Rod insulator with elastic overcoats and conducting paths straddling joint portions of adjacent overcoats

A synthetic resin insulator comprising a fiber-reinforced plastic rod provided at its both ends with holding metal fittings; a plural number of overcoats fitting to and covering the plastic rod, each overcoat consisting of an elastic insulating material and provided at its outside with a shed; and a conducting path formed straddling the joint portion of adjacent overcoats flows leakage current through the conducting path, is not eroded at the joint portion of adjacent overcoats, and is very long in the life.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to an improvement of synthetic resin 
insulators comprising a fiber-reinforced plastic rod or pipe (hereinafter, 
referred to as reinforced plastic rod), overcoats consisting of an elastic 
insulating material, and holding metal fittings. 
(2) Description of the Prior Art 
A reinforced plastic rod, reinforced with bundles of fibers or knitted 
fiber bundles in their axial direction, has a resistance against very high 
tensile stress and an extremely high strength-to-weight ratio. While, 
elastic insulating materials, such as silicone rubber, ethylene-propylene 
rubber, polyethylene, polypropylene, ethylene-propylene copolymer, 
cycloalipatic epoxy, acrylic, polyfluoroethylene and the like, 
occasionally mixed with an inorganic filler having a low decomposition 
temperature, such as alumina trihydrate or the like, have excellent 
weather resistance and tracking resistance, recently there have been made 
various investigations for producing light and high-strength synthetic 
resin insulators by combining these elastic insulating materials. As a 
typical synthetic resin insulator, there has been known an insulator 
comprising a reinforced plastic rod 1, a large number of superposed 
overcoats 3 made of an elastic insulating material and fitted to the rod 
1, each overcoat 3 being provided at its outside with one shed, and grease 
6 filled in the interface 4 between the reinforced plastic rod 1 and the 
overcoats 3, as illustrated in FIG. 1, FIG. 2a and FIG. 2b. 
However, the conventional synthetic resin insulator, wherein a large number 
of individual overcoats 3 are superposed, is assembled in the following 
manner in order to prevent the leakage of grease 6 from the interface 4 or 
the penetration of water or the like into the interface 4. That is, 
overcoats 3 having an inner diameter smaller than the outer diameter of a 
reinforced plastic rod 1 are used in order to fasten tight always the 
reinforced plastic rod 1 and further the overcoats 3 are compressed in 
their axial direction between both holding metal fittings 2 and 2 to cause 
pressure between adjacent overcoats 3 and 3. As a result, the overcoats 3 
are always elongated to the circumferential direction. Such elongated 
state promotes the breakage of molecular chain of elastic insulating 
material due to oxygen, ultraviolet ray and the like, and the electric 
insulating material in elongated state is apt to be easily deteriorated. 
Particularly, the shoulder x at the contact portion 5 of adjacent 
overcoats 3 is easily deteriorated by oxidation due to its large specific 
surface area as shown in FIG. 2a. Moreover, as the overcoats 3 are 
compressed in their axial direction, stress is concentrated into the 
shoulder x.sub. 1 and the shoulder x.sub.1 is elongated in a large amount 
and is apt to be deteriorated more easily. In general, this erosion 
proceeds in a direction perpendicular to the stretching direction. In 
addition, the shoulder x.sub.1 is eroded by the minute discharges due to 
leakage current, which flows on the overcoat surface during rainfall, as 
shown by the mark x.sub.2 in FIG. 2b, and the erosion grows rapidly in the 
form of a groove in a direction perpendicular to the stretching direction, 
that is, towards the interface 4 between the reinforced plastic rod 1 and 
the overcoats 3 in combination with the above-described deterioration of 
the shoulder. This directional erosion reaches the interface 4 between the 
overcoat 3 and the reinforced plastic rod 1 in a very short period of time 
to cause leakage of the grease 6 and penetration of water easily, and to 
promote insulation breakdown of the interface 4, and further to erode and 
break the reinforced plastic rod. As a result, the function of the 
insulator is lost. In this case, the deterioration speed of the function 
of the insulator due to erosion depends upon the erosion speed at the 
contact portion of adjacent overcoats 3. 
