Method for the production of an insulator

An electrical insulator coated with electrically insulating material is disclosed. An electrically insulating material is applied on a primer coat, in which the uniformity of the applied primer coat can be checked in a comparatively simple manner. A fluorescent material is dissolved in the primer solution before being applied to the component. Once dry, the primer coat is exposed to ultraviolet light, which causes the fluorescent material to fluoresce. Points on the surface of the component where the primer defectively coats the components are optically detected from the absence of fluorescent material. Primer is then reapplied to these points.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a method for the production of an insulator. 
2. Discussion of Background 
EP-A1-0,545,038 discloses a surge voltage protector which is encapsulated 
in an electrically insulating material. This surge voltage protector has 
two end fittings braced relative to each other by slightly axially 
resilient plastic strips. The plastic strips are positively inserted into 
the end fittings. The surge voltage protector has cylindrically designed 
varistor elements superposed to form a stack. A spacer plate is provided 
between the stack and the respective connection end fitting, which spacer 
plate is pressed against the stack by means of a threaded bolt screwed 
into the end fitting. The end fittings form the ends of the stack of 
varistor elements. Grooved disks for providing better contact are provided 
between the individual varistor elements and between the varistor elements 
and the electrically conducting spacer plates. The abovedescribed 
component is potted with insulating material. 
Care must be taken that the component surface that comes into contact with 
the insulating material is not contaminated by grease or other materials 
which could impair the adhesion of the insulating material. Poor adhesion 
would produce the risk of formation of an undesired track along the 
surface of the varistor elements. Adhesion is often improved with the aid 
of a primer coat which is applied onto the varistor elements. Conventional 
primers are colorless, so that is not possible in a simple manner to check 
the quantity applied and its uniform distribution. In particular, points 
which have not received any primer application are also not simple to 
detect. Moisture can diffuse in at these points, which sooner or later 
leads to longitudinal electrical breakdowns in the boundary layer between 
the varistor elements and the insulating material. 
SUMMARY OF THE INVENTION 
Accordingly, one object of the invention, as it is described in the 
independent claims, is to provide a novel method for the production of an 
insulator having electrically insulating material applied on a primer 
coat, in which the uniformity of the applied primer coat can be checked in 
a comparatively simple manner. An insulator produced according to this 
method is also provided. 
Checking of the uniformity of the applied primer coat is carried out by 
optical means and is free of interference. Defective points in the primer 
coat are simple to repair. The checking method can be used economically 
both for large and for comparatively small production runs. Insulators 
which are provided with a uniform primer coat have improved operational 
reliability, so that distribution networks equipped with such insulators 
have a considerably lower number of mains failures attributable to 
insulator faults. In particular, the operational reliability of surge 
voltage protectors is also increased if their active parts are provided 
with a uniform primer coat which is easy to check, before they are 
encapsulated in an insulating material. 
Further developments of the invention constitute the subject-matter of the 
dependent claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, wherein like reference numerals designate 
identical or corresponding parts throughout the several views, FIG. 1 
shows a schematic representation of a longitudinal section through a surge 
voltage protector provided with an outer insulation. The surge voltage 
protector has two metal connection end fittings 1, 2. Connection end 
fitting 1 is, like connection end fitting 2, provided with a fastening 
facility (not represented) for an electrical conductor. A threaded bore 3, 
in which a pressure screw 4 is arranged, is centrally provided in the 
connection end fitting 2. The two connection end fittings 1, 2 are 
connected together by two rigid, but to some extent axially extensible, 
glass-fiber reinforced plastic strips 5. These plastic strips 5 are 
fastened to the connection end fittings 1, 2 by means of screws 6. The 
plastic strips 5 have, for example, a rectangular cross section and are 
arranged symmetrically relative to the longitudinal axis of the surge 
voltage protector. The rectangular plastic strips 5 are positively 
embedded in the surface of the respective connection end fitting 1 or 2. 
