DPST switch question

Discussion in 'Electrical Forum discussion & Blog' started by beekerc, May 1, 2009.

  1. beekerc

    beekerc IT Consultant / Network Engineer

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    Location:
    Seattle
    are the two circuit lines in a double pole, single throw (DPST) switch electrically isolated? specifically, could I run A/C across one line and D/C across the other?

    here's my application.
    i have a garage door openers. in the old days, when i would go on a trip, i'd unplug it from the wall so as to prevent the doors from being opened by accidental or intentional remote signals. with my current openers, I have hard-wired the openers, so instead of an A/C plug, there's a SPST switch in the wall to cut power. the issue is there's a battery backup in the unit which is great for power outages,but if i wanted to really kill power to the unit, i have to climb up and disconnect the battery as well. what i'd like to do is run some low-voltage wire from the opener to where the A/C switch is and install a DPST switch so i can cut the A/C and disconnect the battery with one switch.

    is this feasible to do?
    are there any NEC issue with putting a D/C circuit and an A/C circuit on the same switch (even though the poles should be isolated)?

    yes, i know with modern openers and rolling codes, it's highly unlikely that my door could be opened by a rogue transmitter, but i'm old school when it comes to security - no power, no open.

    thanks
  2. Thatguy

    Thatguy Homeowner

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    Yes, as long as the switch contact load ratings are sufficient. For a given contact lifetime, it's easier to switch a given amount of AC amps than DC amps because the alternating current goes through zero 120 times/second.

    Ratings are for motor/inductive load, tungsten (lamp) load and resistive load. A common wall switch rated at 15A could be rated at 12A for a motor load.
    Last edited: May 1, 2009
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    You shouldn't have high and low-voltage circuits in the same box even though the switch will handle it. Adding a loop to DC circuits will add resistance, and likely require increasing the wire gauge to maintain low voltage drops, so take that into consideration if you put in a separate switch for the low-voltage circuit. FWIW, some openers have a 'vacation' switch that essentially disables the RF section. With the newer rolling code, multi-billion possibilites remotes, somebody hacking into the system to open the door is really hard to do, and if they can do that, they'd get in one-way or another anyway.
  4. Thatguy

    Thatguy Homeowner

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    With a single switch and relay(s) you can keep the hi/lo separation and still make your design work.

    Coming at it from the other end, find out how to comply with the NEC and then we'll design circuitry to meet your desired function.

    One possible way: the relay that cuts battery power can have a 120vac coil or a 24vac coil. If it has the low voltage coil you need to switch on and off a 120v:24v xformer.
    These parts are all available from Hosfelt.com for a few bucks.
  5. Billy_Bob

    Billy_Bob In the Trades

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    An energy saving design would be to just use two separate switches. One for AC and another for battery. Then zero electric use when off.

    I don't know about you, but I'm getting fed up with "parasitic loads". Everything you buy these days is "always on" and always using a little electricity. All these things add up.

    Here is more on this...

    Leaking Electricity: Individual Field Measurement of Consumer Electronics
    http://enduse.lbl.gov/info/ACEEE-Leaking.pdf
  6. Thatguy

    Thatguy Homeowner

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    The product marketing people have decided that the consumer's utility for "instant" is more than his/her disutility for wasting some power.
    They're probably right; everybody wants instant nowadays. These people would not have been able to withstand the vacuum tube and no-cell-phone era.
  7. hj

    hj Moderator & Master Plumber Staff Member

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    Location:
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    door

    Why don't you have a wall switch with the "off" circuit that kills the radio receiver and only allows the wall switch to open the door?
  8. Thatguy

    Thatguy Homeowner

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    Verily, Sire.
    It seems that there is more than one way to skin a cat.

    Listing all possible options as a first step is what Decision Theory recommends for solving these type problems.
  9. beekerc

    beekerc IT Consultant / Network Engineer

    Messages:
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    Location:
    Seattle
    agreed, but while i do take advantage of may high tech security options, some habits just die really hard.

    i'm now thinking may be i should go with a single-gang, dual-switch device, then maybe use some plastic and glue to tie the switches together, so it acts like a double breaker. sure, there's both high and low voltage coming into the same box, but at least the circuits are isolated.

    i'm pretty sure the RF receiver is integrated into a circuit board somewhere, being able to put a switch to interrupt its function would probably require some serious "surgery".

    this wouldn't work. if i understand how relays work, as long as there is 120v flowing through the relay, it closes the circuit that the low voltage circuit. if a/c power is cut, then it breaks the d/c circuit. is this correct?

    the only problem is that if the power goes out, then the relay would kill the battery backup. i only want battery backup cut when i flip the switch manually.
  10. Thatguy

    Thatguy Homeowner

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    You can have it either way, with Normally Open or Normally Closed contacts.
    http://www.rowand.net/Shop/Tech/images/RelayWiringGuide.jpg
    Last edited: May 2, 2009
  11. manny1602

    manny1602 New Member

    Messages:
    6
    The relay doesn't know if he flipped the switch or if it's a power outage.
    Why not just a pad lock through the holes on one of your tracks so the rollers can't move?
  12. Thatguy

    Thatguy Homeowner

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    It begins to sound like this problem is unsolvable, and that is not possible.
  13. killavolt

    killavolt In the Trades

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    Location:
    Southington CT
    That's what I do...keeps the motorcycle safe when I'm away.
  14. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
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    Location:
    Cave Creek, Arizona
    door

    EVERY electric door motor, and wall pad, that I have had since 1977, has had a lockout circuit on the pad to cut out the radio receiver circuit and ONLY allow the wall pad to operate the door. How old is your operator?
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