Application Note - Rev. A
P/N 650-0002-01
August 1997
ECO No. 798
Installation Standards for Grounding Requirements
by Paul Linser
Introduction
This application note addresses the grounding requirements of modern Telecommunications Systems and
Facilities.
Proper ground connections are required for all parts of a Telecommunications System to ensure the
correct range of operation for ground-return circuits and prevent signal degradation caused by distortion,
severe crosstalk, and noise interference. Proper ground connections also prevent dangerous voltage
potentials that can injure personnel and damage equipment. The point where a telecommunications
facility or system makes metallic contact to earth is commonly referred to as station ground or site ground,
but is sometimes referred to as man-made ground, buried ground, or main ground. This contact point is
where the equipment ground, the cable shielded ground, and the signal ground are tied together.
This application note is divided into five sections:
•
Section 1 Equipment and Signal Grounds
•
Section 2 Interior Signal Ground Network
•
Section 3 AC Protective Network (Equipment Fault Protective Network)
•
Section 4 Tower, Waveguide Bridge, and Waveguide Grounding
•
Section 5 Grounding Practices for Shielding Buildings and Rooms
Section 1
Equipment and Signal Grounds
Equipment ground, sometimes referred to as the protective grounding system, is required to connect all of
the following equipment:
•
Chassis
•
Equipment cabinets and racks
•
All ferrous shields and covers
•
All conduits
•
Raceways
•
Cellular floor shields and transformer cases
•
All local signaling power supplies
1
The signal grounding system—composed of insulated conductors—connects the ground side of all loop
signal supplies and all the nonferrous shields of signal and signal control cables.
Normally, facility grounding systems are installed during the original construction of a facility. Either a wire
grid or a metal plate—usually copper—is imbedded in the foundation when the concrete is poured. This
wire grid or metal plate is then connected to an underground metal ring (that surrounds the facility) with
vertical metallic poles extending up from the foundation. The actual ground wires are then connected to
the vertical metallic poles.
For a facility without an installed grounding system, earth grounds may be made by driving metal rods into
the ground to form an electrical connection to the earth. The metal rod must be a good electrical conductor, capable of resisting corrosion, and have sufficient area in contact with the soil so that the ground
resistance is within the rated limits of 5 ohms. If ground resistance is greater than 5 ohms:
1. Drive the rod deeper into the earth.
2. If the grounding resistance is still greater than 5 ohms, install additional rods spaced at least 6 feet
apart, and connect them in parallel.
3. Treat the soil with magnesium sulfate.
Caution: The use of salt is not recommended because of its corrosive effect.
4. Place a 4-foot-long, 8-inch diameter tile pipe in the ground approximately 4 inches from the ground
electrode. Fill the pipe, within one 1 foot of ground level, with a magnesium and water solution—see
Figure 1. Initially 40 to 90 pounds of chemical will be required to retain an effective ground treatment
for two or three years.
Figure 1 - Chemical Treatment of the Earth Around a Given Electrode
Copperweld 5/8”
Steel Ground Rod
Magnesium Sulphate
and Water
Not Less Than 8’-0”
Removable Cover
with Holes
Approx
4’-0”
Approx
4”
Approx
8”
RT260901
Shielded signal wiring is connected to ground using the braided shield. After properly butting the signal
cable and exposing the braid, the installer forms a pigtail or connection-to-ground.
2
Measuring Ground Resistance
Ground resistance measurement requires two reference rods in addition to the grounding rod or plate
being tested. Measure ground resistance by using the triangular method or the fall of potential method.
The triangular method measures the series resistance of each pair of grounding rods. Reference rods are
installed at least 25 feet from the grounding rod under test, and 25 feet from each other—see Figure 2.
in Ω A
im B
um
25
M
’
5
BC 2
Ω um
m
ini
M
’
Figure 2 - Triangular Method of Measuring Resistance of Earth Electrode
Ω AC
Minimum 25’
Ground
Under Test
RT263901
Use a Megger to obtain the value of resistance between ground rods A & B, A & C, and B & C—see
Figure 3.
Results from the triangular method are not relevant if the reference grounds have a resistance of more
than ten 10 times that of the ground under test. Compute ground resistance with the following equation:
(resistance AB) + (resistance AC) – (resistance BC)
resistance
A =2
Where: A is equal to resistance of ground under test; B is equal to resistance of reference ground, and C
is equal to resistance of reference ground.
