Low impedance surge protective device cables for power line usage

A coaxial cable (10) is for use in a power distribution network (N). The cable connects a surge protective device (SPD) in parallel with feeder lines (W1, W2) of the network. The SPD senses voltage surges on the feeder lines and clamps the voltages to a level at which loads (LD) connected downstream of the SPD are protected from excessive voltage levels. An inner conductor (12) and an outer conductor (14) have a dielectric material (16) separating them. The inner conductor is a round conductor, and the outer conductor forms a hollow cylinder in which the inner conductor and insulation material fit. A ratio of the inner diameter (D) of the outer conductor to the diameter (d) of the inner conductor is approximately 1.05. Thus, the diameter of the inner conductor is relatively large compared with the inner diameter of the outer conductor. A relatively large diameter of the inner conductor serves to minimize the dc resistance of the cable. Also, the dielectric material has a permittivity in the range of 2.0-4.0. Performance characteristics of the coaxial cable are compared with those of other conductors to illustrate the superiority of the coaxial cable in reducing "let through" voltage otherwise passed by a surge protector device to the loads.

BACKGROUND OF THE INVENTION 
This invention relates to surge protective devices (SPD's) and, more 
particularly, to a low impedance, or low-Z cable for use to connect SPD's 
in power line applications. 
A surge protective device, or SPD, is used in power distribution network 
applications to protect loads connected to the network from high voltage 
surges or transients. Examples of the types of installation in which SPD's 
are used include centrifugal fire pumps, HVAC systems, computerized 
numerical control (CNC) machines, PLC's, and uninterruptible power 
supplies (UPS) for computer systems. SPD's use a variety of protection 
technologies. These include zener and selenium diodes, metal-oxide and 
silicon carbide varistors, and crowbar devices such as triggered and 
untriggered spark gaps. 
In use, a SPD is connected across two feeder lines of the power 
distribution network. In a three-phase distribution system this would be 
one of the phase lines, and neutral; or, between phases, phases-to-ground 
and neutral-to-ground. An SPD can be connected on either the service side 
or load side of a service distribution buss. It can also be located on 
branch service busses and at distribution panels. Often, SPD units consist 
of a collection of SPD modules parallel wired to terminal blocks, as well 
as to disconnects inside a unit. When a voltage surge propagates down the 
conductor lines, it is sensed by the SPD. If the surge voltage exceeds the 
threshold level of the SPD, the SPD then presents a short-circuit across 
the conductors until the surge level falls back below the threshold. The 
downstream loads, especially those of relatively high impedance, are thus 
protected from the surge voltage. 
It will be appreciated that in an ideal network, the SPD would present a 
perfect short-circuit in front of the loads, and would divert all of the 
current back to the source. However, because most configurations are less 
than ideal, the SPD is not necessarily exposed to all of the transient 
voltage. This is because while power distribution systems are designed to 
efficiently transmit 60 Hz power, they are not designed to transmit fast 
transient surges; i.e., voltage spikes of about 10 microsecond (10.sup.-6 
sec.) or faster rise time. Consequently, some of the surge voltage is "let 
through" to the loads. Subjecting the loads to these high voltage 
transients is harmful to them. One culprit in this regard is the wiring or 
cabling used to connect the SPD in parallel with the network conductors. 
Conventionally, this cable is a shielded twin conductor cable. Shielded 
twin cables include two parallel conductors of radius r embedded in an 
insulator material with a distance w between the longitudinal axis of the 
conductors. A shield (typically conduit) encloses the conductors and 
insulator. The transient voltage drop across the wiring used in these 
shielded twin cable applications is sufficiently high that the SPD is not 
exposed to the full amplitude of a voltage surge. Accordingly, either the 
SPD is not switched into operation; or if it is, switching occurs at a 
higher transient voltage level than that to which the device is ultimately 
designed. Having available a lower impedance cable specifically for use in 
these configurations would allow the SPD's to be more effective in 
protecting downstream loads from exposure to excessively high voltages. 
