برق. قدرت. کنترل. الکترونیک. مخابرات. تاسیسات.

دایره المعارف تاسیسات برق (اطلاعات عمومی برق)

Traffic Detector Handbook:Third
Edition—Volume II

APPENDIX N. GROUNDING (DESIGN GUIDELINES)


SECTION I—REASONS FOR GROUNDING


1. SAFETY GROUNDING







(1) Grounding of all metallic electrical enclosures  is required for safety. If
a live conductor touches the metal, a large ‘short  circuit current’ flows to
ground, thus tripping the circuit breaker. If the  metal were not grounded, it
would assume the same voltage as the touching  conductor and remain so until
discharged to ground. When touched, the discharge  could occur through the
person’s body to ground depending on the resistance of  gloves, boots and the
material the person is standing on.

2. SYSTEM GROUNDING







(1) A low voltage system is grounded throughout to  ensure that any
line-to-ground fault is cleared by the circuit breakers prior  to doing any
permanent power system damage such as melting of cables, etc. The  system ground
is usually tied to the safety ground. If the two grounds are  separated, the
following disadvantages occur:



















(a) ‘Resistance to ground’ of both system and safety  grounds is greater than
would be the case if the two were connected together.
(b) High currents could still flow in the safety  ground in the event of cable
insulation failures in the enclosure.
(c) A high degree of coupling, through the earth, is  difficult to avoid if the
ground rods are in the same local area.
(d) Where decoupling is possible, voltages (often  dangerous) can be possible
between the nearby ‘grounded points’.

3. LIGHTNING DISCHARGE







(1) Lightning induced currents on cables must be  given a fast and easy path to
ground through protective devices such as  lightning arresters, varistors and
gas-tube arresters. If the path to ground is  not provided properly, the voltage
surge ‘spikes’ and resultant current and  energy will damage components.
Electronic components are particularly  susceptible to damage since they operate
at very low voltages and high speeds  and are not designed to physically absorb
any significant energy.

SECTION II—CALCULATION OF RESISTANCE TO GROUND


1. GENERAL











(1) A minimal resistance to ground is desirable.  Older versions of the Code
called for 10 ohm (Ω)  maximum to ground. This requirement is now replaced with
a description of the  physical grounding materials, or, in the case of a
substation, limiting the  voltage rise due to a fault to 5000 V. The requirement
for 10 Ω  was difficult to design and perhaps even more difficult to obtain
during  installation.
(2) The resistance to ground depends on several  non-exclusive
factors:




















(a)The number and length of ground rods
(b)The number and length  of connecting ground wires in the ground
grid
(c)The quality of wiring connections
(d)The resistivity of the earth
(e)The temperature of the earth
(f)The water content of the earth





(3)The last three factors are somewhat weather  dependent and are therefore
beyond precise design.

2. SOIL RESISTIVITY







(1) The resistivity of the soil, at the site under  consideration, is a measure
of the resistance to conducting electrical current  and is measured in
Ohm-meters (Ωm). Representative values are given Table
N-1.

























Table N-1. Representative values of soil
resistivity.
Soil TypeResistivity ρ (Ωm)

Clay, saturated silt
100

Sandy or silty clay
250

Clayey sand or saturated sand
500

Sand
1500

Gravel
5000

Dry sand, rock
>5000






(2) The soil classification and ρ  values in Table N-1 are left purposely vague
since environmental effects can  drastically change the resistivity of the soil.
Table N-2 shows the typical  variation of a nominal resistivity with different
soil temperatures.





























Table N-2. Variation of resistivity with ground
temperature.
Ground Temperature (°C)Resistivity ρ (% of nominal)
2073
10100
0+139
0– (freeze)303
– 5798
–103333










(3) Resistivity varies widely with moisture content as  well as temperature,
with values a factor of 350% higher for soil in the ‘dry’  state than in the
‘wet’ state.
(4) In order to custom design a grounding system,  the designer would need to
know not only the type of soil and its resistivity,  but also the condition of
future measurements. For this reason, a resistivity  of ρ =  100 Ωm  is selected
as the basis of design for grounding systems. (The system is field  measured
upon installation and any deficiencies can be made up by installing 
supplementary facilities.) It should also be noted that Ontario  has little or
no lightning activity during months when the ground temperature  is below the
freezing point.

GROUND ELECTRODE RESISTANCE TO GROUND


3.1 Ground Rods

(1) Resistance to ground for a single ground rod may  be calculated from


1st equation just after 3.1 Ground Rods (1) Resistance to ground. Capital R subscript Capital G equals Capital R subscript Capital R which in turn equals the product of parenthesis lowercase rho divided by parenthesis 2 times pi times Capital L subscript Capital R parenthesis times parenthesis the summation of the natural logarithm of the quotient of 4 times Capital L subscript Capital R divided by A subscript Capital R parenthesis minus 1 parenthesis.

where













RG =  Resistance to ground in Ohms (Ω)
ρ = Soil  resistivity in Ohm-meters (Ωm)
LR = Rod length in meters (m)
aR = Rod radius in meters (m)
RR = Resistance to ground of one rod in Ohms
(Ω).

Example  1: for 20 mm dia. x 3 m Rod


Given ρ = 100 Ωm, LR = 3 m,
αR = 0.01 m


Equation 2 Just after “Example 1: for 20 mm dia. X 3 m Rod”. Given lowercase rho equals 100 ohm-meters, Capital L subscript Capital R equals 3 meters, A subscript Capital R equals 0.01 meters. Capital R subscript Capital G equals the product of parenthesis 100 divided by parenthesis 2 times pi times 3 parenthesis times parenthesis the summation of the natural logarithm of parenthesis 4 times 3 divided by 0.01 parenthesis minus 1 parenthesis which in turn equals 32.2 ohms.

If the soil is wet and ρ  decreases to ρ = 50 Ωm


Capital R subscript Capital G equals the product of parenthesis 50 divided by parenthesis 2 times pi times 3 parenthesis times parenthesis the summation of the natural logarithm of parenthesis 4 times 3 divided by 0.01 parenthesis minus 1 parenthesis which in turn equals 16.1 ohms.

If the soil is dry and ρ  increases to ρ = 300 Ωm


Capital R subscript Capital G equals the product of parenthesis 300 divided by parenthesis 2 times pi times 3 parenthesis times parenthesis the summation of the natural logarithm of parenthesis 4 times 3 divided by 0.01 parenthesis minus 1 parenthesis which in turn equals 96.6 ohms.

It may be seen that the nominal resistance to  ground of 50 Ω usually quoted
for a single ground may vary  substantially depending on soil type or
conditions.