Furthermore, when the insulator is practically used in the power 
transmission line, the insulator is exposed to the direct ray of the sun 
to cause temperature rise of the insulator, and grease 6 filled in the 
interface 4 is expanded due to the temperature rise to expand the overcoat 
3. In this case, since airtightness between adjacent overcoats superposed 
one upon another is secured merely by the action of compression force in 
the axial direction of the overcoats, the expanded grease 6 leaks from the 
contact portion 5 of adjacent overcoats 3. Moreover, when a hot-line 
washing is carried out by the use of high-pressure water in order to wash 
away pollutant adhered to insulators used in a substation or the like in a 
region wherein insulators are violently polluted, the overcoats 3 are 
forcedly moved by the high-pressure water blown thereto to form gaps at 
the contact portion 5 of adjacent overcoats 3 and 3, and water is 
penetrated into the interface 4 through the gaps. As described above, 
there are many problems. In order to overcome these problems, there has 
been proposed an insulator, wherein a reinforced plastic rod 1 is bonded 
with overcoats 3 at the interface 4 with an adhesive and adjacent 
overcoats 3 are bonded with each other at the contact portion 5 with an 
adhesive. However, in this insulator, since the adhesive is generally an 
active material, the adhesive, even after solidified, is apt to be 
deteriorated more easily than the overcoat materials, and when the 
adhesive is exposed to the external atmosphere at the contrast portion 5 
of adjacent overcoats, the adhesive layer is firstly deteriorated by the 
action of the above-described ultraviolet ray and oxygen and water in the 
external atmosphere, followed by the erosion due to minute discharges, to 
form gaps in the adhesive layer; and the shoulder x.sub.1 which has a 
large specific surface area and is liable to be oxidized and deteriorated, 
is successively eroded and deteriorated. This erosion reaches the 
interface 4 in a short period of time similarly to the above-described 
insulator, wherein grease 6 is filled in the interface 4, to cause 
insulation breakdown at the interface 4 and further to erode gradually the 
reinforced plastic rod 1, resulting in the separation of the insulator. 
Therefore, the insulator has serious drawbacks. 
Further, there has been known an insulator produced by molding directly an 
individual overcoat 3 having one shed 8 on a reinforced plastic rod 1 by 
means of a mold 12 and repeating this molding to form the whole overcoats 
into substantially a unitary structure as illustrated in FIGS. 3a and 3b. 
However, in this insulator, the bonded plane 13 of adjacent overcoats 3 
and 3 formed in every molding is weak chemically and mechanically and is 
apt to be oxidized and deteriorated, and moreover when the reinforced 
plastic rod 1 is elongated by the load applied to the insulator, the bond 
plane 13 of the overcoats 3 is often exfoliated, and therefore the 
insulator has serious drawbacks similarly to the above-described 
insulator. In order to solve the above-described drawbacks and problems, 
there has been proposed an insulator having a seamless unitary overcoat. 
However, a large size mold is required for producing the overcoat 3 
corresponding to the increase of the length of insulator, and moreover it 
is very difficult to mold a long, slender, shed-shaped, peculiar overcoat, 
and mass production of the overcoat 3 having a length of more than 1 m is 
considered to be difficult. 
Recently, the transmission voltage is raised more and more in order to 
obtain a high transmission efficiency, and an insulator having a long 
insulating distance has become necessary corresponding to the high 
transmission voltage. 
