 8 times each mile (1.6 km) while
3/8\" (9.5 mm) and 7/16\" (11 mm) should be grounded four times each
mile (1.6 km).

**8.1.1** The conductors of exposed aerial inserts in buried wire or
cable should be isolated from the buried portions on both sides of the
insert in order to maintain the unexposed status of the buried plant.
Isolation of the conductors of exposed aerial inserts is necessary so
that fuseless-type station protectors may be used and isolation may be
accomplished by any of the following procedures:

**8.1.1.1** [If the conductors of the buried plant on both sides of the
aerial insert are 24 AWG copper or smaller, no fuse links are
required]{.underline}. Buried type wire or cable may be used for the
aerial insert following construction practices for aerial cable.

**8.1.1.2** If the conductors of the buried plant are coarser than
24 AWG copper, the preferred construction is to install 24 AWG cable as
the aerial insert. The 24 AWG insert then provides the necessary fusing
coordination with the connected stations on both sides of this insert.
Where the aerial insert extends for more than a few spans, the effect of
the use of 24 AWG on transmission should be checked.

**8.1.1.3** If the conductors of both the buried plant and the aerial
insert are coarser than 24 AWG, isolation of the exposed aerial insert
should be accomplished by providing 24 AWG copper fuse links between the
aerial insert and the buried portions of the cable. These fuse links
should be applied at both ends of the exposed insert, as shown in
Drawings 951 and 952 of RUS Bulletin 345-150, RUS Form 515a. In order to
minimize the cost of isolating aerial inserts, the 24 AWG links should
usually be installed in terminal housings that are required for other
purposes at points nearest to each end of the aerial insert. Color-coded
24 AWG leads at least 8 inches (20.32 centimeters) long should be
spliced in series with each conductor of the cable between the end of
the buried portion of the cable and the adjoining end of the aerial
insert. The color code should be preserved. If subscriber\'s services or
buried wire branch tap leads are distributed from the buried plant
terminal housings involved, the leads should be connected to the
unexposed side of the fuse link. Exposed aerial taps should be connected
to the exposed side of the fuse links. The shields of all cables or
wires in the buried plant terminal housings should be bonded together
and grounded to the same ground electrode to which the support strand
(messenger) is grounded. A typical exposed aerial insert is shown in
Drawing 951 of RUS Bulletin 345-150.

**8.2 [Aerial Line Extensions]{.underline}:** All aerial noncable-type
line extensions from buried plant should be connected to the buried wire
or cable through a wire link capable of safely fusing at currents less
than the ampacity (current carrying capacity in amperes) of the
subscriber\'s service. The 24 AWG copper leads connected to terminal
blocks or 20 AWG bridle wires are adequate for this purpose. The term
\"noncable-type circuits\" as used here refers to wire circuit
facilities not enclosed in a metallic shield. Buried wire is considered
to be a \"cable type\" facility.

**8.2.1** Where buried plant facilities are extended by exposed aerial
wire facilities, power contact protection should be applied to the
aerial facilities in accordance with applicable RUS practices. Where the
applicable practices require protection devices at the junction with
buried plant, the low impedance ground required for the power protection
device should be utilized to provide additional lightning protection to
the buried plant. A low impedance path to ground can be accomplished by
bonding the buried cable or wire shield to the grounding conductor or
ground electrode of the power contact protector.

**8.3 [Aerial Drop Wires Connected to Buried Plant]{.underline}:** Where
economically feasible, the connection of aerial drop wires to buried
wire or cable should be avoided in favor of buried services.

**8.3.1** Aerial drop wires exposed to the possibility of contact with
electric distribution facilities operating in excess of 300 volts to
ground should be connected to the buried facilities through a 24 AWG
copper fuse link or a 20 AWG. This fuse link is recommended as a
protective measure to prevent large fault currents from reaching other
stations served by the buried wire or cable. It will also prevent damage
to coarser gauge buried facilities. The station being served should be
protected in accordance with applicable station protection practices.

**9. GROUNDING OF BURIED WIRE AND CABLE SHIELDS**

**9.1** **[Grounding at Junctions]{.underline}:** As previously
indicated, grounding of buried wire or cable shields at junctions with
other types of facilities is recommended by RUS and it helps in
compliance with the NESC requirements to effectively ground non‑current
carrying metal facilities.

**10.** **METALLIC HOUSINGS ON VERTICAL POWER POLES**

**10.1** **[Bonding]{.underline}:** When a metallic buried plant housing
is mounted on a power pole, the grounding conductor of the housing
should be bonded with at least a #6 AWG bare copper wire to the vertical
pole ground wire, if present, on the pole. The purpose of this bond is
to maintain the ground wire and the buried plant housing at the same
potential, thereby preventing a shock hazard that otherwise might exist
during a fault condition on the power line.

