Bosch Power Tools 1375-01, 1375AK manual Electrical Safety

ELECTRICAL SAFETY.

For high-frequency electric tools, electrical safety is assured by the protective conductor as per EN 50144 based on protection class I. When the secondary coil of the converter is switched in the star, the star point or neutral point is extended outwards. This neutral point is earthed (earthing resistance RB Π2 ohms) and connected to the metal housing of the electric tools by way of the protective conductors so that, at an operating voltage of 265 V, the dangerous voltage between phase and earth is only

265 V = 153 V

1.73

At operating voltages of 135 V or 72 V, on the other hand, the dangerous voltage is only

135 V	= 78 V or
1.73
72 V	= 42 V
1.73

The effectiveness of the protective earthing is assured by using corre- spondingly robust plug fixtures of faultless electrical construction

together with suitably resistant cables. Careful maintenance is equally important. The electric tool itself must, in its construction design, be capable of satisfying the stringent demands of indus- trial manufacture. Under normal circumstances, the description provided above, e.g. the protec- tion measure “Earthing" as per VDE 0100 – 10 N is adhered to.

The possible protective measures may be classified and subdivided as follows:

1.0protective measures without switching device

1.1protective insulation (VDE 0100 – § 7 N)

1.2low voltage, 42 V (VDE 0100 – § 8 N)

1.3protective separation (VDE 0100 – § 14 N)

2.0protective measures with switching device

2.1protective earthing (VDE 0100 – § 9 N)

2.2reduction to zero voltage (VDE 0100 – § 10 N)

In cases 2.1 and 2.2, switch-off is effected by fuses or correspond- ing thermo-magnetically triggered station circuit breakers.

The greatest possible degree of protection is reached by using fault current (FI) circuit breakers in addition.

Protective insulation as per 1.1 is not applied to high-frequency electric tools. Low voltage as per 1.2 finds application only in special cases in which, due to existing regulations, it cannot be avoided. Because of the high currents involved, this measure is very problematic with respect to cable cross-section, switches, plugs etc. when applied to the transmission of large outputs. Small screw drivers comprise an exception. In this case, one is better off using protective sepa- ration as per 1.3, according to which every tool must have its own separator-converter. Protec- tive separation should be restric- ted to circumstances in which it is absolutely necessary.

Here we wish to focus on the protective measure “reduction to zero voltage as per 2.2" since it finds application primarily with high-frequency electric tools.The voltage reduction should continuously prevent excessive contact potential on installation parts that do not belong to the operating circuit (see Fig. 5). It requires direct earthing of a mid- point or star point conductor and is effected by connecting the in- stallation parts that are to be pro- tected either to the neutral con- ductor or to a special protective

conductor that is in turn connec- ted to the neutral conductor.

The protective measure “reduction to zero voltage" thus switches off defective installation parts since the inline fuse directly before the defective location is activated.

If the fuse is really to be activated, certain voltage reduction condi- tions as per VDE 0100 – § 10 N must be complied with. The most important voltage reduction requirement: The cross-section of the leads from the power

Fig. 4

Fig. 5

70 71