Why were electrical insulators made from glass

A insulator is an electrical engineering component that prevents the flow of current between electrical conductors.

Insulators therefore consist of insulating materials, i.e. H. from substances with low electrical conductivity. They are used for the insulated fastening of electrical conductors or components of overhead line networks, substations, antennas or as bushing insulators on high-voltage transformers.


The higher the specific resistance of the insulator body, the better its insulating properties.

The glass, which is often used for insulators, has a specific resistance of ρGlass = 1010…1014 Ωm.

Porcelain or plastic materials, which are also often used, sometimes have an even higher insulation capacity.

The properties of insulators can depend on environmental variables such as temperature and humidity. Usually, however, the conductivity of an insulator only decreases slightly at higher temperatures.

However, moisture can reduce the insulation on the surface of an insulator to such an extent that it no longer functions. In the case of high humidity or rain, the creepage path along the surface of the insulator or its resistance to creepage can no longer be sufficient. Insulators are therefore often designed with ribs, with the ribs being appropriately bevelled in a roof shape for outdoor use so that the underlying surfaces remain dry.

Materials for electrical insulators

  • Ceramic (steatite, porcelain)
  • Glass
  • also partially glass fiber reinforced plastics indoors
  • Also hydrophobic plastics outdoors (the hydrophobicity prevents the formation of conductive droplet traces, which can lead to a foreign layer flashover.)


Button-shaped ceramic bodies are usually used as insulators for telephone overhead lines and low-voltage overhead lines, around the cap of which the conductor cable is looped with a special loop. They are putty standing on metal hooks with which they are mounted on masts or walls.

For medium voltage (range 1 kV to 30 kV), insulators made of glass or ceramic, which are ribbed to increase the creepage distance, are usually used. Leakage currents “creep” over (mainly dirty) surfaces of insulators or insulating material. If this leakage current persists, a sliding discharge and flashover, an electric arc and, consequently, a short circuit can occur.

The conductors are fastened with special clamps, the design of which differs depending on the application (suspension or suspension clamp). In this voltage range, there are both standing and hanging insulators on the cross-beams of the masts.

Standing insulators enable lower mast heights and, thanks to the mast construction, offer security against the conductor cable falling down, while hanging insulators evade greater transverse forces through lateral deflection so that they do not experience any bending stress.

Insulators can also be arranged twice (next to one another) for increased mechanical stress. In this case, e.g. B. with high safety requirements a hanging insulator in the event of an insulator break often carry the conductor cable alone.

Insulators for fastening overhead lines (contact lines) do not differ fundamentally from those for overhead lines, but must be designed for the special mechanical loads on the overhead line. Insulators for busbars have to support the heavy busbar. Often, an existing protective cover also serves as insulation for isolated fastening, as is the case with the Berlin S-Bahn.

High-voltage insulators are often equipped with a spark gap as a surge arrester in order to keep the arcing away from the insulating material in the event of an overvoltage (lightning strike) and to extinguish it through a specific design.

Insulators for high voltage (30 kV to 150 kV) are only used in a hanging version. The technique of fastening the conductor cables does not differ from the technique used in the medium-voltage range. Double insulators are often used. The same types are used for traction power lines as for three-phase power lines.

Insulators for high voltages (> 150 kV) are often manufactured as chains of two or more insulators for high voltage (insulator chain). Long rod insulators are also used. In addition to glass and porcelain, high-strength plastics are also increasingly being used. In Germany, double insulators are generally used for 380 kV lines. For very high static requirements, three or four parallel long rod insulators or insulator chains can be used.

Insulators for high-voltage direct current transmission do not differ in principle from the types used for alternating voltage, their stress at the same voltage is even lower, since the pre-discharges (e.g. in damp weather) are lower.

Special requirements are placed on the isolators of self-radiating transmission masts, because these must be able to carry voltages of up to 300 kV and loads of up to 1000 tons at high transmission powers. For this purpose, steatite webbing insulators are used for the insulation of the guys and for the insulation of the towers and masts, hollow or solid steatite bodies are used, on which the overlay body, which carries the tower or mast, is fastened exactly. The isolator must lie in a pressing device until it is installed. To install the isolator, the tower or mast is lifted hydraulically and slowly set down on the isolator.
Guy ropes for transmission masts and overhead lines, but also wire antennas, are insulated with egg-shaped insulators that have holes and grooves to accommodate the ropes. The rope loops move such a ceramic Isolating egg exclusively under the compressive stresses that it can tolerate more easily, as they interlock when separated from the egg.


An isolator museum can be found in Lohr am Main in Haaggasse in a listed former transformer house and is referred to as "Germany's smallest museum" along with a number of other candidates.

Other isolator components

Optical isolator

Under a optical isolator one understands a so-called optical diode, thus a kind of "valve" for light. This component only allows polarized light to pass in one direction: in certain materials located in a magnetic field, the direction of polarization is rotated by 45 °, in the other also, but in the same direction of rotation and not backwards. The rotated part can be removed using polarization filters.

Isolator "valves" in high frequency technology

In high-frequency technology, an isolator is a component with two connection ports. ports), which only allows electromagnetic waves to pass in one direction, while ideally no power is transmitted in the opposite direction. The transfer ratio between the forward and reverse direction, which is finite in practice, is called isolation and is usually given in decibels.

Such an isolator is often implemented with the aid of a circulator in which one of the three ports is provided with a terminating resistor. In this way, the signals are only forwarded between the remaining two gates in one direction, in the other direction they are diverted to the terminating resistor, where they are converted into heat.

Another type of construction are Faraday rotators, which in principle work in the same way as optical isolators, except that the rotating material is a ferrite and the polarization-limiting elements consist of slots.

See also

electrical resistance

Categories: Electrotechnical Material | Solid state physics