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Siemens TYPE VBII Series Application Manual page 53

Safety switches
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Siemens Type VBII Safety Switch
Multiple-Voltage
Horsepower Ratings
A switch may have additional standard
and maximum horse-power ratings for
different voltages. A switch that is
horsepower rated at 240V AC or 250V
DC may also have horsepower ratings
for motors on 120V AC or 125V DC
circuits.
Multipole Horsepower Ratings
A switch may have horsepower ratings
applicable to the same current and
voltage ratings but with fewer poles
if the switch is investigated and found
suitable for the assigned rating.
Short-circuit Withstandability
UL test procedures for switches and
fuses have been expanded to provide
realistic standards of performance
with respect to clearing high-level
fault currents.
These revised standards deal with the
control of destructive energy in the
shorted circuit. Two types of potential
damage are characteristic of high-level
short-circuits: mechanical and thermal.
Mechanical damage is caused by the
electromagnetic force surrounding
conductors; thermal damage is the
result of excessive current during
the fault-clearing time.
Short-circuit Withstand Ratings
Fuse Rating
Fuse Class
Fusible
Plug
H or circuit breaker
K
J
R
T
L
Non-Fusible ➀
H or circuit breaker
K
J
R
L
➀ With fuses or circuit breaker in series with switch. Fuse or circuit breaker ampere rating cannot exceed switch ampere rating.
➁ 60A compact switches are rated 100,000 with 60A max.
Class J or R fuses in series with switch.
The UL fuse standard defines maximum
instantaneous peak let-through current
(lp) and maximum destructive energy
let-through (I2t) for each fuse (except
Class H). Applications of fuses and
safety switches on systems having
more than 10,000A are available.
Short circuits require selections of
the proper UL-listed fuses and switches
capable of withstanding I2t let-
throughs. See chart below for with-
stand ratings on Siemens switches.
I2t Rated
Suitability tests for service with Class
H, R, J, L and T fuses have been
conducted. Representative switches
with test fuses connected in series
with each switch were subjected to
I2t let-through values in excess of
the capacities of the largest fuses
acceptable by the switches.
To pass the test, the switch must
remain operable after being
closed into a high-amp available
short-circuit current. The test fuse
is sized so that higher levels of
let-through current and energy will
be reached than would ever occur
during normal usage in the field.
See the chart below for I2t ratings
on Siemens switches.
Short Circuit Rating (RMS Symmetrical Amperes)
General Duty
Heavy Duty
10,000
10,000
10,000
10,000
10,000
100,000
200,000
100,000
200,000
100,000
200,000
––
200,000
10,000
10,000
10,000
10,000
200,000 ➁
100,000
200,000 ➁
100,000
––
200,000
I
2
t and I
p
Switch
Rating
Ampere
30
60
100
200
400
600
800
1200
Ratings
2
I
t Rating
Ip Rating
(Amp2 x sec.)
(Amps)
50,000
14,000
200,000
26,000
500,000
32,000
2,000,000
50,000
6,000,000
75,000
12,000,000
100,000
10,000,000
80,000
15,000,000
120,000
53

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