Accordingly, when it is intended to obtain a desired insulating distance by 
using relatively short seamless unitary insulators, a large number of the 
insulators must be connected. There are many problems, namely, the 
insulating distance must be long in an amount corresponding to the lengths 
of respective holding metal fittings. Therefore, a tall steel tower which 
is required is expensive. Moreover, the weight of the insulator assembly 
increases corresponding to the increase of the number of holding portions, 
and further the respective holding metal fitting portions form weak points 
due to concentration of mechanical stress and electric stress, and hence 
the reliability of the insulator is lost when a large number of the weak 
points are formed. 
SUMMARY OF THE INVENTION 
The object of the present invention is to obviate the above-described 
drawbacks and problems in the conventional synthetic resin insulators. 
That is, the feature of the present invention is the provision of a 
synthetic resin insulator, comprising a fiber-reinforced plastic rod, 
holding metal fittings which hold both ends of the fiber-reinforced 
plastic rod, a plural number of overcoats which consist of an elastic 
insulating material and cover the total surface of the fiber-reinforced 
plastic rod located between the holding metal fittings, and conducting 
paths formed straddling the joint portion of adjacent overcoats in order 
that leakage current, which flows on the surface of the insulator when the 
insulator is wetted, flows through the conducting path and does not flow 
through the joint portion of the overcoats.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention will be explained in more detail by the following 
examples referring to FIGS. 4a-19. Among the references in these figures, 
the same references as those shown in FIGS. 1-3b represent the same 
portion as or corresponding portion to those shown in FIGS. 1-3b. 
The synthetic resin insulator of the present invention, as illustrated in 
FIG. 4a, comprises a reinforced plastic rod 1 produced by impregnating 
bundles of fibers, such as glass and the like, arranged in their axial 
direction or knitted fiber bundles with a synthetic resin, such as epoxy 
resin, polyester resin or the like, and curing the resin; holding metal 
fittings 2 and 2, which are fixed at one end to both ends of the 
reinforced plastic rod 1, and are provided at their another end with a 
structure, for example, an eye-ring, clevis or mounting base for linepost 
insulator, fitting member, for fitting directly or indirectly the holding 
metal fitting to conductor or steel tower arm or other supporters; a 
plural number of overcoats 3 consisting of a rubber-like elastic 
insulating material, such as silicone rubber, ethylene-propylene rubber or 
the like, and covering substantially the total surface of the reinforced 
plastic rod 1 located between the holding metal fittings 2 and 2, each 
overcoat 3 being provided at its outside with a shed 8 unitarily formed; 
and conducting paths 9a, each made of a conductive material, such as metal 
or the like, and having a proper shape, and being formed straddling the 
joint portion 5 of adjacent overcoats 3 and 3 so that leakage current, 
which flows on the surface of the insulator when the insulator is wet, is 
locally short-circuited not to flow through the joint portion 5 of the 
overcoats. 
The conducting path 9a has a long length l enough to straddle the joint 
portion 5 of adjacent overcoats, which are contacted to each other or 
apart from each other at the ends, as illustrated in FIGS. 4b and 4c in an 
enlarged scale. 
In the present invention, a conducting path 9a having a shape illustrated, 
for example, in FIGS. 5, 6 or 7 can be optionally used. The conducting 
path 9a illustrated in FIG. 5 is made of two metal rings arranged 
concentrically and connected with each other into a unitary structure 
through a rod-shaped conducting member; that illustrated in FIG. 6 is made 
of a metal plate having a given width and curved along the surface of an 
insulator in the peripheral direction; and that illustrated in FIG. 7 is 
made of a metal or other conductive material formed into a hollow 
cylinder. Further, the cross-sectional shape of the conducting path 9a 
along the center axis is formed into the following shape. For example, in 
a hollow cylindrical conducting path, a smooth inner side surface as 
illustrated in FIG. 8 can attain the object of the present invention. 
Further, a conducting path having a projection at the center portion of 
inside so that the projection can be fitted into the recess formed at the 
edge of the joint portion 5 of adjacent overcoats as illustrated in FIG. 
