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Ratings and Test Requirements
Siemens Type VBII Safety Switch
Enclosed Switch Load Ratings
The primary functions of a fusible
enclosed switch are to carry current
continuously, to provide over
current and short-circuit protection, to
be capable of disconnecting the circuit,
and to provide means for mounting
fuses. Safety switches may also have
other capabilities covered by load break
ratings (in contrast with no-load dis-
connect switches), such as standard
and maximum horsepower ratings
and the ability to withstand the
maximum I2t energy let-throughs
of fuses.
All Siemens safety switches are capable
of continuously carrying their full-rated
nameplate current at rated voltage.
This capability is directly attainable
in no-fuse switches and in fusible
switches when the fuses are replaced
with copper bars, without exceeding
permissible temperature rise.
Fuses are capable of carrying their
rated current in open air. Under this
condition the fuses will not open and
will not exceed permissible tempera-
ture rise. When fuses are used in a
switch or other enclosure, a higher
ambient temperature is caused by the
switch heat and fuse-generated heat.
Since fuses are thermal acting by
design, they will not carry full current
rating at higher ambient temperatures.
To assist users of fusible equipment,
UL requires each fusible switch to carry
the statement "Continuous load current
not to exceed 80% of the rating of the
fuses employed." Good electrical
practice may require even further de-
ratings depending on the type of fuse,
load, altitude and ambient temperature
of the switch location.
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Load Break Ratings
All Siemens safety switches are
load break rated. The load break
rating is assigned by UL after the switch-
ing unit has successfully performed the
following tests for general use enclosed
switches:
Load Break Ratings
Number of
Number of Operations
Switch
ON, OFF
Ampere Operations
With
Rating
per Minute
Current
30-100
6
6000
200
5
6000
400
4
1000
600
3
1000
800
2
500
1200
1
500
12X Current Rating
In addition to the required UL overload
testing, all Siemens VBII Safety Switches
have been tested at twelve times rated
current at 600V AC to assure compliance
to automotive and other heavy industry
requirements.
Horsepower Ratings
All Siemens safety switches, where
appropriate, are horsepower rated.
The assignment of such ratings is made
by UL only after the switching unit has
undergone tests to determine its accept-
ability. In addition, the unit must
successfully perform on an overload
test series which includes repeated
interruption of the locked rotor current
of the motor for which it is to be rated
as follows:
Horsepower Ratings
Number of
Max HP
ON/OFF Ops
Number of Cycles
Rating
per Minute
100
6
500
1
Most switches have two or more
ratings for a particular voltage and
current. Siemens safety switches are UL
listed for design E horsepower ratings.
With Siemens safety switches no de-
rating is required in most cases.
Depending on the switch and its
application, various ratings have been
achieved. All Siemens switches include
a complete list of the ratings on the
inside of the cover.
Horsepower Rating Charts
The number and variety of horsepower
ratings that can be applied to a
switching unit makes it impractical, in
most instances, to list all such ratings
on the front of the unit. Siemens does,
however, provide this data by means of
Without
Current
Total
a chart on the inside cover.
4000
10000
2000
8000
Horsepower Rating Range
5000
6000
UL test procedure include ratings up to
4000
5000
500 HP. Siemens safety switch units in
3000
3500
2000
2500
appropriate sizes have successfully
passed the locked rotor current inter-
rupting test series for ratings through
500 horsepower at both 480 and 600
volts AC and through 50 horsepower
at 600 volts DC.
Maximum Horsepower Ratings
The maximum horsepower rating is
based upon the largest rating of a time
delay fuse: 1) which can be incorporat-
ed in the switch and 2) which will
permit the motor to be started. Since
the fuse has extra time delay, it can
hold the starting current of a larger
motor longer than a standard fuse.
Standard Horsepower Rating
This rating is assigned to a switch after
it has successfully completed the locked
rotor test series, on the basis of the
largest standard fuse rating: 1) which
can be incorporated in the switch and
2) which will permit the motor to be
started. The standard fuse does not
have a designed time delay to allow for
of Operation
motor starting currents.
50
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