The frame formed by the connection end fittings 1, 2 and the plastic strips 
5 encloses cylindrically designed varistor elements 7 superposed to form a 
stack. ZnO, for example, may be used as the varistor material. A matching 
metal plate 8 is inserted into a recess in the connection end fitting 1. A 
cylindrically designed grooved disk 9, which has a central bore 10, is 
placed between the plate 8 and the next varistor element 7, as well as 
between neighboring varistor elements 7. A grooved disk 9, which rests on 
a pressure plate 11, is likewise provided after the bottom varistor 
element 7. The pressure screw 4 acts on the pressure plate 11 and guides 
the current that flows when the surge voltage protector operates from the 
pressure plate 11 to the connection end fitting 2. When the described 
active parts are introduced into the frame, care should be taken that no 
gaps, in which insulent can enter during potting, remain open between the 
individual parts of the cylindrically designed stack. The actual contact 
force between the active parts and the connection end fittings 1, 2 is 
produced by the pressure screw 4, which is tightened with a predetermined 
torque and subsequently secured in one of the known ways. A preassembled 
component is in this way created. 
This preassembled component then has all possibly accumulated grease and 
salt residues cleaned off with the aid of a solvent. Acetone or 
isopropanol, for example, are used as the solvent. The cleaning of the 
component is followed by a drying process. The preassembled component is 
thereupon coated with a primer solution, and specifically, all regions, in 
particular including the plastic strips 5, which are covered with the 
insulating material during the subsequent potting, are provided with a 
comparatively thin, only a few .mu.m thick, continuous primer coat. 
Approximately 0.5 .mu.m to 5 .mu.m is generally desired as the thickness 
of the dry primer coat. In FIG. 1, for the sake of better clarity and 
substantially thicker than what would correspond to the scale of the 
drawing, only the primer coat 12 is represented, which externally covers 
the cylindrically designed varistor elements 7, the grooved disks 9 and 
the pressure plate 11. 
The component thus provided with the primer coat 12 is placed in a mold and 
potted without gaps and holes with a covering 13 of electrically 
insulating plastic. Suitable plastics in this case are, for example, 
silicone elastomers. During potting, insulating shields 14 are 
simultaneously molded onto the covering 13. The entire component is 
surrounded with the covering 13 and only those parts of the connection end 
fittings 1, 2 that are required for electrical connections remain metallic 
blank. 
FIG. 2 shows a simplified partial section through a hollow post insulator. 
This insulator could, however, for example also be used as the enclosure 
of an arcing chamber or of an interrupter chamber of outdoor switch gear. 
Insulators are also possible which do not have a cavity in their interior. 
In the case of this post insulator, a cylindrically designed, 
fiber-reinforced insulating tube 15, which has an axis 16, is the only 
supporting element which is connected at each end to connection flanges 
(not represented) which are intended for mechanical connection of the post 
insulator to neighboring modules. The insulating tube 15 is, after a 
cleaning process employing acetone or isopropanol, provided on the outside 
with a primer coat 12 which is, for the sake of better clarity, 
represented as being very thick. This primer coat 12 is, however, also in 
this case only a few .mu.m thick. Approximately 0.5 .mu.m to 5 .mu.m is 
generally desired as the thickness of the dry primer coat 12. The 
insulating tube 15 provided with the primer coat 12 is externally potted 
as far as the connection flanges with a covering 13 of insulating 
material, shields 14 being simultaneously molded onto this covering 13. 
FIG. 3 represents as a block diagram the method for the production of an 
insulator having at least one encapsulated component. This method has the 
following procedural steps: 
a) dissolving a material which fluoresces under ultraviolet light in the 
primer solution; 
b) applying the thus modified primer solution onto the previously cleaned 
surfaces of the component, so as to create a continuous coat; 
c) drying the coated component; 
d) exposing the primer layer 12 using a UV lamp; 
e) detecting the points on the surface of the component that are 
defectively covered with the primer coat 12; 
e) if appropriate, applying primer solution onto the points that are 
detected as being defectively covered; 
f) placing the coated component in a mold; 
g) encapsulating the coated component with a corresponding insulating 
material; 
h) curing the insulating material and removing the finished insulator from 
the mold. 
