© Siemens AG • 08/2016 5
Arrester type | Designation | Rated voltage | Nominal discharge current | Repetitive charge transfer rating | Continuous operating
voltage | Rated short-circuit current | Serial number | Year of manufacture
Fig.
Rating plate
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 metal-oxide (MO) resistors are tightly enclosed by the fibre-reinforced plastic (FRP) rods (as if in a
cage). In the event of resistor overload – 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 considerable number of FRP 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 under normal circumstances only
the so-called leakage current of just a few milliamps will flow.
In the case of overvoltages due to lightning or to switching operations, the resistors become conduc-
tive (ohm range) thereby allowing a discharge current to flow to ground and the overvoltage to be re-
duced to the value of the voltage drop at the arrester (“discharge voltage”). Here the discharge cur-
rents may range up to 2 kA in the case of switching surges and 1 - 10 - 20 kA in the case of lightning
surges.