Example  2: for 20 mm dia. x 6 m Rod


Given ρ = 100 Ωm, LR = 6 m,
αR = 0.01 m


Given lowercase rho equals 100 Ohm-meters, Capital L subscript Capital R equals 6 meters, A subscript Capital R equals 0.01 meters. Capital R subscript Capital G equals the product of parenthesis 100 divided by parenthesis 2 times pi times 6 parenthesis times parenthesis the summation of the natural logarithm of parenthesis 4 times 3 divided by 0.01 parenthesis minus 1 parenthesis which in turn equals 18.0 ohms.

or for a 100% rod depth increase (over example  1), the resistance to ground
is decreased by 44%.


Example  3: for 25 mm dia. x 3 m Rod


Given ρ = 100 Ωm, LR = 3 m,
αR = 0.0125 m


Given lowercase rho equals 100 ohm-meters, Capital L subscript Capital R equals 3 meters, A subscript Capital R equals 0.0125 meters. Capital R subscript Capital G equals the product of parenthesis 100 divided by parenthesis 2 times pi times 6 parenthesis times parenthesis the summation of the natural logarithm of parenthesis 4 times 3 divided by 0.01 parenthesis minus 1 parenthesis which in turn equals 18.0 ohms.

or for a 25% increase in rod diameter (over that  of Example 1), the
resistance to ground is decreased by 3%.


3.2 Pedestals

(1) Using the same formula as for a single ground rod,


Capital R subscript Capital G equals Capital R subscript Capital R which in turn equals the product of parenthesis lowercase rho divided by parenthesis 2 times pi times Capital L subscript Capital R parenthesis times parenthesis the summation of the logarithm of parenthesis 4 times Capital L subscript Capital R divided by A subscript Capital R parenthesis minus 1 parenthesis.

we have the following examples.


Example  4: for Steel Footing (220 mm dia. x 2300 mm)


Given ρ = 100 Ωm, LR = 2.30 m,
αR = 0.110 m


Given lowercase rho equals 100 ohm-meters, Capital L subscript Capital R equals 2.30 meters, A subscript Capital R equals 0.110 meters. Capital R subscript Capital G equals the product of parenthesis 100 divided by parenthesis 2 times pi times 2.30 parenthesis times parenthesis the summation of the natural logarithm of parenthesis 4 times 2.30 divided by 0.0110 parenthesis minus 1 parenthesis which in turn equals 23.7 ohms.

or 26 % "better" than a single rod.


Example  5: for Steel Footing (85 mm dia. x 1830 mm)


Given ρ = 100 Ωm, LR = 1.830 m,
αR = 0.043 m


Given lowercase rho equals 100 ohm-meters, Capital L subscript Capital R equals 1.830 meters, A subscript Capital R equals 0.043 meters. Capital R subscript Capital G equals the product of parenthesis 100 divided by parenthesis 2 times pi times 1.830 parenthesis times parenthesis the summation of natural logarithm of parenthesis 4 times 1.830 divided by 0.043 parenthesis minus 1 parenthesis which in turn equals 36.0 ohms.

or 12 % "worse" than a single ground  rod.


3.3 Plate Electrodes

General


(1) For a single plate,


Capital R subscript Capital G equals Capital R subscript Capital P which in turn equals the product of parenthesis lowercase rho divided by parenthesis 2 times pi times Capital L subscript Capital R parenthesis times parenthesis the summation of the natural logarithm of parenthesis 8 times Capital W subscript Capital P divided by parenthesis 0.5 times Capital W subscript Capital P plus Capital T subscript Capital P parenthesis parenthesis minus 1 parenthesis.

where











RP  = Resistance of plate to ground in Ohms
LP  = Length in meters
WP =  Width in meters
TP  = Thickness in meters.

Example  6: for 610 x 610 x 6 mm Plate


Given ρ = 100 Ohm, LP = 0.61 m,
WP = O.61 m, TP = O.OO6 m


Given lowercase rho equals 100 ohms, Capital L subscript Capital P equals 0.61 meters, Capital W subscript Capital P equals 0.61 meters, Capital T subscript Capital P equals 0.005 meters. Capital R subscript Capital G equals the product of parenthesis 100 divided by parenthesis 2 times pi times 0.61 parenthesis times parenthesis the summation of the natural logarithm of parenthesis 8 times 0.61 divided by parenthesis 0.305 plus 0.006 parenthesis parenthesis minus 1 parenthesis which in turn equals 45.8 ohms.

3.4 Wire Grids

General







(1) For the case of a grounding system consisting of  a wire grid only, the wire
shape forms a ground plane (similar to antenna  design), which if buried deep
enough, can constitute the most effective part of  the grounding system. (Ground
rods are normally driven, in any event, in order  to penetrate below the frost
line.)

The resistance to ground for a grid system is approximated by


Capital R subscript Capital G equals Capital R subscript Capital W which in turn equals the product of parenthesis lowercase rho divided by parenthesis pi times Capital L subscript Capital W parenthesis times parenthesis the summation of the natural logarithm of the quotient of 2 times Capital L subscript divided by the square-root of the product of D subscript Capital W times Capital Z subscript Capital W parenthesis plus parenthesis the quotient of 1.4 times Capital L subscript W divided by the square-root of Capital A subscript Capital W parenthesis minus 5.6 parenthesis.












RW  = Resistance of wire grid in Ohms
LW  = Total Length of grid wires in meters
dW =  Diameter of wire in meters
ZW  = Burial depth of grid in meters
AW  = Plan area covered by grid in square
meters.

Example  7: Using 3 x 3 m grid with cross-tie






This shows a 9.84-foot by 9.84-foot (3-meter by 3-meter) square grid bisected horizontally by a straight line.Given ρ = 100 Ωm, LW = 5 x 3 = 15 m,
ZW = 0.3 m
AW =  3 x 3 = 9 sq m,
dW =  0.0105 m (#2/0)
Given lowercase rho equals 100 ohm-meters, Capital L subscript Capital W equals 5 times 3 which in turn equals 15 meters, Capital Z subscript Capital W equals 0.3 meters, Capital A subscript Capital W equals 3 times 3 which in turn equals 9 square meters, D subscript Capital W equals 0.0105 meters. Capital R subscript Capital G equals Capital R subscript Capital W which in turn equals the product of parenthesis lowercase 100 divided by parenthesis pi times 15 parenthesis times parenthesis the summation of the natural logarithm of parenthesis 2 times 15 divided by the square-root of the product of 0.0105 times 0.3 parenthesis plus parenthesis 1.4 times 15 divided by the square-root of 9 parenthesis minus 5.6 parenthesis which in turn equals 16.4 ohms.