SUMMARY OF THE INVENTION 
Among the several objects of the present invention may be noted the 
provision of a cable for use in power distribution applications for 
connecting SPD's in parallel with power distribution network conductors, 
so the SPD's can protect loads connected to the network from high voltage 
surges or transients; the provision of such a cable which is a low 
impedance, or low-Z cable so the voltage drop across the cable is minimal, 
minimal voltage drop insuring the SPD is subjected to substantially all 
the transient voltage; the provision of such a low impedance cable whose 
use limits the amount of voltage "let through" to which loads downstream 
of the SPD are subjected; the provision of such a cable whose low 
impedance is based upon optimizing cable geometry, cable dimensions, and 
the materials from which the cable is fabricated; the provision of such a 
low impedance cable having a minimized series inductance and DC resistance 
so to have a minimum impedance at the frequencies at which surges or 
transients occur; the provision of such a low impedance cable comprising 
parallel conductors separated by an insulator providing a negligible shunt 
conductance between the conductors; the provision of such a low impedance 
cable to be a coaxial cable having a compact form and aspect ratio 
slightly greater than 1.0; the provision of such a low impedance cable 
which allows a greatly improved SPD clamping voltage rating; the provision 
of such a cable which is usable to connect any type SPD in parallel with 
the conductors; and, the provision of such a cable which is easy to make, 
readily connected in a power distribution network, and safe in use. 
In accordance with the invention, generally stated, a coaxial cable is for 
use in a power distribution network. The cable connects a SPD in parallel 
with feeder lines of the network. The SPD senses voltage surges on the 
feeder lines and clamps the voltages to a level at which loads connected 
downstream of the SPD are protected from excessive voltage levels. An 
inner conductor and an outer conductor of the cable have a dielectric 
material separating them. The inner conductor has a circular 
cross-section, and the outer conductor forms a hollow cylinder in which 
the inner conductor and insulation material fit. A ratio of the inner 
diameter of the outer conductor to the diameter of the inner conductor is 
approximately 1.05-1.56. Thus, the diameter of the inner conductor is 
nearly as large as the inner diameter of the outer conductor. A relatively 
large diameter of the inner conductor serves to minimize both the dc 
resistance and inductance of the cable. Finally, the dielectric material 
has a permittivity in the range of 2.0-4.0. Other objects and features 
will be in part apparent and in part pointed out hereinafter.

DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring to the drawings, a power distribution network is indicated 
generally N in FIG. 1. Electrical voltage from a source S is applied to 
various loads LD through electrical wires or lines W. Although only two 
such lines W1 and W2 are shown in FIG. 1, it will be understood that in a 
poly-phase power distribution network such as a three-phase network, there 
will be more than two lines supplying power to a three-phase load. It is 
not unusual for voltage surges or transients T to propagate down the lines 
and be impressed on a load. As is well-known, if the transients are large, 
the loads can be severly damaged by the high-voltage levels to which they 
are subjected. To reduce this possibility, surge protective devices 
commonly referred to as SPD's are connected across wires W so to be in 
parallel with the load. Although only one SPD is shown in FIG. 1, it will 
be understood that in multi-phase networks, there may be an SPD connected 
in parallel across each phase. In addition, or alternatively, an SPD may 
be connected between each phase and neutral, between each phase and 
electrical ground, or between neutral and ground. The SPD is connected 
across the phase lines typically at a service distribution box B. 
Connector lines A1 and A2, which represent a coaxial cable of the present 
invention, are respectively attached to lines W1 and W2 at respective 
terminals or junctions J1 and J2 within the distribution box. 
With a SPD connected in the network, as shown, when a transient T 
propagates along lines W, it is sensed by the SPD. Each SPD is designed 
for a predetermined voltage level above which the SPD operates. If the 
transient voltage exceeds this threshold, the SPD presents a near 
short-circuit across the lines W until the surge voltage level falls back 
below the threshold. Downstream loads LD, and especially relatively high 
impedance loads are protected from the surge voltage by operation of the 
SPD. Ideally, the SPD presents a perfect short-circuit and diverts all of 
the current back to the source. Because wires A1 and A2 are less than 
ideal, the SPD is not subjected to all of the transient voltage. Some of 
the higher level surge voltage gets through or is "let through" to the 
loads. As is described hereinafter, it has been found that the cables A 
used to connect the SPD across the lines are one reason why high levels of 
surge voltage get through to loads LD. Referring to FIG. 3, the cables A 
currently used in the hook-up shown in FIG. 1 are shielded twin type 
conductors, constructed of THHN wire. 
