The use of auxiliary interrupting devices as attachments on disconnecting switches has been the practice for many decades in North America and is becoming more common elsewhere.
Arcing horns are designed to prevent arcing at the main switch contacts. They have no interrupting rating, however they are commonly used to interrupt small values of current such as transformer magnetizing current or the charging current of a short length of line as shown in Figure, is commonly used. As the disconnecting switch opens, the current is first commutated to the arcing horn and then to the whip via its fixed horn. The whip is restrained by the fixed horn and releases at a certain point achieving a fast motion interrupting the current with minimal or no arc development. The device plays no role on closing the
disconnecting switch, and any prestriking will occur on the main arcing horns.
Many utilities, again primarily in North America, practice disconnecting switch loop switching between transmission loops. In contrast to bus-transfer loop impedances of
less than 0.2 ohm according to IEC 62271-102, transmission loop impedances are in the order of tens of ohms.