Example  8: Using 3 x 3 m triangular grid






This shows a 9.84-foot by 9.84-foot by 9.84-foot (3-meter by 3-meter by 3-meter) equilateral triangle grid.Given ρ = 100 Ωm, LW = 3 + 3 + 3 = 9 m,
ZW = 0.3 m
AW =  0.5 x 3 x 3 sin(60o) = 3.90 sq
m, dW = 0.0105 m (#2/0)
Given lowercase rho equals 100 ohm-meters, Capital L subscript Capital W equals 3 plus 3 plus 3 which in turn equals 9 meters, Capital Z subscript Capital W equals 0.3 meters, Capital A subscript Capital W equals 0.5 times 3 times 3 sin of 60 degrees which in turn equals 3.90 square meters, D subscript Capital W equals 0.0105 meters. Capital R subscript Capital G equals Capital R subscript Capital W which in turn equals the product of parenthesis lowercase 100 divided by parenthesis pi times 9 parenthesis times parenthesis the summation of natural logarithm of the quotient of 2 times 9 divided by the square-root of the product of 0.0105 times 0.3 parenthesis plus parenthesis the quotient of 1.4 times 9 divided by the square-root of 3.9 parenthesis minus 5.6 parenthesis which in turn equals 23.3 ohms.

3.5 Multiple Rods

General


(1) The combined effect of several rods is similar to the rod resistance
acting in parallel and is given by


Capital R subscript Capital G equals Capital R subscript Capital M Capital R which in turn equals the product of parenthesis the quotient of lowercase rho times 2 times pi times N times Capital L subscript Capital R parenthesis times parenthesis the summation of the natural logarithm of parenthesis the quotient 4 times Capital L subscript Capital R divided by A subscript Capital R parenthesis minus 1 plus parenthesis the quotient of 2.8 times Capital L subscript Capital R times parenthesis the square-root of parenthesis N parenthesis minus 1 parenthesis squared divided by the square-root of Capital A subscript Capital R parenthesis.

where











RMR = Combined resistance of multiple rods to  ground in
Ohms
LR = Rod length in meters
n = Number of rods
AR = Area covered by the n rods in square
meters.

For 20 mm x 3 m rods,


Capital R subscript Capital M R equals the product of parenthesis the quotient of lowercase rho divided by the product of pi times N parenthesis times parenthesis 1 plus the quotient of 1.4 times parenthesis the square-root of parenthesis N parenthesis minus 1 parenthesis squared divided by the square-root of Capital A subscript Capital R parenthesis.

Example  9: Using four rods on 3 m square


Given ρ = 100 Ωm, αR = 0.01 m, LR = 3 m, n = 4,  AR = 3 x 3 = 9 sq m


Given lowercase rho equals 100 ohm-meters, A subscript Capital R equals 3 meters, N equals 4, A subscript Capital R equals 3 times 3 which in turn equals 9 square meters. Capital R subscript Capital M R equals the product of parenthesis the quotient of 100 divided by the product of pi times 4 parenthesis times parenthesis 1 plus the quotient of 1.4 times the square of parenthesis the square-root of parenthesis 4 parenthesis minus 1 parenthesis divided by the square-root of 9 parenthesis which in turn equals 11.7 ohms.

(Note: rods not connected by wire)


3.6 Combination Rod and Wire Grids

General


(1) It is may be necessary to include both rod and wire grids for service
grounds, substations, etc.


The resistance to ground of the combined system  is given by


Capital R subscript Capital G equals the quotient of parenthesis Capital R subscript Capital W times Capital R subscript Capital MR minus Capital R subscript Capital W R squared parenthesis divided by parenthesis Capital R subscript Capital W plus Capital R subscript Capital M R minus parenthesis 2 times Capital R subscript Capital W R parenthesis parenthesis.










RG = Total system resistance to ground in Ohms
RW = Resistance of wire grid in Ohms (Subsection 
3.4)
RMR = Resistance of multiple rods in Ohms  (Subsection
3.5)
RWR = Mutual resistance factor of the wires to the 
rods

Which in turn equals Capital R subscript Capital W R which in turn equals parenthesis lowercase rho divided by the product of pi times Capital L subscript Capital W parenthesis times parenthesis the natural logarithm of parenthesis the quotient 2 times Capital L subscript Capital W divided by Capital L subscript Capital R parenthesis plus the quotient of 1.4 Capital L subscript Capital W divided by the square-root of parenthesis Capital A subscript Capital W parenthesis minus 4.6 parenthesis.

For 20 mm x 3 m rods,


Which in turn equals Capital R subscript Capital W R which in turn equals parenthesis lowercase rho divided by the product of pi times Capital L subscript Capital W parenthesis times parenthesis the natural logarithm of parenthesis 0.67 parenthesis plus the quotient of 1.4 times Capital L subscript Capital W divided by the square-root of parenthesis Capital A subscript Capital W parenthesis minus 4.6 parenthesis.

Example 10: Using 3 x 3 m grid with cross-tie  and rods






This shows a 9.84-foot by 9.84-foot (3-meter by 3-meter) square grid bisected horizontally by a straight line.Given ρ = 100 Ωm, LR = 3 m, αR = 0.01
m,
AW = AR = 3 x 3 = 9 sq m, n = 4,
ZW = 0.3 m, dW = 0.0105 m (#2/0),
LW = 5 x 3 = 15 m,
Given lowercase rho equals 100 Ohm-meters, Capital L subscript Capital R equals 3 meters, lowercase A subscript Capital R equals 0.01 meters, Capital A subscript Capital W equals Capital A subscript Capital R which in turn equals 3 times 3 which in turn equals 9 square meters, N equals 4, Capital Z subscript Capital W equals 0.3 meters, D subscript Capital W equals 0.0105 meters, Capital L subscript Capital W equals 5 times 3 which in turn equals 15 meters. Capital R subscript Capital W R equals parenthesis lowercase rho divided by the product of pi times Capital L subscript Capital W parenthesis times parenthesis the natural logarithm of parenthesis 0.67 parenthesis plus the quotient of 1.4 times Capital L subscript Capital W divided by the square-root of parenthesis Capital A subscript Capital W parenthesis minus 4.6 parenthesis.
Substituting the given data in the formula, we 
have
Capital R subscript Capital W R equals parenthesis 100 divided by the product of pi 15 parenthesis times parenthesis the natural logarithm of parenthesis 0.67 times 15 parenthesis plus the quotient of 1.4 times 15 divided by the square-root of parenthesis 9 parenthesis minus 4.6 parenthesis which in turn equals 10.0 ohms.
Capital R subscript Capital M R equals parenthesis 100 divided by the product of pi times 4 parenthesis times parenthesis 1 plus the quotient of 1.4 times parenthesis square-root of parenthesis 4 parenthesis divided by the the square-root of parenthesis 9 parenthesis parenthesis which in turn equals 11.7 ohms.
(from Example 9).
Capital R subscript Capital W equals parenthesis 100 divided by the product of pi times 15 parenthesis times parenthesis the natural logarithm of parenthesis the quotient of 2 times 15 divided by the square-root of parenthesis 0.0105 times 0.3 parenthesis plus the quotient of 1.4 times 15 divided by the square-root of parenthesis 9 parenthesis minus 4.6 parenthesis which in turn equals 16.4 ohms.
(from Example 7).
Capital R subscript Capital G equals the quotient of parenthesis Capital R subscript Capital W times Capital R subscript Capital M R minus the square of Capital R subscript Capital W R parenthesis divided by parenthesis Capital R subscript Capital W plus Capital R subscript Capital M R minus the product of 2 times Capital R subscript Capital W R parenthesis.
Substituting the above results in the formula  for
RG,  we have
Capital R subscript Capital G equals the quotient of parenthesis16.4 times 11.7 minus 100 parenthesis divided by parenthesis16.4 plus 11.7 minus 20 parenthesis which in turn equals 11.3 ohms.
If the site soil was clayey sand instead of  clay, ρ
would be 500 Ωm  instead of 100 Ωm (Table 1) and the resistance to ground would 
be
Capital R subscript Capital G equals 11.3 times 500 divided by 100 which in turn equals 56.5 ohms.