9; or a conducting path, wherein recesses are formed on each of adjacent 
overcoats 3 and 3 at a position apart from the joint portion 5 and 
projections are formed on the upper and lower sides of the inner side 
surface of the conducting path so that the projections can be fitted into 
the recesses as illustrated in FIG. 10; are preferably used. The 
arrangements of conducting path as illustrated in FIGS. 9 and 10 are free 
from the shifting of the positions to the overcoats and the conducting 
path in the fitted state, and are preferable. 
The insulator having a conducting path 9a arranged on the joint portion 5 
of adjacent overcoats according to the present invention has the following 
merits contrary to the conventional insulator. In the conventional 
insulator, when the overcoat surface is wetted during rainfall, leakage 
current flows on the surface of the overcoat to generate minute discharges 
by leakage current on the overcoat surface, and the overcoat is eroded by 
the minute discharges to lose the function of insulator. However, in the 
insulator of the present invention, the leakage current flows selectively 
through the conducting path 9a arranged on the joint portion 5, and minute 
discharges do not generate in the joint portion 5. Therefore, the 
insulator of the present invention has a remarkably prolonged life. 
The above-described effect will be explained based on the test results 
shown in the following Tables 1 and 2. Samples A, B and C shown in Table 1 
are conventional insulators having no conducting path 9a. Sample A 
contains grease filled in the interface 4 in the structure shown in FIG. 
1; Sample B has bonded overcoats 3 with adhesive at the joint portion 5 in 
the structure shown in FIG. 1; and Sample C has overcoats 3 formed by 
repeated moldings shown in FIGS. 3a and 3b. Samples D, E and F shown in 
Table 1 are insulators of the present invention. Sample D has a conducting 
path 9a arranged on the joint portion 5 of Sample A; Sample E has a 
conducting path 9a arranged on the joint portion 5 of Sample B; and Sample 
F has a conducting path 9a arranged on the joint portion of Sample C. All 
the Samples A to F have overcoats made of ethylene-propylene rubber. 
Samples G and H shown in Table 2 are conventional insulators having no 
conducting path 9a. Sample G has overcoats 3 made of polyethylene and 
contains grease 6 filled in the interface 4 in the structure shown in FIG. 
1, and Sample H has overcoats 3 made of cycloaliphatic epoxy and formed by 
repeated moldings shown in FIGS. 3a and 3b. Samples I and J shown in Table 
2 are insulators of the present invention. Samples I and J have a 
conducting path 9a arranged on the contact portion 5 of samples G and H, 
respectively. 
As the conducting path 9a, there was used a conducting path having a length 
l of 30 mm, which consisted of two copper wire rings connected unitarily 
with each other through a conducting member, such as copper wire or the 
like. Each sample insulator had a dimension of an outer diameter of the 
shell portion of 36 mm, a diameter of the shed of 138 mm, a distance in a 
straight line between both holding metal fittings of 200 mm, the number of 
sheds of 3, and a shed pitch of 60 mm. A brine was sprayed intermittently 
on the insulator under a condition that a voltage of 20 KV was applied. 
Spray procedure was repetition of 10 seconds spraying at a flow rate of 
120 ml/min and 20 seconds intermission. This cycle was repeated 
continuously to flow forcedly leakage current on the overcoat surface, to 
cause minute discharges on the overcoat surface, and to erode the 
overcoat. The portion, at which the erosion developed, and the time until 
the erosion reached the interface were measured. The obtained results are 
shown in Tables 1 and 2. 
