Bosch High-frequency production tools professional blue Page 49
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when at any position of the cable network a complete restricted only to cases where it is absolutely neces- short-circuit occurs between an outer conductor and sary. the neutral conductor. In the main, the “zeroing as per 2.2” protective In addition, residual current protective switching in ac- measure should be observed somewhat more closely cordance with Fig. 5 can be used (to simplify this, the because it is predominantly used for high-frequency drawing shows single-phase current). The residual cur- tool systems. Zeroing is intended to prevent constantly rent protective switch obtains its pulse from a current excessive touch voltages at system parts not belong- transformer which all feed cables including neutral ing to the operating circuit (see Fig. 4); it requires a conductor are fed through. The secondary coil of the directly earthed central point or star point conduc- current transformer delivers the operating current for tor and is established by means of connection of the the relay coil of the residual current protective switch. system parts that are to be protected to the neutral The cables enclosed by the current transformer gener- conductor or to a special protective earth conductor ate an alternating magnetic field in the transformer connected to the neutral conductor. core when the sum of all currents is not cancelled out (Fig. 7). Using the “zeroing” protective measure thus ensures that faulty parts of the system are switched off be- In the fault-free status of the residual current protec- cause the fuse directly upstream of the fault position tive switch, the current flowing to the consumer is is active. equal to the current flowing back from it. The currents thus cancel each other out. No induction takes place To ensure that the fuse really does respond, certain onto the secondary coil of the current transformer, so zeroing conditions in accordance with VDE 0100 – that the relay coil of the residual current protective § 10 N must be met. The most important zeroing con- switch remains currentless (Fig. 5). ANGLE GRINDERS transformer. The use of protective isolation should be STRAIGHT GRINDERS Guide | High-frequency technology | 49 dition is as follows: the cross-sections of the cables between current generator or transformer and current ing current IA of the next upstream overload protec- HIGH-FREquENCy TECHNOLOGy tion element as per Panel I VDE 0100 – § 9 N flows ACCESSORIES consumer must be rated such that at least the break- Fig. 5
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