3.7 Single Wire

General


(1) A  single wire or counterpoise directly buried in earth has a resistance
to ground  of


Capital R subscript Capital G equals Capital R subscript Capital C which in turn equals the product of parenthesis lowercase rho divided by parenthesis 2 times pi times Capital L subscript Capital R parenthesis times parenthesis the summation of the natural logarithm of parenthesis Capital L subscript Capital W divided by A subscript Capital W parenthesis plus the natural logarithm parenthesis Capital L subscript Capital W divided by Capital Z subscript Capital W parenthesis minus 2 plus parenthesis 2 times Capital Z subscript Capital W divided by Capital L subscript W parenthesis minus parenthesis the square of Capital Z subscript Capital W divided by the square of Capital L subscript Capital W parenthesis parenthesis.

where RC =  Resistance to ground of
buried conductor in Ohms.


Example  11: Using #6 AWG wire


Given ρ = 100 Ωm, ZW = 0.6 m,
LW = 50 m, αW = 0.00252
m,


Given lowercase rho equals 100 ohm-meters, Capital Z subscript Capital W equals 0.6 meters, Capital L subscript Capital W equals 50 meters, A subscript Capital W equals 0.00252 meters. Capital R subscript Capital G equals the product of parenthesis 100 divided by parenthesis 2 times pi times 50 parenthesis times parenthesis the summation of the natural logarithm of parenthesis 50 divided by 0.00252 parenthesis plus the natural logarithm parenthesis 50 divided by 0.6 parenthesis minus 2 plus parenthesis 2 times 0.6 divided by 50 parenthesis minus parenthesis the square of 0.6 divided by the square of 50 parenthesis parenthesis which in turn equals 3.93 ohms.

3.8 Summary of Calculations

General Formulae












Single Rod Only:Capital R subscript Capital G equals Capital R subscript Capital R which in turn equals the product of parenthesis lowercase rho divided by parenthesis 2 times pi times Capital L subscript Capital R parenthesis times parenthesis summation of the natural logarithm of parenthesis 4 times Capital L subscript Capital R divided by A subscript Capital R parenthesis minus 1 parenthesis.
Single Plate Only:Capital R subscript Capital G equals Capital R subscript Capital P which in turn equals the product of parenthesis lowercase rho divided by parenthesis 2 times pi times Capital L subscript Capital P parenthesis times parenthesis summation of the natural logarithm of parenthesis 8 times Capital W subscript Capital P divided by parenthesis 0.5 times Capital W subscript Capital P plus Capital T subscript Capital P parenthesis minus 1 parenthesis.
Wire Grid Only:Capital R subscript Capital G equals Capital R subscript Capital W which in turn equals the product of parenthesis lowercase rho divided by the product of pi times Capital L subscript Capital W parenthesis times parenthesis the summation of the natural logarithm of parenthesis 2 times Capital L subscript Capital W divided by the square-root of the product of subscript Capital W times Capital Z subscript Capital W parenthesis plus parenthesis 1.4 times Capital L subscript Capital W divided by the square-root of Capital A subscript Capital W parenthesis minus 5.6 parenthesis.

Multiple Rods Only:


Capital R subscript Capital G equals Capital R subscript Capital M R which in turn equals the product of parenthesis lowercase rho divided by the product of 2 times pi times N times Capital L subscript Capital R parenthesis times parenthesis the summation of the natural logarithm of parenthesis 4 times Capital L subscript Capital R divided by the A subscript Capital R parenthesis minus 1 plus parenthesis the product of 2.8 times Capital L subscript Capital R times the square of parenthesis the difference of the square-root of n and 1 parenthesis divided by the square-root of capital A subscript Capital R parenthesis.





Multiple Rods and Wire Grid:Capital R subscript Capital W R equals the quotient of parenthesis Capital R subscript Capital W times Capital R subscript Capital M R minus the square of Capital R subscript capital W R parenthesis divided by parenthesis Capital R subscript Capital W plus Capital R subscript Capital M R minus the product of 2 times Capital R subscript Capital WR parenthesis.

where


Capital R subscript Capital W R equals the quotient of parenthesis Capital R subscript Capital W times Capital R subscript Capital M R minus the square of Capital R subscript capital W R parenthesis divided by parenthesis Capital R subscript Capital W plus Capital R subscript Capital M R minus the product of 2 times Capital R subscript Capital WR parenthesis.

Single Wire Only:


Capital R subscript Capital G equals Capital R subscript C which in turn equals the product of parenthesis lowercase rho divided by the product of 2 times pi times Capital L subscript Capital W parenthesis times parenthesis the summation of the natural logarithm of the quotient of Capital L subscript Capital W divided by A subscript Capital W plus the natural logarithm of the quotient of Capital L subscript Capital W divided by Capital Z subscript Capital W minus 2 plus parenthesis 2 times capital Z subscript Capital W divided by Capital L subscript W parenthesis minus the quotient of the square of Capital Z subscript Capital W divided by the square of Capital L subscript Capital W parenthesis.