TABLE 1 
______________________________________ 
Time until erosion 
reached interface 
Eroded portion 
(days) 
______________________________________ 
Conventional 
Sample A contact portion 
20 
insulator 
Sample B contact portion 
28 
Sample C contact portion 
30 
Insulator of 
Sample D upper portion of 
not less than 200 
this invention conducting path 
Sample E upper portion of 
not less than 200 
conducting path 
Sample F upper portion of 
not less than 200 
conducting path 
______________________________________ 
TABLE 2 
______________________________________ 
Time until erosion 
reached interface 
Eroded portion 
(days) 
______________________________________ 
Conventional 
Sample G contact portion 
25 
insulator 
Sample H contact portion 
20 
Insulator of 
Sample I upper portion of 
not less than 200 
this invention conducting path 
Sample J upper portion of 
not less than 200 
conducting path 
______________________________________ 
It can be seen from the test results shown in Tables 1 and 2 that, in the 
conventional insulators of Samples A, B, C, G and H, erosion develops at 
the contact portion of overcoats, and the erosion reached the interface 
between the reinforced plastic rod and the overcoat in 20-30 days; while, 
in the insulators of the present invention of Samples D, E, F, I and J, 
the joint portion is not at all eroded, and erosion develops at a portion 
other than the joint portion, and not less than 200 days are required 
until the erosion reaches the interface, which illustrates that the 
insulator of the present invention is expected a life as long as not less 
than 10 times the life of the conventional insulator. 
In the above-described insulators, the conducting path formed straddling 
the contact portion of overcoats is made of two metal rings connected 
concentrically to each other through a conducting member. Also, the 
conducting path may be made of a metal plate having a given width and 
curved along the insulator surface in the peripheral direction as 
illustrated in FIG. 6. This conducting path can be easily mounted on the 
joint portion 5 of overcoats, prevents generation of minute discharges at 
the joint portion 5 of overcoats, and further interrupts the ultraviolet 
ray, whereby the conductive path prevents the deterioration of the 
insulator due to these phenomena. Therefore, the conducting path is 
preferably used. Furthermore, a hollow cylindrical conducting path 
illustrated in FIG. 7 is particularly preferably used, because the 
conducting path can cover completely the joint portion 5, and therefore 
the conducting path can prevent surely generation of minute discharges, 
interrupt the ultraviolet ray and further prevent the penetration of water 
and the like into the interface 4 between an overcoat 3 and a reinforced 
plastic rod 1. 
In the curved conducting path 9a illustrated in FIG. 6, when the conducting 
path is mounted on along the surface of an insulator in the peripheral 
direction, an opening 10 is formed along the center axis in the peripheral 
direction. In this case, when the opening 10 has a width in the peripheral 
direction of not larger than 1/4 of the total peripheral length of the 
conducting path 9a as illustrated in FIG. 12, the joint portion 5 of 
overcoats 3 can be substantially protected from the erosion due to leakage 
current. 
Further, erosions k.sub.1 and k.sub.2 are formed due to leakage current at 
the both ends of the conducting path 9a. For example, the case of the 
hollow cylindrical conducting path is illustrated in FIG. 11. The upper 
end a is located at the back side of the shed 8 of the upper overcoat 3, 
one of the adjacent overcoats 3 and 3. The lower end b is located at the 
front side of the shed 8 of the lower overcoat 3, the other of the 
adjacent overcoats 3 and 3. The overcoat 3, which contacts with the lower 
end b of the conducting path 9a, is apt to be eroded more easily than that 
which contacts with the upper end a of the conducting path 9a. Therefore, 
when the length of the upper portion and that of the lower portion of the 
conducting path 9a measured from the joint portion 5 of the adjacent 
overcoats 3 and 3 are represented by references A and B respectively, the 
following conditions 
EQU A.gtoreq.5 mm and A.ltoreq.B 
are preferred to be satisfied in order to prevent the growth of the erosion 
up to the interface 5 due to the leakage current which flows on the 
overcoat 3 in a small amount not to cause deterioration of the function of 
the insulator. 
Further, it is preferable that the overhung length H of a shed 8 formed on 
a overcoat 3 is not less than 1/2 of the length l.sub.1 of a conducting 
path 9a and the distance l.sub.2 between adjacent sheds is not more than 2 
times the overhung length H of a shed as shown in FIG. 13, because the 
decreasing of an effective length of the insulator due to the arrangement 
of the conducting path 9a can be compensated by the above-described 
limitation of l.sub.1, l.sub.2 and H. 