**10.2** **[When Carrier Equipment is not Grounded]{.underline}:** With
certain types of cable carriers, the carrier equipment manufacturers
have recommended that the carrier equipment not be connected to an
electric system ground. In such instances carrier equipment housings
and/or metallic buried plant housings enclosing carrier equipment should
be bonded to vertical pole ground wires as recommended in Paragraph
10.1, but the carrier circuitry and chassis should be isolated from the
metallic housing by insulation having at least 20 kV dc dielectric
strength. The provision of this dielectric between the carrier circuitry
and the housing makes it possible to use floating by-pass protection or
protection grounded to a remote separate ground, and still maintain the
buried housing at the same potential as the vertical pole ground wire.
It is the responsibility of the carrier equipment suppliers to provide
the 20 kV dielectric strength between the carrier circuitry and chassis,
and the metallic housing. See TE&CM Section 822 (proposed conversion to
RUS Bulletin 1751F-822) for additional details on carrier protection.

**11. EFFECTIVE GROUNDING OF CABLE SHIELDS**

**11.1** **[General]{.underline}:** Application of an effective
grounding system is recommended for all locations. Both the National
Electrical Code (NEC) and the National Electrical Safety Code (NESC)
cite a 25 ohm resistance-to-ground for grounding systems. See
Section 250-84 of the NEC and Rule 96B of the NESC. Note by attempting
to obtain at least four grounds per mile (1.6 km), attention to
obtaining 25 ohms at individual grounds is not necessary (except at
special equipment sites) as the multiplicity of grounds helps to achieve
an overall low impedance to ground.

**11.2** **[The Purpose of Grounding Cable Shields]{.underline}** is to
protect telephone plant from the effects of ground potential rise (GPR)
caused by power system faults. Grounding telephone cable shields helps
to direct excessive voltages and currents induced on the shields to
earth. This can often be achieved before these currents and voltages
reach the location of plant or equipment requiring protection.

**11.3** **[The Application of an Effective Grounding
System]{.underline}** can increase the flow of current in the shielding
circuit and help to reduce noise. The shield should be continuous with
no opens or bonding problems, so that the maximum benefits of effective
grounding can be realized.

**11.4** **[Isolating Damage Caused by a Lightning
Stroke]{.underline}:** An effective grounding system can isolate damage
by dissipating the current through multiple paths to ground along the
cable shield. Because of the high magnitude of current in a lightning
strike and the associated GPR, it is not feasible to protect the entire
telephone plant from damage. As a result it is desirable to isolate
damage from a near or direct lightning strike to the least plant length
as possible.

**11.5** **[Obtaining a 25 Ohm Ground]{.underline}** Provision of a
ground at every location with a resistance-to-ground of 25 ohms or less
cannot always be accomplished with a 5 foot (1.5 meter) ground rod. Use
of a longer rod or multiple rods connected together may be necessary.
This is especially true in areas of the country where there is an
extremely high earth resistivity. In many areas of the country the
winter frost line exceeds 18 inches (45.7 cm) or more. In such areas of
the country use of eight foot (2.4 meter) rods should be made standard
practice.

**12. EFFECTIVE GROUNDING THEORY**

**12.1** **[Effective Grounding]{.underline}** is based on the theory
that multiple grounds along a cable shield will provide a low
resistance-to-ground. This low resistance-to-ground provides for the
dissipation of high voltages and currents induced or conducted on the
cable shield.

**12.2** **[Spacing of Electrodes]{.underline}:** For protection
purposes, it is desirable to have the earth electrodes spaced every
quarter mile (0.4 km) but this is not practical in most buried cable
plant. In aerial plant, especially in situations of joint use or joint
occupancy, grounding of messengers at four or eight times a mile
(1.6 km) is required by the NESC and is most beneficial because the
lower the value of effective resistance-to-ground, the better the
overall system will perform during power cross situations.

**12.3 [Selection of Electrode]{.underline}:** Once the earth
resistivity at a location has been determined, selection of the proper
electrode can be made. A 5 foot (1.5 meter) rod is normally used. If the
earth resistivity at the location is extremely high, an 8 foot
(2.4 meter) rod should be used. An 8 foot (2.4 meter) rod should also be
used in areas where the average frost line is 18 inches (45.7 cm) or
deeper.

[^1]: All references to the National Electrical Safety Code in this
    document pertain to the 1993 edition. Readers are urged to refer to
    the edition of the NESC and any, more stringent, local codes.

[^2]: ^2^ All references to the National Electrical Code in this
    document pertain to the 1993 edition. Readers are urged to refer to
    the edition of the NEC and any, more stringent, local codes.