The symbols that appear in the above formulae  are defined as:















































































RG = Total resistance to ground of the system in  Ohms
RR = Resistance to ground of a single ground rod  in Ohms
RP = Resistance to ground of a single ground  plate in Ohms
RW = Resistance to ground of a single ground wire  in Ohms
RMR = Resistance to ground of multiple ground rods  in Ohms
RWR = Mutual resistance factor of wires to rods in  Ohms
RC = Resistance to ground of a single buried  wire in Ohms
LR = Length of ground rod in meters
LW = Length of wire in meters
LP = Width of plate in meters
TP = Thickness of plate in meters
AW = Area of wire grid in square meters
AR = Area covered by several ground rods in  square meters
aR = Radius of ground rod in meters
aW = Radius of wire in meters
dW = Diameter of wire in meters
ZW = Burial depth of wire in meters
n = Number of ground rods
ρ = Soil resistivity in Ohm-meters.

Useful Formulae


(1) Since  the Ministry’s grounding system uses common components of



  • 20 mm dia. x 3.0 m long ground rod

  • #2/0 and #6 AWG ground wire,

then the general formulae can be reduced to reflect the physical parameters
of the common items as follows:



  • Single Rod Only: RR =  O.32ρ

  • Single 220 x 2300 mm Steel Footing: RR = 0.24ρ

  • Single 85 x 1830 mm Steel Footing: RR = O.36ρ

  • Single 610 x 610 x 6 mm Plate: RP
    = 0.46ρ

  • Single #6 wire, 3 m length (ZW =
    0.30): RW = 0.40ρ

  • Single #2/0 wire, 3 m length (ZW =
    0.30): RW = 0.36ρ

  • 3 x 3 m grid c/w cross-tie (#2/0 wire and 4  rods): RG =  O.11ρ

  • 2 Rods @ 3 m space with #6 tie: RG
    = O.19ρ

  • 2 Rods @ 3 m space with #2/0 tie: RG = 0.19ρ.

The foregoing formulae are approximate and may be used where special
conditions apply.


3.9 Application

(1) The  Ministry’s designs for grounding systems are based on the following
premises:



























(a) A resistance to ground of 10 Ω  should be obtained in accordance with good
practice.
(b) In cases where 10 Ω to  ground is impossible to obtain using practical
methods, 25 Ω to  ground is the minimum requirement provided that adequate steps
are taken to  ensure that step and touch voltages do not present a safety
problem to workers  or the public.
(c) Every effort is to be made to meet the 10 Ω to  ground requirement, where
practical, by addition of ground electrodes and wire  in the field.
(d) Since soils and their resistivity vary widely  with location and environment
respectively, the Ministry’s standard criteria  for design is ρ = 100 Ωm. The
resistance to  ground of the designed system is field checked and any required
alterations are  made at that time. Where it is obvious to the designer that
increased grounding  facilities will be required (sand, gravel, rock, etc.), the
required facilities  can be estimated from Table N-3 and included in the
design.
(e) Table N-3 is derived from the general formulae  of Subsection 3.8.
(f) The following notes apply to Table N-3:






















(i) Configuration No.9 may be used with continuous  #6 ground wire commonly used
for lighting. Grounding at every 5th  pole applies.
(ii) Configurations No. 11 or 12 may be used for  grounding in clay or areas that
remain damp. Configuration No. 13 should be  used as the ‘standard’
design.
(iii) Configurations No. 15 to 18 indicate the results  of adding more wire and
rods to the grid. If dry sand, gravel, or rocky areas  are unavoidable, the
principles illustrated may be extended by manual  calculation using the formulae
given.
(iv) Values shown in brackets are for information as  a simpler grid would
normally be required.
(v) Values shown are ‘stand alone’ values (isolated  ground). An approximation
of resistance to ground for any number of the  systems, which are tied together
with ground wire, may be made by considering  the values to be in parallel.

3.10 Problem Areas

Problem areas are identified as:


(1) Bedrock  or shallow overburden of less than 1 m depth over bedrock:







(a) It will be necessary to drill 150 mm (min) holes  in the bedrock and
backfill these with a cementous iron slag slurry mixture  (trade name:
‘Embico’). Note that previously used methods used rock salt as the  chief
conductor and that this method is no longer recommended due to corrosion. 
Difficulty in obtaining (and measuring) proper resistance to ground will be 
encountered as the ground depends, to some extent, on the number of seams 
between rock layers that are encountered. In this situation, the first design 
choice would be to locate the object to be grounded away from the rock area. If 
this is unavoidable, configuration No. 18 in Table N-3 should be used for 
design and added to during construction if necessary.

(2) Soil  overburden of 1 m to 2 m depth over bedrock:







(a) Plates may be used as ground electrodes with the  same configurations shown
in Table N-3 for rods (depending on type of soil  overburden). A minimum of 300
mm of soil should be left between the rock and  plate and the #2/0 wire
grid.

(3) Rock Fill







(a) Areas of rock fill can be assumed to have a  resistivity in excess of 10,000
Wm. A previously used method was to run two  parallel runs of #2/0 wire through
the voids in the rock fill to a location  suitable for use of normal grounding
methods. The method causes large voltages  to appear at the cabinet due to the
high inductance of the leads to ground and  should be avoided by placing at
least 2.0 m of earth fill over the rock fill.















































































































































































Table N-3. Resistance to ground for various ground system
configurations and soils.
Ground System ConfigurationDescriptionNormal UseResistance to Ground (Ω)in Clay (ρ=100Ωm)Resistance to Ground (Ω)in Sandy Clay