3
Figure 3 - Use of Megger Ground Tester
Direct Reading
Scale in Ohms
Turn Crank
at 100 RPM.
G
C
P
Ground
Under Test
A
Reference
Ground
B
RT263902
The fall of potential method measures a known current that passes between the current return electrode
and the ground being tested.
Conduct the fall of potential method as follows:
1. Place the potential measuring electrode midway between the electrodes—see Figure 4.
Figure 4 - Fall-of-Potential Method - Field Setup
To A-C Power Supply
Variable Res.
Transformer
A
Test Leads
Test Leads V
P
Movable
G1
Fixed
P Sets Various Points in a Straight Line
Between G1 and the Ground Mat Under Test.
Ground Mat
Being Test
Distances 5 to 10 Times the Maximum Diagonal Dimension
of Ground Mat, But Never Less Than 100 Feet.
RT263903
2. Place an ammeter across the known source.
3. Place a voltmeter between the potential measuring electrode and the ground being tested.
4. Record values and compute resistance from the equation R = E/I.
4
Grounding Connecting Cables
To connect grounds, use the following standard insulated cables:
•
Signal Grounds, 6 to 12 AWG
•
Equipment Grounds, 8 AWG
•
Protective Grounds, 4 AWG
•
Power Grounds, 4 to 8 AWG (depending on application)
See Table A for recommended Ground Reference Wire Sizes for DC Power Supplies and Battery Facilities.
Table A - Ground Reference Wire Size for DC Power Supplies and Battery Facilities
Full Load Power Supply or
Battery Facility Output
Rating (Amps DC)
Recommended Wire Type and Size
All Conductors are Copper
(AWG or equal)
0-25
No. 12 solid/stranded/insulated, Yellow*
25-100
No. 6 solid/stranded/insulated, Yellow*
100-200
No. 4 solid/stranded/insulated, Yellow*
200-400
No. 2 stranded/insulated, Yellow*
400-800
No. 0 stranded/insulated, Yellow*
800
No. 2/0 stranded/insulated, Yellow*
* If the DC reference wire exits the building to the exterior earth ground electrode
network, the minimum wire size should be No. 4 AWG. Proper splices are permitted.
RT278901
5
Section 2
Interior Signal Ground Network
Introduction
The Signal Reference Ground Network is designed to:
•
Control static charges
•
Establish a common reference for signals between sources and loads for proper operation
•
Minimize interference
The Signal Reference Ground Network can be configured to serve a facility:
•
Operating in the low frequency or low data rate range
•
Operating primarily at the higher frequencies or high data rates
•
Where both low and high frequency and data rate equipment operates
Composition
The Signal Reference Ground Network is a composite of various parts. The configuration of these networks depends on the following:
•
The frequencies and types of signals involved
•
The functions being performed by the equipment
•
The signal amplitude between the communications wires or in the cables
Modern telecommunications systems seldom include a single ground network capable of satisfying all
grounding requirements with optimum effectiveness, while preventing coupling or conduction of undesirable currents and voltages into the network. To minimize interference from the many potential sources,
separate networks are required at complex sites. These networks consist of the following:
•
VF (Voice Frequency) signal and shield ground
•
Switching DC signal and shield ground
•
Power supply (station and switching DC) reference ground conductors
Keep VF signal and shield ground conductors separate, so that the VF shield ground wire can be directly
connected to a known low noise point—such as the exterior earth electrode network. Likewise, keep the
switching DC signal and shield ground conductors separate to prevent the electrical noise of the switching
DC shields from being conducted to lower-level VF signal equipment. High-level DC switching shields are
especially noisy and should be bussed to the earth ground electrode system separately.
Some facilities—such as a telephone exchange—might consist of a single interconnected network where
all components form one complex ring/grid network. Such a network spreads electrical noise over many
parallel paths, reducing its overall effect on the communication facility.
Keep electrically noisy equipment such as telephone switching, switching DC, and DC power separate
from susceptible, low-level VF equipment. Separate ground networks are required with a common interconnection only within the earth ground electrode network.
6
Separation of Signal and AC-Protective Ground Conductors
The Signal Reference Ground Network must be separate from the AC power distribution system to the
extent possible with current communications equipment.
Never ground the AC Neutral to the signal reference ground. The AC-protective connection at the equipment cabinet is a design limitation: expectations are that future equipment design will provide separate
protective and signal ground points. DC-powered equipment requires no AC-protective grounding.