The above-described facts will be explained referring to FIGS. 14, 15 and 
16. FIGS. 14 and 15 illustrate withstand voltage properties of insulators 
with and not with the conducting path 9a. FIG. 16 illustrates the sample 
insulator being made on experiment. The distance l.sub.3 between the 
electrodes of the sample insulators is 1,000 mm and the length l.sub.1 of 
a hollow cylindrical conducting path 9a in the axial direction is 30 mm. 
In the above experiment, in order to make the effective length uniform, an 
arcing horn 11 is arranged, which has an overhung length 10 mm larger than 
the overhung length H of the shed. 
FIG. 14 illustrates a relation between the ratio of H/l.sub.1 shown in 
abscissa and the withstand voltage shown in ordinate in the case where 
l.sub.1 is substantially equal to l.sub.2 and H is varied. The solid line 
(a) in FIG. 14 illustrates the relation when the conducting path 9a is 
used, and the dotted line (b) illustrates the relation when the conducting 
path 9a is not used. It can be seen from FIG. 14 that, when the overhung 
length H of a shed is not less than 1/2l.sub.1, the decrease of withstand 
voltage of an insulator due to the use of a conducting path 9a does not 
appear. Further, FIG. 15 shows a relation between the ratio of l.sub.2 /H 
shown in abscissa and the withstand voltage shown in ordinate. In FIG. 15, 
the solid line (c) illustrates the relation when the conducting path 9a is 
used, and the dotted line (d) illustrates the relation when the conducting 
path 9a is not used. It can be seen from FIG. 15 that, when the ratio of 
l.sub.2 /H is less than 2, wherein l.sub.2 represents the distance between 
adjacents sheds and H represents the overhung length of a shed, the 
decrease of withstand voltage property due to the use of the conducting 
path 9a does not appear. 
Further, as to the distances L.sub.1 and L.sub.2 between the holding metal 
fittings 2 or electrode-forming portions, which are fitted to the holding 
metal fittings 2 and have an arcing horn, (hereinafter, the holding metal 
fitting or the electrode-forming portion is referred to as electrode) and 
the conducting paths 9a nearest to each of the electrodes shown in FIG. 
17, when at least the distance L.sub.2 between the electrode at the 
electric power-supply side and the conducting path nearest thereto is at 
least 20% based on the distance L.sub.3 between the opposite electrodes, 
the deterioration of insulating performance due to the use of the 
conducting path 9a can be substantially prevented. This fact will be 
explained referring to FIG. 18. 
FIG. 18 illustrates the withstand voltage property of the insulator with 
and not with the conducting path 9a. FIG. 17 illustrates the sample 
insulators being made on experiment. These sample insulators having the 
distance of 6,000 mm between the opposite electrodes are arranged with 
conducting paths 9a at intervals of about 300 mm. In FIG. 18, the solid 
line illustrates the result in the case where conducting paths 9a are 
arranged at intervals of about 300 mm and the conducting path 9a nearest 
to the electrode at the energized end side is adjusted to vary the 
distance L.sub.2 between the electrode at the energized end side and the 
conducting path 9a nearest to the electrode. 
It can be seen from FIG. 18 that, when the ratio in percentage of L.sub.2 
/L.sub.3 is at least 20%, wherein L.sub.2 is the distance between the 
electrode at the energized end side and the conducting path 9a adjacent 
thereto and L.sub.3 is the distance between the opposite electrodes, the 
withstand voltage of the insulator does not substantially decrease. 