In analyzing the cables A used to interconnect a SPD with feeder wires W, 
it will first be understood that with respect to these conductors, they 
run parallel to each other with a dielectric material separating them. 
Second, it will be understood that their characteristics cannot be treated 
as lumped characteristics, but must be considered as distributed. 
Accordingly, they can be considered as a series of small, but finite 
circuit elements each of which represents some value per given unit 
length. Referring to FIG. 2, a circuit model of a transmission line A is 
shown. In the transmission line model, an inductance L and dc resistance R 
are in series between source S and the load LD'. A capacitance C and 
conductance G are in parallel with the load. For the model, the following 
represent the lumped circuit values: 
.zeta.=L/l=series inductance per unit length, 
R=r/l=series DC resistance per unit length, 
C=c/l=parallel capacitance per unit length, and 
G=g/l=parallel conductance per unit length, where l=cable length. 
Of the representative lumped circuit elements, the series connected 
elements L and R produce a voltage drop and resist current flow. They also 
increase the overall load impedance. The parallel connected components C 
and G divert current and decrease the overall load impedance. The values 
for the inductance and capacitance are a function of transmission line 
geometry, with capacitance also being dependent upon the dielectric 
constant of the material separating the conductors A. The value of 
resistance R is a function of the resistivity and cross-sectional area of 
the conductor material. I.e., r=.rho.l/A where .rho. is resistivity of the 
material used, l is length as noted above, and A is the cross-sectional 
area of the conductor. The shunt conductance is a function of the 
conductivity of the insulating material separating the conductors. Using 
the representation of FIG. 2, is characteristic impedance Z.sub.c of 
transmission line is given by: 
##EQU1## 
where i represents the .sqroot.-1, and w is the frequency. Often, to 
minimize losses, the characteristic impedance of the transmission line is 
impedance matched with both the voltage source S and load LD'. 
In evaluating the operation of a SPD with cables A, the factors to be 
evaluated are the voltage drop across the inductance .zeta. and resistance 
R, and the currents drawn through capacitance C and conductance G. When a 
transient T propagates through the network, the voltage drop across the 
inductance and resistance is expressed as: 
EQU V.sub..zeta.+R (t)=V.sub..zeta. (t)+V.sub.R (t), or 
EQU V.sub..zeta.+R (t)=.zeta.1(dI(t)/dt)+R1(I(t)), 
where I(t) is total current through the transmission wire. Current flowing 
through the shunt components C and G is expressed as: 
EQU I(t)=I.sub.C (t)+I.sub.G (t), or 
EQU I(t)=C/(dV.sub.LOAD (t)/dt)+GLV.sub.LOAD (t) 
where V(t) is the total voltage impressed on the system and is given by the 
expression 
EQU V(t)=V.sub..zeta.+R (t)+V.sub.LOAD (t) 
The above equations can be combined to produce a second order differential 
equation for the voltage across the load. This equation is a function of 
time and is expressed as: 
EQU d.sup.2 /dt.sup.2 V.sub.LOAD (t)+(R/.zeta.)d/dtV.sub.LOAD 
(t)+(1/.zeta.C)V.sub.LOAD (t)=(1/.zeta.C)V(t). 
Referring to the table of FIG. 3, different transmission line geometries 
are shown. Expressions for the different elements for each particular 
geometry are listed in the table. The information in the table can be 
found in Theory of Guided Electromagnetic Waves, by R. A. Waldron, Van 
Nostrand, Reinhold Co., 1970, Chapter III. Some cable geometries are not 
listed in FIG. 3 because their characteristics are similar to those 
already listed. Or, the geometry of the cable construction is impractical. 
These geometries include striplines, tri-plate lines, and geometries based 
on placement of cylindrical wires between plates. 