Minimizing Electrical Noise
Signal Reference Ground Networks should be designed and installed so that interference coupling can be
controlled. However, not all system and circuit noise problems will be solved by correctly designed and
installed ground systems, whether they are low- or high-frequency systems. Measures such as reducing
the levels of the interference sources through relocation, insulation, shielding, filtering, or alternate design
and installation techniques are more effective in limiting electrical noise to a non-interfering level. Communications equipment can also be designed to have high-noise immunity using the following methods,
methods which are currently being applied to new equipment designs:
•
Balanced inputs and outputs
•
Twisted, shielded pairs
•
Higher-level signal operation
7
Single-Point Signal Grounding Networks
See Figure 5 for a configuration of a Single-Point Signal Reference Grounding Network.
Single-Point Signal Reference Grounding Networks are useful inside equipment and for low frequency
and DC switching grounding applications. For this plan, low frequencies include those below 30 kHz.
Single-point grounding consists of a main ground point at a central location to provide a single point of
interconnection among all grounded components.
Figure 5 - Example of Single-Point Signal Grounding Network - Profile View
Lightning Rod
(3 PLs-TYP)
C-E Eqpt.
Lightning Rod
Downlead
(2 PLs-TYP)
Sig Gnd
Sig Gnd
Note
Sig Gnd
C-E Eqpt.
AC Pwr PNL
Sig. Gnd
Eqpt Wires
To Other
Equipment
Locations
AC Prot.
Wires
AC Pwr
Dist PNL
Sig. Gnd
BR Wires
Main AC Pwr
Entry PNL
Sig. Gnd
Box
Sig. Gnd
Trunk Cable
To AC
Pwr House
Ground Rod
(3 PLs)
Continuation
of Earth Gnd
Conductor
Earth Ground
Conductor Network
RT263904
Note: This method requires separate signal and AC protective ground points within the equipment.
8
Multiple-Point Signal Grounding Networks
See Figure 6 for an example of a Multiple-Point Signal Grounding Reference Network—a high frequency
signal reference subsystem. This multiple-point grounding network utilizes many conductive paths from
the earth electrode network to the various electronic facilities and equipment within the site. Thus,
numerous parallel paths exist between any two points in the facility’s ground network.
Figure 6 - Example of Multiple-Point Signal Grounding Reference Network - Oblique View
#6 AWG. STR.
INS. Copper Wire
Interior Ring GND
#2 AWG. STR. INS
Copper Wire
#8 AWG. STR.
INS. Downleads
Lightning Rod
(4 PLs TYP)
WG To
Tower
Lightning Rod
Downlead
(4 PLs TYP)
Main AC
Power
Entry Panel
GND Rod
(Every 20’-25’)
#2 AWG. STR. INS
Copper Wire
AC Prot.
Wires
AC Prot. to
AC Pwr House
2’4’
Earth Ground
Electrode Network
RT263905
The multiple-point configuration is very effective in minimizing potential differences between RF and DC
equipment. It is also useful in minimizing the effects of RFI (Radio Frequency Interference). This network
generally employs two concentric rings. An exterior earth electrode ring network surrounding buildings,
towers, antennas, etc., is used in conjunction with an inner ring inside each building that surrounds
communications equipment in the building interior. Multiple bonds between exterior and interior rings—
plus connections to antenna towers and metallic objects both in the ground and in the building—form an
equal potential plane. By design, most communications facilities are of the multiple-point variety. Numerous metallic contacts between equipment cabinets, racks, ducts, cable trays, conduits, and their supports
from the building structural elements, plus the AC power grounding interconnections, comprise the
multiple paths to ground.
Building structural metal, cable tray, ladders, and ducts can be bonded in an approved manner to augment the deliberate copper wire network—especially in new construction, where the necessary bonding of
structural metal can be assured. In older facilities, the communications electronics equipment and cable
supports can still be used to fill in the deliberately installed ground wire networks.
9
Composite Single and Multiple-Point Grounding Networks
In many facilities, both high- and low-frequency equipment operates in the same general area. In such
facilities, a signal reference network is required which consists of a hybrid of both configurations. If
engineered and installed meticulously, an isolated single-point Signal Ground Network can be provided
for low-frequency or electrically noisy equipment, and a high-frequency ring/grid can be provided for VF,
video, and microwave equipment. In a hybrid system, the earth electrode network serves as the tie point
between the single- and multiple-point ground networks. See Figure 7 for an illustration of a composite
Signal Grounding Network.