The synthetic resin insulator of the present invention, for example, one 
having a structure to be filled with grease or an adhesive, can be 
assembled by the following method. A reinforced plastic rod 1, a necessary 
number of overcoats 3, having been individually produced and having a 
given length are provided, and a number of conducting paths 9a having a 
hollow cylindrical shape or the like having an inner diameter larger than 
the outer diameter of the end portion of the overcoat 3 are required the 
same as the number of joint portions 5. One end of each overcoat 3 is 
fitted into a conducting path 9a, and then the overcoat 3 having a 
conducting path 9a are fitted to the reinforced plastic rod 1 together 
with grease or an adhesive. In this case, it is preferable that the inner 
diameter of each overcoat 3 is not excessively larger than the outer 
diameter of the reinforced plastic rod in order not to expand the surface 
of the overcoat towards the peripheral direction at the fitted state. 
Then, the conducting path 9a is uniformly compressed in the centripetal 
direction at a given position by means of a hydraulic press arranged 
radially and is deformed and reduced so that the conducting path 9a is 
tightly fixed to the end portion of the overcoat 3 to press it. 
After the overcoats 3 are fitted to the reinforced plastic rod 1 together 
with grease 6 or an adhesive and then the conducting paths 9a are fitted 
to the joint portions 5, holding metal fittings are fixed to both ends of 
the reinforced plastic rod 1 to assemble a synthetic resin insulator of 
the present invention. When a frame capable of molding an individual 
overcoat 3 having one shed is used to mold directly the overcoat 3 on a 
reinforced plastic rod 1 as illustrated in FIGS. 3a and 3b and this 
molding is repeatedly carried out to produce an insulator having 
substantially a unitary structure, a conducting path 9a is fitted to the 
overcoat in every molding similarly to the production of an insulator 
having the above-described structure containing grease filled therein, and 
after the total moldings are completed, the conducting path 9a is 
compressed in the centripetal direction on a given position, that is, on 
an adhering plane 13 of adjacent overcoats 3, whereby the conducting path 
9a is deformed and reduced so that the conducting path 9a is tightly 
contacted to the surface of the overcoat 3. Then, holding metal fittings 
are fixed to both ends of the reinforced plastic rod 1 to assemble a 
synthetic resin insulator of the present invention. 
The present invention can be variously modified from the above-described 
embodiments without departing from the scope of the present invention. For 
example, in the above-described example, the end portion of a holding 
metal fitting 2 is surrounded with an overcoat 3. Further, in the present 
invention, an insulator having the following structure is preferably used 
due to the reason that minute discharges at the joint portion 5 of 
adjacent overcoats 3 can be prevented, the end portions of adjacent 
overcoats can be mutually and firmly fixed and an overcoat 3 can be 
airtightly isolated from the external atmosphere at the interface 4 
between the reinforced plastic rod 1 and the overcoat 3 to prevent surely 
the penetration of water and the like into the interface 4. That is, in 
this structure, a sleeve 9b which receives the end portion of an overcoat 
3 and is contacted thereto, is airtightly fixed to a holding metal fitting 
2 at the side for receiving a reinforced plastic rod by a threaded 
engagement or unitary working through a seal tape or O-ring as illustrated 
in FIG. 19, and further a conducting path 9a straddling a joint portion 5 
of overcoats 3 is formed by bending a metal plate into a cylindrical shape 
closely adhering to the surface of the insulator along the peripheral 
direction as illustrated in FIG. 7, whereby the end portion of the 
overcoat 3 is received in the conducting path, and the conducting path is 
compressed uniformly in the centripetal direction and is deformed and 
reduced to press the end portion of the overcoat 3. When the outer 
diameter of an overcoat 3 or the inner diameters of a conducting path 9a 
and sleeve 9b are adjusted so that the conducting path 9a and sleeve 9b 
are contacted with the surface of an overcoat 3 at the inlet portion and 
are pressed against the overcoat 3 at the inner portion, the elongation of 
the surface of the overcoat 3 is small in the portion exposed to the 
external atmosphere and the growth of groove-shaped erosion can be 
prevented. 