With respect to the table of FIG. 3, it will be noted that the real portion 
of the impedance (Z.sub.c) corresponds to the real part of the complex 
impedance in the equation: 
##EQU2## 
When G=O, the expression can be reduced to: 
##EQU3## 
Preferably, a cable 10 of the present invention, which is shown in FIGS. 8A 
and 8B, is for use in power distribution networks for connecting SPD's in 
parallel with network conductors. The cable is a coaxial cable having an 
inner conductor 12 which is a round wire conductor of diameter d. It 
further has an outer, hollow cylindrical conductor 14, which, as shown in 
FIG. 8A is a braided wire conductor having an inner diameter D. A ratio of 
the outer conductor's inner diameter to the diameter of the inner 
conductor; i.e., D/d, is referred to as the aspect ratio for the coaxial 
cable. An insulation material 16 is annular in cross-section. The material 
fills the space between an outer surface 18 of inner conductor 12 and an 
inner wall 20 of the outer conductor. The coaxial cable also has an outer 
jacket of insulation material 22. 
As is now discussed, the material used to fabricate cable 10 is such that 
the cable has a DC resistance that minimizes voltage drop to the clamping 
elements (not shown) within the SPD which react to the sensed transient 
voltage condition. These clamping elements are typically metal-oxide 
varistors, or MOV's. Referring to FIG. 4, a characteristic voltage-current 
curve for a MOV is shown. From this graph, the clamping voltage dependency 
on surge current will be understood. That is, an MOV will clamp the 
voltage within a narrow range around 200 V for a current range which 
covers six orders of magnitude. 
Cable 10 also includes minimal series inductance. However, it further has a 
maximum shunt capacitance to divert surge current from the clamping 
elements and reduce clamping voltage. These cable 10 features are 
important because, as discussed above, it is important in protecting loads 
LD to minimize the "let through" voltage to the loads. This means it is 
important, in turn, to minimize the voltage drop from the junction points 
J to the points at which the cable is connected to the SPD, since the SPD 
will then be the most responsive to transients. Finally, shunt conductance 
is small for most materials which may be chosen for insulator/dielectric 
16. The conductivity of most commonly used dielectrics is on the order of 
10.sup.-14 mho/cm. If the cable run for cable 10 is ten feet, for example, 
and a 10 KV transient pulse propagates down the cable, the total current 
drawn through the cable, due to conductivity of material 16, is on the 
order of 30 nA. This current level is insignificant. Accordingly, shunt 
conductance can be generally disregarded in choosing the appropriate 
materials for cable 10. With regard to the materials chosen, in addition 
to their selection for the electrical properties they possess, they are 
also chosen on the basis of manufacturability, connectability, safety, 
overall cable 10 size (cross-sectional area, etc.), and safety. 
To minimize dc resistance in cable 10, inner conductor 12 is chosen to have 
as large a diameter d as is practical. This maximizes the cross-sectional 
area of the conductor. Next, the material from which the conductor is made 
is selected for its low resistivity. Copper has a resistivity of 
approximately 1.72 micro-ohms/cm. For silver, this value is 1.59 
micro-ohms/cm. In choosing which of these preferable materials to use, the 
decision is a function of the approximately 7.6% improvement in 
resistivity using silver versus the price of a coaxial cable 10 made with 
more expensive silver wire. If SPD protection of loads from transients is 
very critical, then silver is the material of choice. Otherwise, copper 
can be used. 
It will be appreciated that the inductance and capacitance of cable 10 are 
functions of the cable geometry. Capacitance is also a function of the 
dielectric relative permittivity of the insulation material used in the 
cable. Generally, materials which could be used for insulation 16 have a 
permittivity which ranges from 1.0-8.0. For use in cable 10, it has been 
found that the materials which provide the best results have 
permittivities ranging from 1.5-8.0. Typically, insulation material 16 has 
a permittivity of approximately 3.0. 
The geometry of the currently used THHN cables is a shielded twin geometry. 
For this construction, inductance is minimized for a minimum (w/r) 
.gamma.. Parameter .gamma. is minimized for a maximum value of w/2R. This 
occurs at 2R=2w, when an insulated, twisted twin conductors are tightly 
wrapped with the shielding for the cable. Thus, 
EQU (w/2R).sub.max =1/2, which implies, .gamma..sub.min =3/5. 