(ρ=200Ωm)
Resistance to Ground (Ω)in Clayey Sand
(ρ=500Ωm)
Resistance to Ground (Ω)in Sand (ρ=1500Ωm)Resistance to Ground (Ω)in Sand, Gravel
(ρ=5000Ωm)
Row 2—Very small black circle representing a single 20-millimeter (0.8-inch) by 3-meter (9.8-foot) rod.1.Single 20 mm x 3 m rodAdditionto system32801604801610
Row 3—Small black circle representing a single 220-millimeter (8.7-inch) diameter by 2300-millimeter (90.6-inch) steel footing.2.Single 220 mm dia. x 2300 mm steel footingPoles(requires additional system)28701404201400
Row 4—Very small black circle representing a single 85-millimeter (3.3-inch) diameter by 1830-millimeter (72.0-inch) steel footing. 3.Single 85 mm dia. x 1830 mm steel footingPoles,cabinets (requires additional system)401002006002000
Row 5—Very small black square representing a 610- by 610- by 6-millimeter (24.0- by 24.0- by 0.2-inch) plate.4.610 x 610 x 6 mm plateRockoverburden 0.6 to 2.0 m461152306902300
Row 6—The number 3 above a short black horizontal line representing a single number 6 wire, bare, 3 meters long.5.Single #6 wire, bare, 3 m longAdditionto system411032056152050
Row 7—The number 3 above a short black horizontal line representing a single number 2/0 wire, bare, 3 meters long.6.Single #2/0 wire, bare, 3 m longAdditionto system38951905701900
Row 8—The number 3 above a short black horizontal line representing a single number 6 wire, 2 rods.7.Single #6 wire, 2 rodsService193895290950
Row 9—The number 3 above a short black horizontal line connected on the left side to a small black square, all of which represents a single number 2/0 wire, 2 plates.8.Single #2/0 wire, 2 platesServicein overburden27541404101400
Row 10—The number 1 above a short black horizontal line connected on the left side to a small black circle and on the right side to a very small black circle, all of which represents a 220-millimeter (8.7-inch) diameter by 2300-millimeter (90.6-inch) steel footing, number 6 wire, 1 rod.9.220 mm dia. x 2300 mm steel footing, #6 wire, 1 rodPoles193895285950
Row 11—The number 1 above a short black horizontal line connected on the left side to a moderately small black circle and on the right side to a very small black circle, all of which represents an 85-millimeter (3.3-inch) diameter X 1830-millimeter (72.0-inch) steel footing, number 6 wire, 1 rod.10.85 mm dia. X 1830 mm steel footing, #6 wire, 1 rodPoles163480240800
Row 12—Equilateral triangle with small black circle at the top vertex and very small black circles at the two base vertices, with the numeral 3 printed on each side, all of which represents item 11, with 85 millimeter diameter (3.3-inch) by 1830-millimeter (72.0-inch) steel footing, number 2/0 wire, 2 rods.11.85 mm dia. x 1830 mm steel footing, #2/0 wire, 2 rodsCabinets142870210700
Row 13 —Equilateral triangle with small black circle at the top vertex and very small black circles at the two base vertices; straight lines connect the center of each side of the triangle to the center of the triangle; the numeral 3 is printed at each side. All of this represents item 12, with 85-millimeter (3.3-inch) diameter by 1830-millimeter (72.0-inch) steel footing, number 2/0 wire, 3 rods.12.85 mm dia. x 1830 mm steel footing, #2/0 wire, 3 rodsCabinets132665195650
Row 14 —Square with very small black circles at each corner. A horizontal line across the upper two-thirds has a small black circle hanging from it by a short black line. The numeral 3 is printed below the bottom of the square. All of this represents item 13, with 85-millimeter (3.3-inch) diameter by 1830-millimeter (72.0-inch) steel footing, number 2/0 wire, 4 rods.13.85 mm dia. x 1830 mm steel footing, #2/0 wire, 4 rodsCabinets102050150250
Row 15 —Square with very small black circles at each corner. The numeral 3 is printed just above the bottom line of the square. All of this represents item 14, with number 2/0 wire, 4 rods.14.#2/0 wire, 4 rodsService Anyfor  ρ < 125 Ωm 112255165550
Row 16 —Square with very small black circles at each corner. Straight lines connect the center of each side of the square to the center of the square. The numeral 3 is printed just below the bottom side of the square. All of this represents item 15, with number 2/0 wire, 4 rods, 2 ties.15.#2/0 wire, 4 rods, 2 tiesAnyfor ρ < 125 Ωm112255165550
Row 17 —Square with very small black circles at each corner. Straight lines connect the center of each side of the square to the center of the square. Horizontal lines shoot off to the right and left of the top and bottom of the square. The numeral 3 is printed on the lower left and lower right shooting lines and the bottom line of the square. All of this represents item 16, with number 2/0 wire, 4 rods, 2 ties, 4 tails.16.#2/0 wire, 4 rods, 2 ties, 4 tailsAnyfor 125 < ρ < 150 Ωm(9)1845135450
Row 18 —Square with very small black circles at each corner. Straight lines connect the center of each side of the square to the center of the square. Horizontal lines shoot off to the right and left of the top and bottom of the square. Vertical lines shoot off to the top and bottom of the page from the right and left sides of the square. The numeral 3 is printed on the lower left and lower right shooting lines and the bottom lines of the square. The numeral 3 is also printed on the upper right vertical shooting line and the lower left vertical shooting line. All of this represents item 17, with number 2/0 wire, 4 rods, 2 ties, 8 tails.17.#2/0 wire, 4 rods, 2 ties, 8 tailsAnyfor 150 < ρ < 200 Ωm(6)123090300
Row 19 —Square with very small black circles at each corner. Straight lines connect the center of each side of the square to the center of the square. Horizontal lines shoot off to the right and left of the top and bottom of the square. Vertical lines shoot off to the top and bottom of the page from the right and left sides of the square. The numeral 3 is printed on the lower left and lower right shooting lines and the bottom lines of the square. The numeral 3 is also printed on the upper right vertical shooting line and the lower left vertical shooting line. All of this is bounded by an outer square with 4 small black circles at its vertices. All of this represents item 18, with number 2/0 wire, 8 rods, 6 ties.18.#2/0 wire, 8 rods, 6 tiesAnyfor 200 < ρ < 350 Ωm(5)102575250

3.11 Application Guidelines






































(1) From the examples in the foregoing sections, it  is immediately obvious that
obtaining a 10 Ω resistance to ground  is difficult in soils with high
resistivity.
(2) The effect of ground rod diameter is small.  About 8% less resistance to
ground is obtained by using a 25 mm diameter rod  instead of a 20 mm rod. Much
better results are obtained by making ground rods  longer rather than
thicker.
(3) The effect of electrode material (copper or  steel) has negligible effect on
results since the resistivity of all metals is  much less than that of all
soils.
(4) Ground rod spacing should be kept within one rod  length spacing of each
other.
(5) The effect of the size and type of wire  interconnecting the ground rods has
little effect on results. The #2/0 AWG  cable usually used is sized to withstand
a 50,000 ampere lightning discharge  without complete melting.
(6) The upper 1.0 m of ground rod does not have much  effect, even in wet soil.
A minimum depth of 2.0 m gives about 25% more  resistance to ground than the 3.0
m standard depth rod.
(7) In order to design proper grounding, a soils  classification at the intended
location should be obtained from the Regional  Geotechnical Office (if not on
the ‘Soils Profile’ or indicated on borehole  logs included with contract
drawings) and District personnel should be  consulted.
(8) If the equipment to be grounded will be in a new  fill location, the fill
should not be composed of sand, gravel, rock and the  like (if practical). A
note on the grading drawings should be added where  necessary: ‘Fill in the area
of (equipment) to be cohesive material only or  similar.’
(9) Table N-3 gives the number of ground rods (20 mm  x 3.0 m) and grid
configurations required for various classes of soil. Where  there is not an
apparent site problem, ground designs corresponding to ρ =  100 Ωm  should be
used by the designer. Where necessary after testing, the design may  be adjusted
during construction. Where it is not practically possible to obtain  10 Ω to 
ground, an absolute minimum of 25 Ω may be used.

