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 Supplemental
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RERATING CURRENT FOR SWITCHES WITH 125V AC ORIGINAL RATINGS
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Generally, most switch applications can be classified into one
of the below load categories. Switch capacities can
be mathematically rerated when the application calls
for a category or voltage other than the switch standard
general specification ratings, meaning original current
ratings at 125V AC. Nikkai has not conducted life tests
at these rerated voltages and currents so it is important
to contact the factory in such cases. The candidate
switch should be tested in the actual application in
which it is intended to function.
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FOR CALCULATING RERATED CURRENT AT VARIOUS LOADS
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New Voltage
Rating
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Resistive Load
Multiply by:
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Inductive Load
Multiply by:
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Lamp Load
Multiply by:
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Motor Load
Multiply by:
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Capacitive Load
Multiply by:
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125VAC
250VAC
12VDC
30VDC
48VDC
125VDC
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1
0.66
1.25
1
0.33
0.05
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0.66
0.33
1
0.5
0.25
0.03
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0.25
0.16
0.31
0.25
0.08
N/A
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0.33
0.22
0.41
0.33
0.11
N/A
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0.25
0.16
0.31
0.25
0.08
N/A
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Sample
Calculation for model M2012SS1W01
with 6A @ 125VAC resistive rating.
To use at 48V inductive, multiply
6A x 0.25 = 1.5A @ 48VDC
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Sample
Calculation for model JWL22RCA
with 16A @ 125/250VAC resistive rating.
To use at 30V DC motor load, multiply
16A x 0.33 = 5.28A @ 30VDC
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Resistive
Load
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Resistive loads can be purely resistive or of the tungsten-heater
load type. A resistive load that has no heating element
is the easiest for a switch to handle, and the switch's
rating is based on this type of load. A resistive load
is one in which 100% of the load is composed of resistive
devices. The power factor is high (PF =1) and contact
erosion is low. Consequently, the switch's electrical
life can be anticipated with some certainty.
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Lamp
Load
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When a switch closes on a resistive lamp load, the switch
sees a short circuit because the cold resistance of
the lamp filament is near zero. The surge current as
the switch closes can be many times the steady state
current. As the lamp filament heats up to operating
temperature, the resistance of the filament increases
and the current decreases to the lamp's steady state.
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Motor
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Motor loads present yet another brutal environment for switch
contacts. Closing the switch contact on a motor start-up
load causes very large current surges of about 3 to
8 times the running current. When the switch is opened
and the current decreases, the magnetic field of the
inductor collapses and an electromotive force is induced.
The polarity of the induced voltage is such as to oppose
any change in current flow. This induced voltage aids
the source voltage in striking an arc and maintaining
it as the contacts separate.
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Inductive Load
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Non-motor inductive loads, such as those seen in switching power
supplies, have inrush currents that greatly exceed the
normal operating currents of the equipment. This inrush
current can easily reach 8 to 10 times the steady state
current. As a switch on an inductive load is opened,
the inductor, or transformer, inducesa counter option
"voltage" in the circuit. This voltage opposes any change
in the circuit current and can reach hundreds of volts.
This extremely high voltage can restrike the arc as
the switch contacts open resulting in severely eroded
or welded contacts.
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Capacitive
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With such loads as DC power supplies, welding machines, and
strobe charging units the inrush current is even more
damaging than with inductive loads. To the switch a
capacitive load appears as a dead short as the switch
closes. In the first few milliseconds the inrush current
can sometimes reach 100 times the steadystate current
of the circuit. Even worse for the switch, this inrush
occurs before the contact bounce has subsided. This
produces severe arcing and massive contacterosion. Often
the contacts weld upon closure preventing the switch
from ever opening. In an emergency the operator of the
equipment would know he could not open the circuit.
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In AC circuits the voltage and current are varying in
a sinusoidal pattern; both the voltage and current cross
the zero reference 120 times per second for 60Hz.
Therefore, the chances of closing or opening a switch
when the voltage and current are at their maximum in
AC circuits is remote.