Figure 7 - Example of Composite Signal Grounding Network - Top View
WGs to Tower
VF Sig & Shield
Gnd Box*
VF Sig Gnd Grid
VF Shield
Gnd Wire
Mux Mux Mux Mux MW
VF Patch
Radio
VF Line Cond. & Test
CDF
Gnd Rod
VF Video & MW Eqpt.
Earth Gnd
Electrode
Network
TTY Shield
Gnd Wire
TTY Sig
Gnd Grid
CCFB
TTY Eqpt.
TTY Sig & Shield
Gnd Box*
* Optional
RT263911
Note: (1) The AC protective wires are omitted for clarity. (2) The TTY equipment must be insulated from
the VF, video, and microwave equipment except for the common earth ground connection.
10
Separation of the Various Ground Conductors
Regardless of which signal reference configuration is used, certain equipment operating at high levels will
require special attention to minimize electrical noise. Separate busses for VF and switching DC signal and
shield grounding are advisable. All switching DC equipment generates a significant amount of electrical
noise during normal operation. A separate switching DC signal and shield ground bus grounded directly to
the earth electrode network will be effective in keeping much of the electrical noise off the VF signal
reference ground network. In addition, cable shields from switching DC equipment should be taken to the
earth ground electrode separately.
In a facility containing VF and switching DC equipment in the same general area, the equipment should
be physically separated into two groups:
•
VF and any associated video and/or microwave equipment in one group
•
Switching DC equipment in another group
Placing the equipment in two groups will facilitate separate equipment grounding.
Install separate insulated VF and switching DC ground networks: a grid for the VF and associated equipment, and a separate bus for the switching DC and shields. The switching DC bus can be connected to
the earth electrode at more than one point if required. Avoid direct metal contacts through cable trays,
ducts, or metal flooring (false floor).
Certain low-level circuits may also require special attention. For example, a separate VF cable shield
ground bus can provide lower overall electrical noise on shields if the point at which the bus is connected
(such as the earth electrode subsystem directly) has a lower overall electrical noise level than the general
Signal Ground Network.
11
DC Power Equipment
DC power equipment—such as rectifier-chargers, DC-AC inverters, and DC-AC converters—produce a
lot of electrical noise. This equipment requires only a protective ground conductor to the main AC circuit
breaker, an intermediate panel, or the panel that powers the equipment, in order of preference.
Caution: Never connect DC power equipment to signal ground conductors.
For best results, the DC power equipment should be physically and electrically separate (insulated) from
the communications equipment. Whenever possible, the DC power equipment should be located in a
separate building, a basement, or a separate room in the communications facility. Note that DC power
equipment is acoustically, as well as electrically, noisy.
DC power equipment is grounded for safety only by the AC protective wire. The effect of electrical noise
conducted by the AC protective wire can be minimized by connecting it to one of the following (in order of
preference):
•
The AC protective bus bar in the main AC entry panel
•
An intermediate power distribution panel away from the communications equipment
•
The AC panel supplying the DC power equipment
Where the DC power and communications are in separate areas, the AC power panel supplying the DC
power equipment should be used.
The voltage return load bus bar (plus a bar for a negative facility) must be insulated from the equipment
rack or cabinet. Connect the ground reference wire to the common distribution point closest to the loads
or at one central load which is not likely to be removed. At the other end, connect the ground reference
wire directly to the exterior earth ground electrode network.
The DC ground reference conductor should have a green/yellow insulating covering or be color coded
with green/yellow plastic tape at key points to distinguish it from other ground conductors. Do not connect
any other conductors to the DC ground reference wire. Base the size of the DC ground conductor on the
capacity of the power supply—see Table B.
Table B - DC Ground Conductor Size
Full Load Power Supply or
Battery Plant Output Rating
(Amps DC)
Recommended Wire Type and Size AWG or equal (all conductors are copper
and marked green/yellow)
0-25
AWG 12 solid or stranded/insulated*
25-100
AWG 6 solid or stranded/insulated*
101-200
AWG 4 stranded/insulated
201-400
AWG 2 stranded/insulated
401-800
AWG 0 stranded/insulated
Over 800
AWG 2/0 stranded/insulated
* If the DC reference wire exits the building to the exterior earth ground electrode
network, use AWG 4 wire (minimum wire size). Proper splices are permitted.
