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  • Switch ratings

    I'm looking at McMaster for a DPST On/off switch.

    Some of the switches I see are rated "X" amps @ 120 volts.

    Would that be total both poles or per pole?

    The reason I ask is it's for 240 volts.
    Len

  • #2
    The rating is per contact . If both legs of a double pole switch are used to switch both hots from a 220 supply, each is carrying the same current. If you switch only one hot, you could double up the contacts and essentially double the current rating of the switch, but then you'd only be switching one hot lead, leaving wiring in the machine hot when the switch is off.

    Part of the rating of the contacts has to do with continuous current carrying ability, which doubles when you parallel the contacts, but the voltage rating remains the same. The voltage rating is partially a function of how fast the contacts can separate when the switch is opened, and how far apart the contacts get when open- also how well the contacts can deal with the repeated arcing when opened.

    There is often a spec which says 'for electric motor use', or for 'inductive use'. This makes the distinction between an inductive load or a non-inductive load. For an inductive load, the switch has to be better because it's dealing with a large voltage spike at the moment it is switched off. Adding to all this is the rating for dc, or ac. If ac, the rating can be higher because both contacts are sharing the erosion caused by arcing, whereas in dc, one contact is taking the brunt of the erosion.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

    Comment


    • #3
      To add to Darryl's comments, the AC rating can be higher because the AC arc self extinguishes (mostly) each time the voltage passes through zero. Due to ionized air surrounding the region of the arc, the arc may quickly re-establish as the voltage rises again from zero crossing.

      The DC arc is sustained through a path of ionized air and unfortunately, ionized metal from the contact surface.

      The contacts need to open fast to minimize the duration of arcing, and wide to exceed the air dilectric breakdown.
      Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
      ~Practitioner of the Electromechanical Arts~

      Comment


      • #4
        If you are going to use it for a motor, look for a HP rating, as this will take into account the inductive rating, locked rotor current, etc, etc.

        For a resistive load, like a heater, you can generally use the current rating directly.

        Incandescent lamps are NOT a resistive load in this respect, they have a heavy in-rush current.
        1601

        Keep eye on ball.
        Hashim Khan

        Comment


        • #5
          if the use is for 240 volts you need a switch rated for 240 volts not 120 volts a 120 will work but its not advised.. amps is also something to take into consideration whats the amp rating on your motor ...i would go with a 30 amp switch unless you need less then go less, a 15 amp motor that runs at 15 amps will surge more then 15 amp on start up then settle down to what it needs , so you can use a 15 amp switch with a 15 amp motor but again id go 20 amp switch for 15 amp motor and so on ...

          if the switch has the volts rating for 240 and your running 240 but the switch cant handle the amps of the device then you can and will have problems,

          Comment


          • #6
            This is the on/off switch for a DC speed control.
            240AC to the control to run a 180 volt DC motor.

            Each leg is fused for 8 amps as directed.

            Oh, there's another question!
            Make any difference if I fuse before or after the switch?
            Len

            Comment


            • #7
              Considering the incandescent inrush current, the cf"s are even worse. The first thing the ac source has to do is charge a fully discharged capacitor. That may or may not take as long as warming a tungsten filament (probably not as long) but the current spike is higher. I can readily see and hear this when I turn on my array of four cfs above my hobby bench. There's a very definite momentary dimming of the other lights, and you hear a definite SNAP when the contact is made.

              Cfs don't give the inductive spike when they're turned off, so at least that way they are good. Of course, incandescents don't either.

              Talking of switches- I have an ordinary light switch on my table saw, since the one that it came with only made it a couple years. This is a 3/4 horse motor (a real 3/4 horse) so it probably only draws 20 odd amps during startup, but that light switch has survived it for many years. I have replaced it once in the 35 or so years that I've owned the saw. For a $1 switch that doesn't even have snap action, that's pretty good.

              Fuse before or after the switch- why not before, so you protect the switch as well. That's what I see on any piece of gear I open up- the fuse is always the first thing the hot wire sees before anything else.
              Last edited by darryl; 01-01-2010, 10:45 PM.
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

              Comment


              • #8
                Yes, YES! The fuse is before the switch. It is indeed the first thing the current should pass through after the point where it enters the device. Fuses are often built into the sockets for removable AC cords. I can't cite chapter and verse, but this is probably in the NEC or whatever other code applies. And it is the way to provide maximum protection.
                Paul A.