Referring to FIG. 5, the dependence between w/2R and .gamma. is shown. To 
get a comparison between the shielded twin geometry of currently used 
cables, and that of coaxial cable 10, the minimum .gamma. value is used. 
First, the scale sizes between the two types of conductors is made 
comparable. That is, 
EQU (D/d).sub.coax =(w/2r).sub.S-twin. 
From this relationship, the ratio of inductance for the respective 
conductors is expressed as: 
EQU L.sub.coax /L.sub.S-twin =ln(D/d)/2ln(6D/5d). 
As D/d approaches plus infinity, the ratio expressed above converges on the 
1/2 value. This is shown in FIG. 6. Further, if an unshielded twin-type 
geometry is used, the above expression is restated as: 
EQU L.sub.coax /L.sub.twin =ln(D/d)/2ln(2D/d).ltoreq.L.sub.coax /L.sub.S=twin. 
Consequently, regardless of which type of twin conductor is used in the 
hook-up of FIG. 1, the coaxial geometry of cable 10 provides at least a 
factor of two improvement in the reduction of inductance, for comparable 
sized conductors. 
As noted previously, capacitance in a cable varies inversely with 
inductance. Therefore, based upon the above formulations, there should be 
an increase in capacitance in cable 10 over that in a shielded or 
non-shielded twin conductor. This increase should be by a factor of at 
least two. The dc resistance for cable 10 should, however, be comparable 
with that of shielded twin cable conductors now in use, assuming 1) that 
inner conductor 12 is a similar gage wire to that of either of the two 
inner conductors of the shielded twin cable, and 2) that an equivalent 
gage is also used for outer conductor 14. 
Having established that coaxial cable 10 provides an improvement of at 
least two with respect to certain performance parameters with respect to 
shielded twin cables, the cable's performance is also compared with other 
type conductors shown in FIG. 3. With respect to a parallel plate cable 
geometry, the scale size of the two type cables are first made comparable. 
That is, the aspect ratios for the two types of cable are expressed as: 
EQU (D/d)=(y/x). 
The ratio of inductance for the two type cables is then, 
EQU L.sub.coax /L.sub.p.p. =(1/2.pi.)(D/d)ln(D/d) 
FIG. 7 graphically represents the ratio of cable 10 and parallel plate 
inductances per unit length. These values have been normalized on the 
basis of vacuum permittivity. With respect to FIG. 7, it is shown that 
when D/d is &lt;4.3, the geometry of cable 10 provides better results than 
the parallel plate geometry. Otherwise, very significant parallel plate 
aspect ratios are required in the parallel plate geometry to obtain a 
performance similar to that of cable 10. 
Based on the foregoing, the geometry of cable 10 provides better 
performance characteristics, given normal manufacturing requirements for 
cables to be used in the network/SPD application than either of the other 
two cables. That is, for a reasonable aspect ratio (D/d), better 
low-inductance, high-capacitance performance is available with cable 10. 
As noted, shunt conductance can generally be disregarded. 
For the coaxial cable geometry of cable 10, the inductance L, resistance R, 
and capacitance C characteristics must also be considered in order to 
optimize the performance of a MOV in a SPD. Peak performance of a MOV 
minimizes the clamping voltage. First, it has been found that the 
preferred aspect ratio of cable 10 is 1.05. This means the diameter d of 
inner conductor 12 is substantially the same diameter as the inner 
diameter of outer conductor 14. Or, there is only a thin layer of 
insulation material 16 separating the inner and outer conductors. For this 
aspect ratio, and given other practical considerations such as the overall 
size of cable 10, dielectric voltage hold-off, etc., cable 10 can provide 
the desired MOV performance for a SPD connected to the network. 
With the 1.05 aspect ratio, and a relative permittivity of 3.0, the 
capacitance of cable 10 is approximately 0.0035 microfarads/meter. 