RT278902
12
Additional filtering is generally required when the noise voltages (ripple, impulse, and wideband) across
the supply distribution points of a station VF power supply or battery plant exceed 100 mV p-p. Various
types of filters—capacitive, inductive, or LC—are used depending on the noise identified. For electrical
noise of mainly ripple, capacitive filtering is used; for wideband electrical noise, LC filtering is used. See
Figure 8, Figure 9A, Figure 9B, and Figure 10 which illustrate filter types commonly used.
Today’s technology is rapidly moving toward solid state filtering techniques. Filtering is especially important in large DC-AC inverter use as the inverters generate electrical impulse noise as high as 6 V p-p.
This noise must be isolated from the communications load(s).
Figure 8 - Typical 48V DC Capacitor Filter Panel for Installation in an Existing Fuse Distribution Rack
to Provide Additional Filtering of the Station 48 V DC Power Supply
Part of 48 V DC
Copper Bus Bar
Across Back of
Fuse Holders
Fuse
Panel
Alarm Bus
30 A
Slo-Blo
Fuse
1/4 A Type 70
Alm Fuse
(2 PLs.)
1/4 A
Fuse
30 A
Slo-Blo
Fuse
To Fuse
Alm Pnl
#6 Awg, Ins, Stranded Black
#12 Awg, Ins, Blk
-
-
-
-
-
-
-
-
-
-
+ C1
+ C2
+ C3
+ C4
+ C5
+ C6
+ C7
+ C8
+ C9
+ C10
#12 Awg,
Ins, Wht
#6 Awg, Str,
Ins, Wht
Part of 48 V DC Return Bus
Ear at Top of 48 V DC Fuse
Distribution Rack
RT263906
Note: (1) Construct using a suitable panel for rack mounting. (2) Install in fuse distribution rack. (3) C1 to
C10, 10,000 - 20,000 µF, 75 WV DC. (4) The capacitor filter is very effective for frequencies below
approximately 100 kHz.
13
Figure 9A - Typical LC and PI Filter
Fuse Alm
Typical
-48.8 V DC
Input
L1
Typical -48.5 V DC
@ 10 A Output
Attenuation:
20 dB @ 200 Hz
45 dB @ 4 kHz
15 A
10 mH
+
C1
20,000 uF
75 WV DC
RT263907
Note: Typical LC communications equipment decentralizing filter for loads up to 10 A DC. LC filters for
other voltages and larger loads of 25 A, 50 A, and 100 A are available or can be constructed. The LC
filter should be used where an input capacitor already exists and wideband (high -and low- Frequency) noise filtering is required.
Figure 9B - Typical LC and PI Filter
Fuse Alm
Typical
-48.8 V DC
Input
L1
30 A
Typical -48.5 V DC
@ 25 A Output
C1
+
5 - 20 mH
10,000 - 20,000 uF
75 WV DC
C2
10 Hz to 25 MHz
Noise < 100
mV p-p
RT263908
Note: Typical 48 V DC P1 communications equipment decentralizing filter for loads up to 25 A DC. P1
network filters for other voltages and all types of loads are available or can be constructed. The P1
network filter is used where no input capacitor exists or greater effectiveness is required.
14
Figure 10 - Typical 48 V Filtering of Large Inverters
Control Rack
Inverter Cabinet
DC Filter #1 (Noise 2)
Load
Bars
(Note 1)
+
-
150 A
L1
3-5 mH
5-kVA Inverter
(Main)
Slo-Blo
Fuse
C1 C2 C3 C4
(Note 3)
DC Filter #2 (Noise 2)
150 A
L1
3-5 mH
5-kVA Inverter
(Hot-Standby)
Slo-Blo
Fuse
C1 C2 C3 C4
(Note 3)
RT263909
Note: (1) This additional filtering is only required if the electrical noise on the common load bars is
excessive as determined by communications equipment operation. Generally, 1000 mV p-p is permissible where decentralizing filters are also used for the sensitive equipment. (2) Each inverter input
must be filtered separately. There may be enough stored energy in filter inductor L1 to blow the fuse
of the standby inverter in case of failure (open or short) of the main inverter if the two inputs are
connected to one filter. (3) C1 ,C2 , C3, C4 -- 35,000 µF, 75 WV DC.
15
Interior Signal Grounding Conductor Types and Colors
Interior signal ground wires should be copper, solid or stranded, and insulated with green/yellow covering.
