Production of Insulators

Preparation of raw material and production of mass

In the factory three types of porcelain masses are being made in compliance with C-120, DIN, KER 110.1, KER 110.2.
For the production of above mentioned masses both domestic (quartz sand, feld and clay) and imported ( feld, clay, kaolin and alumina ) raw materials are used. Quartz sand, feld and clay are domestic and feld, clay, kaolin and alumina are imported row materials. Due to limited production capacities of the factory and conventional technology for preparation of masses the special care should be paid to on-receipt control of the raw material. Prior to arrival of raw material to the factory the production process control carries out first testing of the prepared raw material for delivery. The samples are taken at the deepest layer. 


Upon filtering, the filter cakes rest for a certain time, afterwards the mass is worked up on the vacuum press. The homogenized mass is applied for drawing out the billets on vacuum presses NETZSCH VPP 250,350, 500 and 750. Depending on the type of mass and size of billets required tools , mode of feeding, speed of feeding and mass moisture are defined. The application of the rules is checked by the production process control. The maximum diameter of the empty billets is 750 mm and of the solid ones Ø 360 mm. Each billet is marked with the direction of drawing out and ordinal number. The drying of billets to reach the prescribed moisture could be natural (in the production department) or by the electric current (electric resistance). In the addition to the verification done by the foreman, the workers keep records of the billets temperature and duration of drying in the check lists. 
Empty billets contain the moisture according to the production process prescriptions and are shaped on vertical profiling lathes (billet normal dimension Ø 750x3000) by means of models and blades of hard metal under prescribed machining conditions. The quality control checks dimensions and quality of manufacture of each first piece and later on every hour. 
Large dimensioned cone insulators (that cannot be shaped on profiling lathe ) are shaped by the application of the convential method of shaping plaster forms and after the treatment of each separate part they are connected by means of suitable electrolyte. 
Gluing is done by standard spindles and on hydraulic bench (maximum diameter Ø 1200 mm). Each insulator is marked with type, ordinal number and worker number. On special check list the quality control keeps records of each completed insulator. Transformer bushings up to 35 KV as well as other insulators having similar dimensions are shaped on horizontal lathe . 
Shaped insulators are stored on special storage racking and then are transported for drying.


The drying is done in drying chambers. Insulators are produced in special natural gas iso jet furnaces. Due to the production of special insulators having good electrical, thermal and mechanical properties, the drying chambers are designed in a way that the various conditions of drying may be established depending on the shape of insulator and quality of porcelain mass. The drying chambers heated by electric heaters are provided with horizontal and vertical circulation and re-circulation of air for drying with the speed of circulation from 0,5 to 3 In/sec.
The heating is either automatic or semi-automatic and the regulation of relative humidity is manual. The check up of the established conditions is automatic via thermocouple on six-color recorder and visual with standard psychorometre (thermometers dry and wet ball)..
The conditions of drying depend on the type of insulator and are set by the Development Department - engineer specialists in co-operation with institutes. For each drying chamber temperature conditions, conditions of relative humidity and speed of air circulation are established. The operators keep records of the process for each drying chamber on the paper for charts and graphs every hour and they register the relative humidity and temperature during the entire drying process. Types and number of insulators which are in the drying chambers are indicated in the same charts. .
At the end of the drying process the production process control registers the quality of drying whereas the quality control carries out visual check up. Production process control files the records in it's documentation
Finally, dried insulators marked by the quality control are subjected to glazing.


Glazing of all types of insulators is done by soaking. Available color is dark brown RAL 8017, gray MUNSELL 5.OBG 7.0/04 and white dust.
Glaze suspension is prepared in special department under direct control of the laboratory. The control of preparation of insulators for glazing is effected by the quality control. Next step in the production process is burning.



Grinding is divided into 3 groups depending on the assortment range:

  1. grinding of empty insulators done by the diamond grinding stones on the following grinding machines:
    -horizontal grinding machine HEINRICH ZEIDLER Ø 800 x 3500 mm
    - horizontal grinding machine HEINRICH ZEIDLER Ø 500 x 2000 mm
    -vertical grinding machine NETZSCH Ø 1600 x 3000 mm
  2. grinding of line and medium voltage insulators is done by diamond grinding stones
  3. cutting of solid insulators is done by diamond grinding stones on grinding machine HEINRICH ZEIDLER.

Since the grinding of insulators is performed at the same time from both ends by one tightening on the machine, parallel faces and coaxiality are completely ensured.

Quality Control


In addition to the production process control there is also a quality control. They perform following operations:

  1. on-receipt control of flanges
  2. intermediate stage control of the process (inspection of dimensions, surface, visual check up after drying and burning)
  3. final check up that embraces visual check up, inspection of dimensions and testing

Quality control carries out following testing:

  1. testing of insulators by ultra-sound

The quality control verifies by this seal that the insulators may proceed to assembly.

  1. electrical testing of the walls of insulators according to IEC prescription (possibility of testing up to 100 kV).

The electrical testing should be performed on all glued insulators and, if the insulators are made from one piece, the testing is carried out if so agreed between the purchaser and manufacturer

  1. thermal testing according to IEC prescriptions (maximum dimension 2.5 m),
  2. mechanical tests to bending, torsion, elongation, pressure and internal pressure
  3. testing of porosity according to IEC prescriptions by means of Fuchsin press.