برق گیرهای منصوب روی بوشینگ ترانسفورماتورها یا rod gap جهت حفاظت سیم پیچی در مقابل امواج اضافه ولتاژ و برش آنها مورد استفاده قرار می گیرد. فاصله هوایی مورد استفاده در این نوع برق گیر تابع ولتاژ شکست مورد نظر و شرایط محیطی نظیر دما ، فشار و رطوبت می باشد.
All types of surge protection aim at attaining the same results, namely that of shunting surges from lines to earth or line to line to prevent their reaching the transformer. Protection may take the form of a rod gap, known as a coordinating gap, connected across the transformer bushings and designed to flash over at a given voltage level, or alternatively surge arresters may be used.
Until quite recently surge arresters employed several spark gaps in series with a non-linear resistor material, normally silicon carbide, and, although this type is still used in significant quantities on rural distribution networks at 33 kV and below, elsewhere these have now been almost entirely superseded by the gapless metal oxide variety. The arresters are connected from each line to earth, or they may be occasionally connected from line to line. When a highvoltage surge reaches the arrester the metal oxide becomes conducting or the spark gaps break down and the disturbance is discharged through the device by reason of the fact that at the high voltage involved the arrester resistance is low. As the surge voltage falls the arrester resistance automatically increases and prevents the flow of power current to earth or between lines. An arrester of this type is therefore entirely automatic in action and self-extinguishing.
The coordinating gap is designed to trigger at a voltage just below that to which the winding may be safely exposed. If it is set too low it will operate too frequently. Set too high it will fail to provide the protection required. Because of the severe dV/dt imposed on the transformer windings by the triggering of a rod gap it has been the practice to test for this condition by means of chopped-wave tests when carrying out impulse tests in the works.
In order to ensure that this gap flashed over as close as possible to the nominal impulse test level, it was the practice to specify that the impulse voltage for the chopped-wave test should be increased by a further 15% over the normal fullwave test level. Specification requires that the gap should flash over between 2 and 6µs from the start of the wave and since the nominal time to peak is 1.2 µs, this means that the peak has normally passed before flashover and the winding has been exposed to 115% of the nominal test voltage. Designers were thus required to design the windings to withstand this 115% as a fullwave withstand. It is now possible to use triggered gaps whose instant of flashover can be very precisely set, so the need to specify that the test be carried out at 115% volts no longer arises and IEC 76, Part 3, which deals with dielectric testing of transformers, now specifies that the chopped-wave tests should be carried out at 100% volts. As far as withstanding the rapidly collapsing voltage wave is concerned, this will, of course, be better dispersed through the winding with a high series capacitance, so that the winding design will follow the same principles as for the full-wave withstand.