SECTION III—EFFECTS OF LIGHTNING


1. GENERAL


The effects of lightning on outdoor electrical  and electronic equipment can
be costly. Damage from lightning may result from:



  • Direct strokes

  • Power surges

  • Inducted transient voltage spikes

  • Capacitive voltages.

Since it is not practical to protect outdoor  equipment against direct
strokes, protective systems apply to the prevention or  handling of surges and
transients. The protective systems consist of the  application of proper ground,
suppression, and shunting devices.


Since weather is somewhat unpredictable,  protection design is based on the
following probabilities:



  • Probability of a storm

  • Probability of a strike

  • Probable potential energy and RF energy

  • Probable rise time of the voltage (open circuit)  wave or current (short
    circuit) wave

  • Probable duration or repetition of a strike.

2. DESIGN CRITERIA


The design criteria adopted for protection of  the Ministry’s electronic
equipment are:



  • Peak voltage = 15,000 V

  • Peak current = 5,000 A

  • Maximum current flow duration = 500 μs

  • Current waveform = 8 x 20 μs

  • Voltage Waveform = 1.2 x 50 μs.

Figure N-1 indicates waveforms and timing of  lightning protection
devices.


FIGURE N-1. VOLTAGE AND CURRENT WAVEFORMS. A curve representing percent of maximum voltage or current declines slowly from 100 percent at approximately 0.9 volts to approximately 50 percent at 50 microseconds to approximately 40 percent at 80 microseconds. A lightning arrestor is shown as firing at 20 microseconds to 400 microseconds. A second curve shows that an MOV fires at 0.007 microseconds and a gas tube at 0.15 microseconds, resulting in a much slower rise in maximum discharge current and a much sharper decline. The maximum discharge percent is only 15 at 50 microseconds and only 3 percent at 80 microseconds.
Figure N-1. Voltage and current
waveforms.

Note that the times needed for protection are  much too fast to allow power
circuit protection devices such as breakers, fuse,  lightning arresters etc. to
operate effectively. However, devices such as gas  tubes and metal oxide
varistors (MOVs) will initiate protection at about 0.15 μs  and 0.007 μs,
respectively.


3. POWER SURGES


Surges in any equipment including cables, poles,  etc. can be induced by
lightning strikes as far as 6 km away. Surges on  overhead high voltage lines
are grounded through lightning arresters at  transformer locations.


Figure N-2 shows the voltage and current  distribution through the earth near
the bottom of the utility pole. For the  design value of resistivity ρ = 100 Ωm,
a voltage of  15,000 volts would be transferred through the earth for a distance
of 5.3 m. It  is therefore necessary to keep the service ground a minimum
distance from the  Hydro ground as indicated in Figure N-3. Since the designer
seldom knows where  the Hydro ground line is located, the convention of 5.5 m to
the center of the  pole is used as a design practice. Note that a large voltage
will appear at the  service ‘SN’ due to the Ldi/dt voltage on the
grounding cable.







Figure N-2 shows the voltage and current distribution in the earth near the bottom of a utility pole. The voltage intensity declines in proportion as the area of a sphere surrounding the base of the pole expands.

Figure N-2 shows the voltage and current distribution in the earth near the bottom of a utility pole. The voltage intensity declines in proportion as the area of a sphere surrounding the base of the pole expands.





Figure N-2 shows the voltage and current distribution in the earth near the bottom of a utility pole. The voltage intensity declines in proportion as the area of a sphere surrounding the base of the pole expands.



Figure N-2. Voltage in earth due to
discharging lightning current at service pole.



Recommended improvement to system ground connections.

Figure N-3. Recommended improvement to system ground
connections.

4. OTHER SOURCES OF POSSIBLE DAMAGE


Traffic signal systems contain many other  sources of transient voltages and
currents within the controller cabinet. These  sources are not considered as
severe as the energy surge through the service  neutral and all have protection
devices installed within the cabinet. Some  sources are:























(a) Detector Loops—inductive loop detector  electronics units are protected
internally with their own lightning arrester  and are also provided with
external MOVs at the input file. Failure rate due to  lightning damage is very
low as the voltage impressed on a loop is caused by  capacitive
effects.
(b) Detector Cable—the possibility of induced  currents caused by transient
voltages in the earth is minimized by shielding  the detector cable and leaving
both ends of the shield cut off.
(c)

Signal Cable—signal cable is  shielded by metal poles (above ground), but is
subject to induced currents  caused by transient voltages in the earth. The load
switches and the  AC-terminals of the cabinet are protected by MOVs and the
failure rate is low.

(d) Direct Hits on Cabinet—although nothing can be  done to ensure a complete
lack of damage, the controller cabinet may be  considered to be protected by an
umbrella cone of 30o from an  overhead line and somewhat protected by a 45o
cone. It is not  desirable, however, to install the cabinet directly under the
lines due to  possible electromagnetic interference. The cabinet location
(Figure N-4) should  be:

  • 11 m minimum from the supply pole

  • 3 m minimum (horizontally) clear of overhead  lines

  • Within the 30° to 45° cone  of protection (within 15 m for normal height
    lines) of the overhead lines.
If the controller is to be situated
across the  road from the hydro lines, then the #6 AWG (green) ground wire and
the"  feeder wires should be run in rigid steel duct, to the nearest electrical 
chamber. These conductors should then be run to the next chamber across the 
road via the under-pavement crossing, and from this chamber to the controller 
in any approved electrical duct, not necessarily of metal.
(e) Direct Hits on Poles or Equipment—this condition  would cause severe damage.
The method of mitigating possible damage effects  consists of installing a #6
AWG RWU 90 (green) system ground wire connecting  all poles and intersection
equipment and installing a ground rod on each  corner. Connection of the system
ground around the intersection should be made  at one point only (the service
ground bus) as indicated in Figure N-5.







Controller cabinet location for best lightning protection.

Controller cabinet location for best lightning protection.

Figure N-4. Controller cabinet location for best
lightning protection.









Figure N-4 shows that the controller cabinet is located at least 5.5 meters from the first grounding rod. The grounding rods are separated at least 3 meters apart and at least 5.5 meters from the electrical supply pole. Wires to the grounding rods are in rigid steel duct located underground.

Figure N-5 shows that the signal grounding system includes no ground electrodes at the cabinet and includes lightning arrestors and hybrid suppressors between the service cabinet and the ground. It also shows that the control equipment in the cabinet is additionally protected by lightning suppressors in the cabinet.

