In vacuum circuit breakers or vacuum contactors, current making and breaking is performed in the entirely sealed vacuum chamber. So, they poses various features such as (1) Excellent arc-extinguishing ability, (2) Less contact wear, and long life, (3) Safety, no fire or explosion hazard, (4) Less maintenance, (5) Compact and lightweight. Then the vacuum circuit breakers and vacuum contactors have been becoming major switching apparatus throughout the countries around the world.In 1965, Toshiba put the first vacuum switching devices in the market. Since then, various efforts have been made to increase their application fields. Regarding switching surge, analysis of generating mechanisms, measurement of the switching surge, development of the surge suppressors have been achieved. Based on those results this application guide was compiled.
Toshiba Low & Medium Voltage Vacuum Contactors are all "low surge type". As contactors are mainly applied for a motor switching, low surge characteristic is essentially required. So Toshiba developed a special contact material of the low chopping level, and employed for all vacuum contactors.
It was considered that as circuit breakers must interrupt a high current, low surge contact material can not be employed. However, Toshiba invented a "axial magnetic field type electrode" which can interrupt a remarkable high current. So by combining this type of electrodes and the low surge contact material, development of low surge type vacuum circuit breakers can be successfully achieved.
As the oil-immersed transformers have high lightning impulse withstand voltage, and insulation life is fairly long, surge protection is considered to be not required. However, in case of interrupting exciting inrush current, or switching dry type transformers or other equipment of which insulation level is considerably low, surge protection is advisable, such as to install surge arresters. However, the limiting voltage of the surge arrester shall be evaluated to be effectively lower than the insulation strength of the load equipment. If there is a possibility of encounter to lightning, the arrester must have enough capacity. Same caution should be taken for gapless surge absorbers using nonlinear resistors. When a vacuum circuit breaker switches a transformer, in which loads of low insulation levels (such as power electronics equipment) are connected to the secondary side, surge protection at primary and/or secondary side is advisable.
As stated above, surge protection is not required for the Toshiba vacuum contactors or the low surge type circuit breakers. However when starting inrush current is interrupted, installation of surge suppressor is recommended. Generally the impulse withstand voltage is not specified for motors, and insulation deterioration is not negligible. Therefore, it is a general rule to install surge suppressors, when the conventional vacuum circuit breakers are used for switching the motors. Toshiba recommends to use CR type surge suppressors in which a capacitor and a resistor are connected in series.