It is preferable that synthetic resin insulators having overcoats made of 
an elastic insulating material, such as ethylene-propylene rubber or the 
like, are free from damage at the fitting to steel tower or the like, and 
are excellent in the handling. On the contrary, overcoats made of these 
rubbers are poor in the erosion resistance due to the structure at the 
joint portion of the overcoats. According to the present invention, the 
joint portion can be protected, and synthetic resin insulators having the 
above-described excellent properties can be obtained. 
Thermoplastic resins, such as polyethylene and the like, do not contain 
--C.dbd.C-- bonds in the chemical structure and are excellent in the 
tracking resistance. However, in the production of overcoats, it is 
preferable that individual overcoats, each having one shed, are 
individually produced, and then superposed to form the overcoats in view 
of the moldability of the thermoplastic resin. Accordingly, the drawbacks 
at the contact portion of overcoats of synthetic resin insulators having 
such overcoats can be overcome by the present invention. Further, when it 
is intended to produce an insulator by a method, wherein an individual 
overcoat having one shed is directly molded on a reinforced plastic rod, 
and this molding is repeatedly carried out to form overcoats having 
substantially a unitary structure, thermosetting resins, such as 
cycloaliphatic epoxy and the like, are used due to their good moldability. 
The present invention can overcome the drawbacks of an interface of 
adjacent overcoats adhered with each other through the above-described 
methods. 
As described above, according to the present invention, synthetic resin 
insulators having excellent erosion resistance can be produced without 
losing excellent properties inherent to each elastic insulating material. 
According to the present invention, conducting paths are arranged to 
synthetic resin insulators, whereby leakage current which is a cause of 
minute discharges is locally short-circuited and does not flow in the 
joint portion of overcoats, which contact portion is apt to be most easily 
eroded by the deterioration due to ultraviolet ray and oxygen in air and 
by the minute discharges generated on the overcoat surface during the 
rainfall, and the joint portion of overcoats are protected from erosion 
due to the minute discharges. Particularly, the conducting path, which is 
produced by curving a metal plate having a given width along the 
peripheral direction of the surface of an insulator, can interrupt 
ultraviolet ray and the like, and protects the joint portion of overcoats 
from the deterioration due to ultraviolet ray. 
Moreover, in the insulator, wherein the joint portion of overcoats is 
airtightly and firmly fixed by a hollow cylindrical conducting path and 
further the end portion of the uppermost and lowermost overcoats is 
airtightly and firmly fixed, penetration of water into the interface 
between the reinforced plastic rod and the overcoat or leakage of grease 
from the interface can be prevented at the same time. 
Further, in the insulator of the present invention, the overhung length of 
a shed of an overcoat adjacent to a conducting path, the distance between 
the sheds of adjacent overcoats, or the length of overcoats having a 
unitary structure at the energized end side or at the earth side are 
properly selected, whereby the deterioration of insulating performance of 
the insulator can be prevented. 
As described above, according to the present invention, there can be 
prevented the deterioration of insulating performance which occurs in a 
very short period of time in the conventional insulators due to 
deterioration by oxidation generated from the seam of overcoats, erosion 
caused by minute discharges, penetration of water into the interface of 
the reinforced plastic rod and the overcoat through the seam of overcoats 
and leakage of grease from the seam. Further, even when a large number of 
short insulators having one-piece overcoats are connected to each other 
and used, there can be decreased the deterioration of reliability, the 
loss of insulating distance and the increase of weight due to the series 
connection of a large number of holding metal portions, wherein the 
concentration of mechanical stress and electric stress are developed, and 
long synthetic resin insulators having excellent insulating property and 
erosion resistance, which are light in weight and are high in strength and 
in reliability, can be obtained. Particularly, the synthetic resin 
insulators of the present invention can be widely used as an insulator for 
ultra-high voltage transmission line and the like, and the present 
invention is very contributive for the development of industry.