ANSI/IEEE C62.41 deals with IEEE Recommended Practices for Surge Voltages 
in Low-Power AC circuits. A category B3 combination waveform set out in 
this document has waveform characteristics of 1.2.times.50 microseconds at 
6 kV, and 8.times.20 microseconds at 3 kA. For this test or specimen 
waveform, the capacitance of cable 10 diverts over 3.0+ amps. I.e., 
EQU I=(CV)/t=3.15 amps. 
Given a 3 kA short-circuit current, a 3.15 amp, or 0.11% current diversion 
is insignificant. This can be readily seen by viewing the MOV operating 
characteristics of FIG. 4. As shown on the V-I curve of this Fig., for the 
normal operating range of the MOV, where the slope of the curve is 
approximately zero, a reduction from 3 kA, for example, down to 2, 997A 
has no significant effect in reducing the clamping voltage of a SPD. If, 
however, much faster transients than those represented by a category B3 
waveform appears, the contribution of the capacitance will become 
increasingly significant. 
The geometry of the cable 10 design further specifies the inductance L. 
From FIG. 3, the inductance per unit length of cable 10 is: 
EQU L=(.mu..sub.0 /2.pi.)ln(D/d)=(.mu..sub.0 /2.pi.)(1+(.delta./r)), 
where .delta. is the thickness of the dielectric material, and r is the 
radius of inner conductor 12. By minimizing the value of .delta./r, the 
inductance of the cable can be minimized. This is accomplished where, as 
in cable 10 with its aspect ratio of approximately 1.05, inner conductor 
12 has a large diameter d, and material 16 comprises a thin annular layer 
between the inner and outer conductors. So long as the dielectric layer of 
material 16 is sufficiently thick to hold off nominal line voltage, it 
does not have to be especially thick. Further, the inner conductor does 
not have to be a solid conductor. As shown in FIG. 8C, coaxial cable 10' 
includes an inner conductor 12' which is a hollow, cylindrical conductor. 
The trade-off is that a hollow inner conductor has less cross-sectional 
area than a solid one. Accordingly, the dc resistance of conductor 12' is 
higher than that of conductor 12 for a same diameter d conductor. Further, 
because the hollow core inner conductor 12' uses less copper or silver, a 
cable using this inner conductor is relatively less expensive. 
In designing cable 10, one factor to be considered for commercial use of 
the cable is obtaining Underwriter's Laboratory (UL) acceptance. A 
fabrication of a test cable 10 is shown in FIG. 8A in which a #10 AWG type 
THHN wire is covered with a #10 AWG tinned copper braiding to form the 
outer conductor of the coaxial cable. Braid pig-tails 24 are formed at 
each end of the cable are covered with a length of shrink tubing 26. For 
purposes of determining a trend in cable 10 behavior, five cables were 
constructed similar to that shown in FIG. 8A. One cable each was 
constructed of #14, #10, #6, #2, and #3/0 AWG. The thickness of the 
dielectric material (.delta.) was kept constant at 0.025" (0.635 mm); 
while, the minor radius r ranged from 0.225" (5.72 mm) to 0.032" (0.81 
mm). The cross-sectional area of the center conductor 12 varied 
proportionately with r.sup.2. The variance in radius also effected the DC 
resistance of the conductor. 
FIG. 10 presents a table listing each of the five cables 10 and the cable 
parameters of each. Lines 1-8 of FIG. 10 list the respective parameters 
discussed above for cable geometry and materials including the various 
resistance, inductance, and capacitance values. Each cable was connected 
to a MOV. The MOV and cable were mounted in a common fixture that was used 
throughout the tests. Each cable was pulsed with a 1,500 V category C 
transient. This transient's characteristics are 1.2.times.50 microseconds 
at 6 kV, and 8.times.20 microseconds at 10 kA. Each are maximum figures. 
Further, for each test cable, five transient waveforms were generated and 
propagated through the cable to the MOV. The clamping voltage and current 
results were averaged and the resulting deviation is shown at lines 13 and 
15 of FIG. 10 for each cable. In addition to these tests, the MOV was 
directly connected to the pulser unit (not shown) used to generate the 
transient waveforms. Transient waveforms generated by the pulser unit were 
then directly applied to the MOV as a voltage surge. The difference 
between the clamping voltages, with and without a test cable connected to 
the MOV, are shown at line 16 of FIG. 10. The peak current for each cable 
is shown at line 14 of the Fig. The variation in peak current is 
approximately 7% which is within an acceptable range for a valid 
experiment. 
