Even though a great number of arresters which are gapped arresters with resistors
made of silicon-carbide (SiC), are still in use, the arresters installed today are almost all
metal-oxide (MO) arresters without gaps, which means arresters with resistors made of
metal-oxide (metal-oxide or MO resistors). The distinctive feature of an MO resistor is
its extremely non-linear voltage-current or U-I-characteristic, rendering unnecessary the
disconnection of the resistors from the line through serial spark-gaps, as is found in the
arresters with SiC resistors. The currents passing through the arrester within the range
of possibly applied power-frequency voltages are so small that the arrester almost behaves
like an insulator. If, however, surge currents in the kiloampere range are injected
into the arrester, such as is the case when lightning or switching overvoltages occur,
then the resulting voltage across its terminals will remain low enough to protect the insulation
of the associated device from the effects of overvoltage