Figure N-5. Signal grounding system (with or without
lighting). No ground electrodes at cabinet.



SECTION IV—SUMMARY OF DESIGN GUIDELINES


1. TRAFFIC SIGNAL SYSTEMS


(1) Design  standard grounding system under normal circumstances (Figures N-3
and N-5):



  • Service ground—4 rods and #2/0 bare ground wire.

  • Controller cabinet ground connected to system  ground at service ground
    bus.

  • Equipment ground—1 rod or steel footing per  intersection corner
    interconnected with #6 insulated wire.

  • System ground—interconnect the controller  cabinet ground and the equipment
    ground to the service ground at service ground  bus.

(2) Use  improved design as per Table N-3 for grounds in sand, gravel, or
rock. Consult  Geotechnical Information and District Maintenance.


(3) Both  ends of the detector cable shield should be cut off and left
unconnected.


(4) Locate  controller at least 11 m from a hydro pole and at least 3 m
horizontally from  overhead lines. Locate controller 1.5 m clear minimum from
metal objects such  as poles, fences, and guide rails.


SECTION V—REFERENCES















































































































































(1) Biddle Instruments, Getting Down to Earth: A Manual on
Earth Resistance Testing for Practical  Man
, 4th ed., 1981.
(2) Bodle, D., Electrical Protection Guide for Land-Based Radio
Facilities
, Joslyn  Electronic Systems, 1971.
(3) Burns, G.A, “Lightning Damage to  Tank-Gauging Equipment Solved
by Modification Instead of Replacement,” Oil and Gas Journal, pg. 93,
September 14, 1981.
(4) Canadian Standards Association, Canadian Electrical Code
Part I, Section 10,  Grounding and Bonding
, 1987.
(5) Carpenter, R.B., “Total Isolation from  Lightning Influences,”
IEEE Transactions  on Industry Applications, Vol. 1A-17, No. 3, pg.
334, May/June 1981.
(6) Cunagin, W.D. and Avoub, N.A., Lightning Protection
Hardware and Techniques  for Electronic Traffic Control Equipment
, Federal 
  Highway Administration, February, 1986.
(7) Dasen, M., Insulation Tester – Megger, Algonquin  
College.
(8) Dasen, M., Meg-Earth Tester, A1gonquin
College.
(9) Denny, H.W. and Rohrbaugh, J.P.,  “Transient Protection,
Grounding, and Shielding of Electronic Traffic Control  Equipment,” NCH RP
Report 317
,  Transportation Research Board, June, 1988.
(10) Edco, Inc., Lightning Protection for Traffic Control,
Edco Technical Bulletin  No. 200-01, May, 1978.
(11) Edco Inc. of Florida, Installation Technical Bulletin #
100484
,  1984.
(12) Epstein. B.M., “For Best Results, Treat  Power and Computer
Requirements as One System,” EC&M, pg. 130, August 1986.
(13) Fink, D.G and Beaty, H.W., Standard Handbook for Electrical
Engineers
, 11th Edition, 1978.
(14) Freund, A, “Protecting Computers from  Transients,”
EC&M, pg. 65, April  1987.
(15) General Electric, Transient Voltage Suppression, 3rd
Edition.
(16) Gunn, R., “Facility Noise Control from the  Ground Up,”
EC&M, pg. 56, April  1987.
(17) Harder, J.E., Hughes, A.E., and Vosicky,  J., “Analytical
Method for Coordination of Surge Arresters with Current-Limiting  Fuses,”
IEEE Transactions on Industry  Applications, Vol. lA-17, No. 5, pg.
445, September/October 1981.
(18) Institute of Electrical and Electronic  Engineers, Inc.,
IEEE Guide for Radio  Methods of Measuring Earth Conductivity, IEEE
Standard 356-1974.
(19) Institute of Electrical and Electronic  Engineers, Inc.,
IEEE Guide for Safety in  Substation Grounding. IEEE Standard
80-1976.
(20) Institute of Electrical and Electronic  Engineers, Inc.,
IEEE Guide for the  Installation of Electrical Equipment to Minimize
Electrical Noise Inputs to  Controllers from External Sources
, IEEE
Standard 518-1982.
(21) Institute of Electrical and Electronic  Engineers, Inc., ”IEEE
Standard Procedures for the Measurement of Radio Noise  from Overhead Power
Lines,” IEEE  Transactions on Power Apparatus and Systems, Vol. Pas
-100, No. 8, August  1981.
(22) Institute of Electrical and Electronic  Engineers, Inc.,
“Modeling Current-Limiting Surge Arresters,” IEEE Transactions on Power
Apparatus and  Systems
, Vol. Pas -100, No.8, August 1981.
(23) Lee, W.R., “The Dangers of Lightning,” ETI
Canada,  pg. 27, November 1978.
(24) Michaels, E.C., “Principles and Techniques  for Grounding and
Bonding in Hazardous( Classified  )Locations,” Plant Engineering, pg.
133, September 17, 1981.
(25) Mims, F.M., “Introducing the Varistor,” Computers and
Electronics
, pg. 88, May  1983.
(26) Ontario Hydro Inspection Department,  “Electrical Inspection,
Provincial Government,” Electrical Inspection Guide 26-4, December
1975.
(27) Ontario Hydro Inspection Department,  “Pub1ic Roads -
Electrical Devices,” Electrical  Inspection Guide 11-3, August
1984.
(28) Ontario Hydro Inspection Department, “Rule  10-208: Grounding
Connections for Two or More   Buildings or Structures,” Bulletin 10-6-0, April
1987.
(29) Plumber, J.A. and Crouch, K.E., Lightning Protection for
Traffic Control  Systems
, National Aeronautics and Space Administration,
1978.
(30) Schwarz, S.J., “Analytical Expressions for  the Resistance of
Grounding Systems,” presented at the AIEE Summer and Pacific  General Meeting,
Los Angeles, June  1954.
(31) Stifter, F.J., “Power-Line Disturbances,” Computers and
Electronics
, pg. 35,  October, 1983.
(32) Thomas, P., Investigative Report - Lightning Problems on
170/332 Traffic Control  Systems
, Ministry of Transportation of Ontario, 
June 1985.
(33) Waterson, A. and Maher, P., “Computer Power  - Problems and
Solutions,” EC&M,  pg. 67, December 1982.
(34) Westinghouse Electric Corporation, Electrical Transmission
and Distribution  Reference Book
, 1950.
(35) “Equipment, Manuals and Procedures  Evaluation for the Design
and Maintenance of Traffic Signal Systems,” Report No. 2, Grounding,
Ministry of  Transportation of Ontario,  May 1988.

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