In both of the embodiments described, a primer solution of the company 
Wacker Chemie GmbH, PO Box 1260, D 84480 Burghausen, type "GRUND G 790" 
was used. This primer solution contains an organosilane compound which is 
dissolved in an aliphatic solvent naphtha. The primer solution contains as 
solvent 50% of aliphatic hydrocarbons and additionally 2% of silicic-acid 
esters. A dye of the company Ciba-Geigy, CH 4002 Basel, type "UVITEX-OB" 
was mixed with this primer solution as material reflecting ultraviolet 
light. This dye is normally used in the paper industry and also in 
detergents as an optical brightener. 0.5 per cent by weight of this dye 
was mixed with the primer solution. If somewhat more dye is admixed, then 
the undissolved dye component settles on the bottom of the vessel, and the 
processability of the primer solution is not thereby negatively 
influenced. The dye is likewise soluble in the solvent naphtha used for 
the primer solution, but only in a comparatively low concentration. The 
addition of only 0.5 per cent by weight of the dye leads to a solution 
that is saturated with "UVITEX-OB". 
After the dye has been dissolved in the primer solution, this solution is 
applied onto the component using one of the conventional methods, 
depending on the dimensions of the component and on the number of 
insulators to be produced. An immersion method or a spraying method could, 
for example, be used for the application, but it is also possible in small 
production runs to use simple application using a brush. The application 
is followed by a first drying process referred to as air drying, in which 
the primer-coated component is kept, for example, for 30 minutes at 
ambient temperature, so that most of the solvent can evaporate. This first 
drying process is followed by a second drying process, in which the 
primer-coated component is further dried, for example, at temperatures in 
the vicinity of 100.degree. C. for one hour. During the vaporization of 
the solvent naphtha used as solvent, which accompanies the drying, 
supersaturation of the solution takes place, and consequently 
precipitation of "UVITEX-OB" particles. These particles have their full 
fluorescence after precipitation. 
After this drying of the applied primer coat 12, the uniformity of this 
coat is checked under dark conditions with the aid of a UV lamp. The 
"UVITEX-OB"dye used here displays particularly strong fluorescence if it 
is exposed to near-UV light of wavelength 340 nm. Wherever fluorescent 
light cannot be detected, the surface of the component has points which 
have not been coated with the primer, and these must be deliberately 
covered up by reapplication of the primer solution in this region, so that 
a continuous and sealed primer coat 12 is provided after the subsequent 
drying. Wherever the fluorescent light that can be detected is 
particularly intense compared to the rest of the surface of the component, 
the thickness of the primer layer 12 is somewhat too thick. Applying the 
primer too thickly somewhat reduces the adhesion of the insulating 
material to the component, and should therefore be avoided. 
Tests were also carried out using a primer solution of the company Bayer 
AG, Olof-Palme-Strasse 15, D 51371 Leverkusen, with the trade reference 
SILOPREN-HALFMITTEL Z 3042, this primary solution having been modified by 
the addition of the above dye, and these tests also produced very good 
results. 
For the potting or coating of the component, the silicone elastomers of the 
company Bayer AG, Olof-Palme-Strasse 15, D 51371 Leverkusen, with the 
trade names SILOPREN LSR 2530 or SILOPREN LSR 2250 are particularly well 
suited. The silicone elastomers of the company RADO Gummi GmbH, PO Box 
1480, D 42463 Radevormwald, with the trade names SILOPREN LB SI 419 AI or 
SILOPREN LB SI 419 AII can also advantageously be used in this case. In 
addition, the silicone elastomers of the company Wacker Chemie GmbH, PO 
Box 1260, D 84480 Burghausen, with trade names POWERSIL 660 or POWERSIL 
661 or POWERSIL 310 or POWERSIL 311 can also be used for this purpose. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.