Yellow should be reserved for shield ground wires, and green for all protective wires. Existing wires need
not be replaced merely for the color of insulation. Instead, use colored tape around the existing wire at
key places to color-code signal, shield, and protective grounding wires. Make ground conductor interconnections with bronze, brass, or copper tap connectors or cross lugs. Where movement due to vibration or
contact with AC and DC power equipment or metal pipes is anticipated, insulate connectors with plastic
tape or covers. Make connections to equipment with terminal lugs and screws, or screws only when a
strap is used. Minimize the use of dissimilar metals.
If the possibility of corrosion exists because of dissimilar metals, use a compatible washer and suitable
covering between the dissimilar metals. Scrape equipment connections clean of insulation or corrosion to
expose bare metal prior to connection. If exposed to weathering, the joint should be covered with silicone
or another suitable covering after connection. See Table C, Table D, and Table E for metal compatibility
and ground conductor sizes for various exterior and interior grounding applications.
Table C - Strap and Wire Types and Sizes for Grounding Applications
Application
Range in Copper Wire
Sizes (AWG)
Typical Recommended
(AWG)
Outside Ring Grounds
#2 to #4/0
30 x 3.5 mm solid galvanized steel strap,
1 inch x 1/16 inch solid copper strap or
#2 solid tinned bare wire
Tower Lightning Rod Downleads
#4 to #1/0
#2 solid bare wire
Waveguide Grounding
#6 wire to 1-inch-wide
woven copper braid
1 inch x 1/16 inch solid copper strap or
#6 solid of coarse stranded wire, bare or
insulated
AC Protective Ground
#14 to 800 MCM
Solid or stranded, insulated, green
AC Neutral Ground Wires
#14 to 800 MCM
Solid or stranded, insulated, white or gray
Overvoltage Protector Ground
Wires
#8 to #4
#6 solid or coarse stranded, insulated,
green
Ring Ground to Inside Ground
Grid
#2 to #4/0
#2 solid or coarse stranded, insulated,
green or yellow
Main Inside Ground Wires (Ring)
#4 to #2/0
#2 stranded, insulated, green or yellow
Spurs from Main Ground (Grid)
#6 to #4
#6 stranded, insulated, green or yellow
Downleads from Main Inside
Ground to Cabinets or Racks
#10 to #6
#8 stranded, insulated, green or yellow
Inside Cabinet or Rack
#18 to #10
#12 solid or stranded, insulated,
green or yellow
CDF Ground Wire Harness
#14 to #10
#12 solid, insulated, yellow
Ground Wires to Terminal Blocks
on CDF
#18 to #12
#18 solid or stranded, insulated, yellow
DC Power Supply Reference
Ground Connectors
#12 to #4/0
Dependent on application
RT278903
16
Table D - Metals
Galvanic Series of Metals
Anodic End, Active,
Sacrificing - Cathodic End, Passive
Magnesium
Magnesium Alloys
Zinc
Aluminum
Cadium
Steel or Iron
Cast Iron
18:8 Stainless Steel (active)
18:8:3 Stainless Steel (active)
Lead Tin Solders
Lead
Tin
Nickel (active)
Incinel (active)
Hastelloy C (active)
Brass
Copper
Bronze
Copper Nickel Alloys
Monel
Silver Solder
Nickel (passive)
Inconel (passive)
18:8 Stainless Steel (passive)
18:8:3 Stainless Steel (passive)
Hastelloy C (passive)
Silver
Graphite
Gold
Platinum
Note: The metals indicated as active or passive
change positions depending on the nature of the
corrosive medium.
RT278904
17
Table E - Size of Equipment Grounding Conductors for Grounding Raceway and Equipment
Rating of Automatic Current
Device in Circuit Ahead of
Equipment, Conduit, etc., Not
Exceeding (Amps)
Copper Wire Size (AWG)
Aluminum or Copper Clad
Aluminum Wire Size (AWG)
15
14
12
20
12
10
30
10
8
40
10
8
60
10
6
100
8
6
200
6
4
400
3
1
600
1
2/ 0
Note: The source for this table is the National Electric Code Handbook.
RT278905
18
Section 3
AC Protective Network (Equipment Fault Protective Network)
The AC Protective Network is designed for protecting personnel and equipment in the event of a malfunction (through an electrical short to a metal equipment enclosure or frame) by causing protective devices to
operate promptly. To accomplish this function, ground connections must be adequate for fault currents ten
times or more the normal value for a limited time period, usually less than one second. In general, the AC
Protective Network must conform to the requirements established in the National Electrical Code.