In DC circuits the voltage and current do not vary
and are always at their rated levels. Compared to AC
circuits with the same voltage and current, DC circuits
handle 1.414 times the power. Therefore, when opening
or closing a switch on a DC load, the arc developed
is more severe, more energetic, and lasts longer causing
more contact erosion and a shorter switch life. A
switch intended for a DC circuit should have its AC
capacity rerated for DC. See previous page for
rerating current.
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DESIGN FOR INDUCTIVE DC LOAD MODELS S800 & SW3800
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Bar magnets are placed at each end of high capacity
switches, and their magnetic field opposes the field
created by the arcing current, thereby extinguishing
the arc and protecting the contacts. Positive (+)
must be connected to end terminals and negative
(-) to common terminals.
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The
TV5 and TV8 ratings are tested and assigned by the Underwriters
Laboratory. The switches are intended to be used as "Power ON"
devices in equipment where a high tungsten inrush current is
anticipated, such as tungsten-filament lamp loads or entertainment
equipment like sound systems and monitors.
An example is the TV8 test which consists of an overload test
and an endurance test. The overload test consists of a switch
closing on a minimum inrush current of163 amps with 50 consecutive
operations at a rate of 10 cycles per minute. The test must
be conducted without any failures. In the endurance test the
current is reduced to 117 amps, and the same switch is subjected
to another 25,000 operations.
The JWL is a product example that has been tested and meets
the TV8 rating. In addition to the electrical testing, the switch
enclosure (housing) must comply with the requirements for classifying
materials as UL94V-0. The insulation material must have arc-tracking
characteristics with a minimum arcing time of 180 seconds when
tested in accordance with the Standard Test Methods for High-Voltage,
Low-Current Arc Resistance of Solid Electrical Insulation. The
JWM and JWL switches are rugged, dependable, and well suited
to high inrush circuits.
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OPERATING
RANGE
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Three contact materials are commonly used in Nikkai switches: gold, silver,
and gold over silver. These materials give the options of low
level, power level, plus combined power and low level ratings.
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Low
Level ~ 0.4VA maximum @ 20V AC or DC maximum
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Gold plated contacts are recommended for dry circuits, which are
defined as very low energy. In circuits where the voltage is
below 28 volts DC and current is below 100 milliamps (dry circuits),
no arc develops as the contacts open or close. So, the tarnish
remains. Eventually without the arc, the contacts become so
encrusted that the switch is unable to close the circuit due
to the high contact resistance. The solution to this is plating
the silver contacts with gold, which does not tarnish. Gold
plated contacts close under low voltage and low current conditions
indefinitely, or for the mechanical life of the switch.
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Power Level ~ 100mA to 10 amps @ 125V AC
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Silver contacts are recommended for electrical levels above 0.4VA.
Although silver tarnishes, it is a good conductor and this electrical
energy is sufficient to break through the tarnish to give reliable
performance. The oxidation which coats the contact surfaces
with a hard layer of insulative contamination is removed by
arcing. In circuits where the voltage is above about 12 volts
DC and the current above .5 amps, an arc develops during opening
or closing of the contacts. This arc keepsthe oxidation cleaned
off.
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Power or Low Level
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Gold over silver contacts are used in applications requiring both
dry and power circuits. Nikkai's gold over silver contacts have
dual ratings as further described below.
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DUAL RATINGS
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The
dual rated option is suitable where identical switches are
used in both a logic and a power level circuit within the
same application. Dual rated switches enable the user to install
the same switch in both a logic level (dry circuit) and a
power level circuit. However, once a code "A" rated contact
switch has been used at a power level, it cannot then be used
at a logic level. There may be advantages to stocking only
a single switch for use in both a logic level and a power
level circuit. Our dual rated contact material option allows
this advantage. However, once a dual rated contact material
switch has been used at a power level it cannot then be used
at a logic level. The gold over silver contact material provides
a reliable, tarnish free, contact surface for logic level
switching. When this same contactmaterial switch is used in
power level circuit, the gold plating is removed by contact
arcing. If an attempt is then made to use this sameswitch
in a logic level circuit (where no arcing occurs). The low
current condition cannot provide adequate contact wiping or
cleaning.
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