The following pictures (Figures 15…17) show other models of MO arresters: a medium-
voltage distribution arrester with polymer housing and two different designs of
high-voltage arresters with polymer housing. The chosen examples differentiate from
each other, in some cases greatly, in their design features, and thus provide an overview
of some of the basic arrester designs in use. Medium-voltage arresters with porcelain
housing are not discussed in more detail here as they have been almost completely superseded
by arresters with polymer housing. Millions of them are nevertheless still in
use. The appendix contains a diagram.
Due in large part to the failures caused by leakage in cheaply designed distribution
arresters with porcelain housings, the first ones equipped with polymeric outer insulation
appeared on the market in the mid 1980’s. Their most remarkable design feature is
the polymer housing located directly on the MO resistor stack. As a result, the gas-filled
gap between the MO resistors and the housing no longer exists, and with the appropriate
constructive realization of the interface between the polymer housing and the end fittings,
a sealing system can be completely omitted. Similarly, in case of an overload, a
pressure buildup and the related risk of housing breakage can be avoided. Nevertheless,
a number of different designs are possible based on this principle1.
In the case of a porcelain insulator, different properties – such as, protection from
environmental impact and provision of sufficient creepage distance on the one hand,
and mechanical strength on the other – are united in a single component. In an arrester
with polymer housing, however, these properties are apportioned to two different components.
Mechanical strength is, virtually without exception, achieved with fiber-glass
reinforced plastic (FRP) materials. In the example shown in Figure 15, several rods
serve this purpose. They are strained, e.g. by crimping, in the end fittings and enclose
the MO resistor stack like a cage. That is why the term "cage design" is used in this
context2. Here, the MO resistors themselves form part of the mechanical structure. This
is how a mechanical high-strength unit out of MO resistors, end fittings and the FRP
structure are created. This module is inserted in a mold, in which silicone rubber is directly
injected. With the appropriate manufacturing techniques, it is possible to obtain a