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Electrical Safety - Bosch 1375-01 Catalogue

High-frequency power tools for industry
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ELECTRICAL SAFETY.

For high-frequency electric tools,
265 V
= 153 V
electrical safety is assured by
1.73
the protective conductor as per
EN 50144 based on protection
At operating voltages of 135 V
class I. When the secondary coil
or 72 V, on the other hand, the
of the converter is switched in
dangerous voltage is only
the star, the star point or neutral
135 V
point is extended outwards. This
= 78 V or
neutral point is earthed (earthing
1.73
resistance RB Π2 ohms) and
72 V
= 42 V
connected to the metal housing
1.73
of the electric tools by way of the
protective conductors so that, at
The effectiveness of the protective
an operating voltage of 265 V, the
earthing is assured by using corre-
dangerous voltage between phase
spondingly robust plug fixtures
and earth is only
of faultless electrical construction
Fig. 4
70
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:
1.0 protective measures
Here we wish to focus on the
without switching device
protective measure "reduction to
1.1 protective insulation
zero voltage as per 2.2" since it
(VDE 0100 – § 7 N)
finds application primarily with
1.2 low voltage, 42 V
high-frequency electric tools.The
(VDE 0100 – § 8 N)
voltage reduction should
1.3 protective separation
continuously prevent excessive
(VDE 0100 – § 14 N)
contact potential on installation
2.0 protective measures with
parts that do not belong to the
switching device
operating circuit (see Fig. 5). It
2.1 protective earthing
requires direct earthing of a mid-
(VDE 0100 – § 9 N)
point or star point conductor and
2.2 reduction to zero voltage
is effected by connecting the in-
(VDE 0100 – § 10 N)
stallation parts that are to be pro-
tected either to the neutral con-
In cases 2.1 and 2.2, switch-off is
ductor or to a special protective
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.
Fig. 5
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
71

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