The AC Protective Network should generally follow a crows-foot configuration from a central or main
ground point which, ideally, should be located at the primary station transformer ground point. It shall be
connected directly to the earth electrode subsystem at the same point as the AC Neutral line from the
communications equipment. Note that the AC Protective Network is in addition to any Signal Ground
Network. The Signal Ground Network cannot be relied upon to operate protective devices promptly. A
path all the way back to the AC power source may not exist. Conversely, the single-ended AC protective
network cannot ensure an equal-potential ground for all equipment, such as a ring/grid network can. Both
are generally necessary to fulfill all functional requirements at the site/station.
To protect personnel from exposure to hazardous voltages, all exposed metallic elements of AC-powered
electrical and electronic equipment are connected by means of the AC protective wire (normally green).
This ensures that in the event of inadvertent contact between the hot lead and the chassis frame (or
cabinet) through human error, insulation failure, or component failure, a good, direct, known, fault current
path is available to quickly remove the hazard.
19
Install a separate AC protective conductor with AC power cables if not already provided in the cable itself.
When run as a separate conductor, place it in the same conduit or duct with the phase and AC neutral
conductors for best results. If this is not possible, it should be installed alongside the AC conduit or duct
and connected to the power panel protective bus bar. Never connect the AC protective conductor to the
signal reference ground conductors. A minimum 2-inch separation should be maintained between AC
protective conductors and communications or signal reference conductors if run parallel and not encased
in conduit or ducts.
Pay special attention to situations that might result in flashover or a capacitive discharge between equipment, cabinets, or racks caused by operating personnel walking between them. This may occur from
power faults or lightning strikes where metal components that could store a static charge are not adequately bonded to a discharge path. Accepted lightning protection practices dictate that, within a metalroofed or metal clad building, all metal within 6 feet of the metal sides, roof, or down conductor must be
connected to the external metal or down conductor network. Also, all cabinets and racks that are close
together must be bonded to each other.
By proper segregation of ground routes, careful location of ground nodes, and elimination of unnecessary
ground loops, one can avoid the flow of objectionable currents between two grounding points having a
given ground resistance or impedance.
This is the basis of ground segregation networks as discussed above. Provide separate ground paths for
circuits which have different functions and sensitivities, and avoid ground loops (mandatory). A typical
ground segregation in a complex system should consider separate grounds for the following:
•
Low-level analog returns
•
High-level signal returns
•
Protective shield grounds
•
RF coaxial shield returns
•
High-speed logic returns
•
Power returns
•
Safety
•
Lightning
Some of these conductors will probably have one point in common. To avoid circulating currents among
the different layers of the grounding hierarchy, make sure they do not have more than one common point.
20
Section 4
Tower, Waveguide Bridge, And Waveguide Grounding
Tower Grounding
Connect towers to the earth ground electrode network underneath at all legs. The tower ground network
should consist of ground rods not less than 8-feet-long and interconnected by means of #2 AWG bare or
tinned, solid or coarse, stranded copper wire or a 30 x 3.5 mm galvanized steel strap. For very tall, square
towers with wide bases, install 10-foot rods and an additional cross from the diagonal legs—see Figure
11. Radial wires extending from each leg can be added for extremely tall towers in areas of severe
lightning activity. Make wire leads or strap connections as short and direct as possible.
Figure 11 - Tower Grounding
#2 Awg Solid Copper Wire
Buried 18” Below Grade
8 FT. (Minimum)
Copper-Clad Steel
Ground Rod
Tower Leg
Base
Note 1
Cross Only For
Very Large Towers
Note 2
Shortest Direct Rout
for Ground Strap to
Ground Electrode
RT263910
Note: (1) Normally, two separate conductors are used to interconnect the tower ground ring with the
building ground ring. One of these should follow the route of the WG support structure. If a second
WG support is used, the second interconnecting ground conductor should follow the same route,
since the WG supports require grounding also. (2) Optional radial wire to lower the ground impedance
(1 to 4 places) - required only in areas of severe lightning activity.
21
Equip all concrete and nonmetal towers with lightning rods. Also equip metal towers with lightning rods
where antennas, solar panels, or obstruction lights are mounted at, or near, the top. Downleads in good
condition are required for metal towers with tubular/flange legs and all concrete or other nonmetallic
construction. For wiring sizes and types, see Table C.
As a minimum, connect waveguide bridges to the exterior earth ground electrode network at the first and
last support columns. Make all wire leads as short and direct as possible. Connections in the earth shall
be mechanically strong and brazed or welded. Above ground connections can be bolted.