                Make it fit.
                You can't win and there is a penalty for trying!

                Comment


                • #9
                  Originally posted by darryl
                  Considering the incandescent inrush current, the cf"s are even worse. The first thing the ac source has to do is charge a fully discharged capacitor. That may or may not take as long as warming a tungsten filament (probably not as long) but the current spike is higher.
                  Maybe. maybe not.

                  The 100W equivalent CFs I have taken apart had only a 22 uF filter capacitor. While that will draw a large spike of current, it is not a large total charge, and the capacitor will be filled up early in the first half cycle after turn-on.

                  A 100W incandescent will draw an initial current of 10x or more its nominal current, and that decreases over several half cycles. The I^2*T impact of the incandescent on switches and fuses is probably quite a bit larger.
                  1601

                  Keep eye on ball.
                  Hashim Khan

                  Comment


                  • #10
                    You're probably right, JT. Although the peak current drawn by the incandescent would be about 9 or 10 amps, and probably last a few ac cycles, the shorter but sharper charge current into the cf would be higher. There's a definite snap when you make the connection to a cf- more like the sound of a static discharge. It's quite pronounced if you make the contact at the peak of the ac cycle.

                    I've had a few of those apart, but I don't recall the size of the cap.

                    I'm starting to assemble some capacitors to build a direct 160vdc power supply. I don't know how many mfds I'm going to end up with, but I'll probably have to use a series resistor to pass the initial charge current through, then bypass the resistor with a relay when the voltage is near nominal. I want to be able to draw 10 or more amps continuously without exploding the capacitor bank, so the combined value of the caps needs to be pretty significant. This isn't something you'd toss at a guy and say, 'here, catch this'
                    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

                    Comment


                    • #11
                      Originally posted by darryl
                      Part of the rating of the contacts has to do with continuous current carrying ability, which doubles when you parallel the contacts, but the voltage rating remains the same.
                      The contacts won't have the same contact resistance, so they won't share the current equally you won't double the current capacity. The actual increase in capacity is a little unpredictable.
                      Paul Compton
                      www.morini-mania.co.uk
                      http://www.youtube.com/user/EVguru

                      Comment


                      • #12
                        Originally posted by darryl
                        I'm starting to assemble some capacitors to build a direct 160vdc power supply. I don't know how many mfds I'm going to end up with, but I'll probably have to use a series resistor to pass the initial charge current through, then bypass the resistor with a relay when the voltage is near nominal.
                        Have you heard of an inrush supressor?

                        High resistance when cold and a low resistance when hot. Common in switch mode power supplies.

                        A plain resistor gives very slow charge as you approach final voltage.

                        Your idea of a bypass is a good one. With an inrush supresssor it allows the suppressor to cool down so you get another soft start if the power is interrupted. It would also work with a lightbulb, which can be used as a crude constant current source for charging your cap bank. It also goes out when the bank is charged and so makes a handy indicator. In my model railway days some 25 years ago, I used to use a bulb in series with the controller to act as a current limiter. I also lit up of course when there was a short.


                        Many fluorescent 'ballasts' are now made with a power factor correcting front end and present a resistive load to the grid with almost no inrush current.
                        Paul Compton
                        www.morini-mania.co.uk
                        http://www.youtube.com/user/EVguru

                        Comment


                        • #13
                          Originally posted by EVguru
                          Have you heard of an inrush supressor?

                          High resistance when cold and a low resistance when hot. Common in switch mode power supplies.

                          A plain resistor gives very slow charge as you approach final voltage.

                          Your idea of a bypass is a good one. With an inrush supresssor it allows the suppressor to cool down so you get another soft start if the power is interrupted.
                          The inrush suppressor needs current to stay hot/low resistance. And they are always hot in use. I like resistors for power circuits, but bypassing the suppressor is just as good.

                          Many fluorescent 'ballasts' are now made with a power factor correcting front end and present a resistive load to the grid with almost no inrush current.
                          Just about all ordinary boost-type PFCs DO have an inrush surge, as the boost topology cannot limit current.

                          There are other types, but they tend to be complex
                          1601

                          Keep eye on ball.
                          Hashim Khan

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