© Siemens AG • 07/2003
5
1
Order number
3
Year of manufacture
2
Rated voltage
4
Unit no.
Fig. 3
Module plate
Description
Design
The active components of the arrester are metal oxide resistors. They are arranged in a stack and
hermetically protected against environmental influences by means of a directly mounted silicon enclo-
sure. The mechanical strength is attained by glass-fibre-reinforced polymer rods which tightly enclose
and compress the resistor stack. Thanks to its hydrophobic properties, the silicon enclosure minimises
electrical discharges on the enclosure surface and thus ensures particularly good operating character-
istics, even under conditions of heavy pollution.
The flanges are made of a lightweight metal alloy suitable for use outdoors and directly connected with
the silicon enclosure.
The MO resistors are tightly enclosed by the GRP rods (as if in a cage). In the event of resistor over-
load – an extremely unlikely case, but which cannot entirely be ruled out – the resultant arc produces
no overpressure, since the resistors are not enclosed by a sealed, mechanically rigid casing. The arc
escapes immediately through the silicon enclosure, without any abrupt rupturing of the mechanical load
bearing enclosing structure. At the same time the MO resistors are held largely in place by a consider-
able number of GRP rods. The risk of parts flying off is therefore minimised.
The arresters are, where necessary, equipped with control fittings and / or shields.
Mode of operation
Metal oxide resistors are markedly non-linear - in other words, they have a strongly curved current-
voltage characteristic, which means that with the continuous voltage obtaining under normal circum-
stances only the so-called leakage current of just a few milliamps will be flowing.
In the case of overvoltages due to lightning or to switching operations, the resistors become conductive
(ohm range) thereby allowing a discharge current to flow to ground and the overvoltage to be reduced
to the value of the voltage drop at the arrester (“discharge voltage”). Here the discharge currents may
range up to 2 kA in the case of switching surges and 1 - 10 - 20 kA in the case of lightning surges.