Ground waveguides to the tower and earth ground electrode network at two points: just above the vertical-to-horizontal transition near the base of the tower, and at the waveguide entry port. In addition, ground
waveguides connected to antennas at or near the top of the tower shall be grounded to the tower near the
antenna—see Figure 12.
Figure 12 - Grounding of Waveguide and Supporting Structures
Lightning
Rod
Waveguide
Steel Tower
Member
Earth Electrode
Ring
#6 AWG
Wire
Ground Rods
#2 AWG Ground
Conductor Buried
18” to 24” Below Grade
Earth Electrode
Ring
RT263912
Note: Only the waveguide to the topmost antenna need be grounded at the top. All waveguides will be
grounded near the bottom of the tower and at the waveguide entry port.
Use solid copper strap or wire equal to #6 AWG to ground waveguides. Coarse-stranded, insulated
copper wire can be used if solid wire is unavailable. Do not use braid or fine stranded wires. Make sure
wire leads are as direct as possible, with no up- or right-angle bends.
22
Section 5
Grounding Practices for Shielding Buildings and Rooms
Because of either the severity of outside interference threats or the vulnerability of the inner system,
electomagnetic shielding is often required at building or room level. Some examples are as follows:
•
Buildings where EMI (Electromagnetic Interference)/RFI (Radio Frequency Interference)
testing is performed
•
Hospitals using NMR (Nuclear Magnetic Resonance) scanners
•
Buildings which host sensitive electronic equipment and are near powerful broadcast transmit
ter, radar, etc.
•
Embassies, government facilities, and industrial headquarters handling confidential data,
where eavesdropping must be prevented.
Whether the entire building, or only the specific room, is to be shielded, will depend on the application.
Currently, shielding an entire building is rare, but this practice may become more common as the number
of EMI occurrences increases. This solution is viable when the building contains many rooms which need
shielding and when target attenuation levels are rather modest—40 dB, up to approximately 100 MHz.
The walls can be covered with aluminum foil, conductive textile, metal mesh, or conductive paint.
In addition to all the mounting precautions to avoid seam leakage, two general grounding practices should
be followed:
1. Bond cable conduits, pipes, shields, etc. to the building shield at their points of penetration,
since the whole building now constitutes a Faraday cage.
2. Connect the building overall shield to the earthing conductor for added safety.
When superior attenuations are required—for example 80, 100, or 120 dB— a real shielded room must
be installed. To avoid alteration of the Faraday cage performance from installation, the following principles
should be observed:
1. Connect the shielded room to the building safety ground conductor. This connection must be
made at the exterior of the room, preferably at the power input box or filter. Metal ducts,
pipes, etc., must be electrically interrupted with a dielectric spacer at the point where they
penetrate the cage. Then, equip the entry hole with a waveguide or honeycomb barrier to
preserve the high attenuation of the room to HF fields.
2. The grounding of the room down to the earthing terminal can be made by a dedicated
conductor, but take precautions to avoid ground loops by making no other ground connection
to the shielded room.
3. Do not use a dedicated earth rod since it would create both a noise problem and a safety
hazard (that is, two earthing resistances in series) in case of a power fault to ground. See
Figure 13 and Figure 14 for examples of some common mistakes in grounding a shielded
room and their corrections.
23
Figure 13 - Bad and Preferred Grounding Practices for Shielded Rooms or Shielded
Enclosures in a Building
Plemum
A/C Heat Ducting
Filters
Disconnect
Box
Excitation &
Monitoring
Equipment
Shielded
Enclosure
Safety (Green) Wire
Panel Service
Entrance
Earthing
Rod
Earth Impedance
a. Typical Grounding of Shielded Enclosure Room
Showing Multiple Ground Loops
A/C Heat Ducting
Break Conduit
& Safety Wire
Asbestos
Bellows
Disconnect
Box
Shielded
Enclosure
Safety (Green) Wire
RF Chokes &
Shielded Power Line
Isolation Transformers
Panel Service
Entrance
b. Eliminating Ground Loops Including Shielded Enclosure (One Approach)
RT263913
24
Figure 14 - Eliminating Ground Loops - Second Approach
A/C Heat Ducting
Break Conduit
& Safety Wire
Asbestos
Bellows
Disconnect
Box
Triple Shielded Isolation
Transformer & Ground
Shielded
Enclosure
Safety (Green) Wire
RF Chokes &
Shielded Power Line
Isolation Transformers
Panel Service
Entrance
RT263914
25