For each test cable, the voltage drops due to cable inductance and dc 
resistance are calculated in accordance with the respective formulas 
previously derived. The dc resistivity of the cable was directly measured. 
Line 9 of FIG. 10 indicates the values for outer conductor 14 and line 10 
for inner conductor 12. 
FIG. 9 represents a comparison between theoretical and actual experimental 
additional clamping voltage (over MOV only clamping voltage) for the 
various cables. The upper and lower bars represent the upper and lower 
limits for each cable based upon the experimental results. In each 
instance it is seen that the theoretical calculations and actual results 
are within an acceptable range of each other. Based upon this test 
information, it is evident that the two critical design parameters of a 
cable 10 are 1) the ratio of thickness of the dielectric material 16 used 
to the radius of center or inner conductor 12, and 2) the cross-sectional 
area of the conductors. The first of these determines inductance, and the 
second dc resistance. 
In addition to the above described test, a second series of tests were 
performed testing the performance of the geometry of coaxial cable 10 with 
that of other cable geometries. Two coaxial cables were used in the test, 
one a #6 AWG cable, and the other a #10 AWG cable. In addition to the 
cables 10, the other geometries included a twisted quad cable with an over 
braid, and a pair of THHN wires in a conduit. Each cable was identical in 
length, i.e., 9.25 ft. (2.82 m). One end of each cable was connected to a 
pulser unit similar to that used in the previous tests, and the other end 
to a MOV. Again, a 1,500 V category C transient was propagated down each 
cable and clamping voltages and currents were measured. Again as before, 
the MOV was tested without a cable connected to it. 
Referring to FIG. 11, the "let-through" voltage for each test cable, in 
addition to the MOV itself, are shown. The MOV, by itself, measures 
slightly over 800 V. The THHN cable, which is shown on the far right of 
the Fig. has a "let-through" voltage which is some 220 V higher than the 
MOV. Next to the THHN cable, the twisted quad with over-braid cable is 
shown to permit a "let-through" voltage over 140 V higher than the MOV by 
itself. With respect to the two sizes of coaxial cables used, the #10 AWG 
cable allows less than 75 V over the MOV by itself. This is threefold 
improvement over the conventional THHN cable. Finally, the #6 AWG cable 
allows less than 50 V. over the MOV by itself. This represents a 4.6 times 
improvement over the conventional cable's performance. 
What has been described is a cable 10 for use in power distribution 
networks N for connecting SPD's in parallel with network conductors W. 
This allows the SPD to protect loads LD connected to the network from high 
voltages surges and transients. The cable is a low impedance coaxial cable 
capable of use with any type SPD and whose use produces a minimal voltage 
drop so the SPD is subjected to substantially all the transient voltage. 
This, in turn, reduces or eliminates the amount of voltage "let through" 
to loads downstream of the SPD. Low impedance of the cable is based on an 
optimal cable geometry, cable dimensioning, and the material used in 
making the cable. In this regard, the cable of the invention has a 
minimized series inductance and dc resistance. The cable has parallel 
conductors separated by an insulator which produces a negligible shunt 
conductance between the conductors. The cable has a compact form with an 
aspect ratio of only 1.05. It will be understood, however, that as shown 
in FIG. 10, cables having an aspect ratio D/d ranging from approximately 
1.05 to approximately 1.56 fall within a range of cable aspect ratios 
contemplated by the invention. Use of the cable allows for a greatly 
reduced SPD clamping voltage rating. The cable is safe in use, and is easy 
to make. 
In view of the foregoing, it will be seen that the several objects of the 
invention are achieved and other advantageous results are obtained. 
As various changes could be made in the above constructions without 
departing from the scope of the invention, it is intended that all matter 
contained in the above description or shown in the accompanying drawings 
shall be interpreted as illustrative and not in a limiting sense.